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// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package runtime
import (
"internal/abi"
"internal/goarch"
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
// throwType indicates the current type of ongoing throw, which affects the
// amount of detail printed to stderr. Higher values include more detail.
type throwType uint32
const (
// throwTypeNone means that we are not throwing.
throwTypeNone throwType = iota
// throwTypeUser is a throw due to a problem with the application.
//
// These throws do not include runtime frames, system goroutines, or
// frame metadata.
throwTypeUser
// throwTypeRuntime is a throw due to a problem with Go itself.
//
// These throws include as much information as possible to aid in
// debugging the runtime, including runtime frames, system goroutines,
// and frame metadata.
throwTypeRuntime
)
// We have two different ways of doing defers. The older way involves creating a
// defer record at the time that a defer statement is executing and adding it to a
// defer chain. This chain is inspected by the deferreturn call at all function
// exits in order to run the appropriate defer calls. A cheaper way (which we call
// open-coded defers) is used for functions in which no defer statements occur in
// loops. In that case, we simply store the defer function/arg information into
// specific stack slots at the point of each defer statement, as well as setting a
// bit in a bitmask. At each function exit, we add inline code to directly make
// the appropriate defer calls based on the bitmask and fn/arg information stored
// on the stack. During panic/Goexit processing, the appropriate defer calls are
// made using extra funcdata info that indicates the exact stack slots that
// contain the bitmask and defer fn/args.
// Check to make sure we can really generate a panic. If the panic
// was generated from the runtime, or from inside malloc, then convert
// to a throw of msg.
// pc should be the program counter of the compiler-generated code that
// triggered this panic.
func panicCheck1(pc uintptr, msg string) {
if goarch.IsWasm == 0 && hasPrefix(funcname(findfunc(pc)), "runtime.") {
// Note: wasm can't tail call, so we can't get the original caller's pc.
throw(msg)
}
// TODO: is this redundant? How could we be in malloc
// but not in the runtime? runtime/internal/*, maybe?
gp := getg()
if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
throw(msg)
}
}
// Same as above, but calling from the runtime is allowed.
//
// Using this function is necessary for any panic that may be
// generated by runtime.sigpanic, since those are always called by the
// runtime.
func panicCheck2(err string) {
// panic allocates, so to avoid recursive malloc, turn panics
// during malloc into throws.
gp := getg()
if gp != nil && gp.m != nil && gp.m.mallocing != 0 {
throw(err)
}
}
// Many of the following panic entry-points turn into throws when they
// happen in various runtime contexts. These should never happen in
// the runtime, and if they do, they indicate a serious issue and
// should not be caught by user code.
//
// The panic{Index,Slice,divide,shift} functions are called by
// code generated by the compiler for out of bounds index expressions,
// out of bounds slice expressions, division by zero, and shift by negative.
// The panicdivide (again), panicoverflow, panicfloat, and panicmem
// functions are called by the signal handler when a signal occurs
// indicating the respective problem.
//
// Since panic{Index,Slice,shift} are never called directly, and
// since the runtime package should never have an out of bounds slice
// or array reference or negative shift, if we see those functions called from the
// runtime package we turn the panic into a throw. That will dump the
// entire runtime stack for easier debugging.
//
// The entry points called by the signal handler will be called from
// runtime.sigpanic, so we can't disallow calls from the runtime to
// these (they always look like they're called from the runtime).
// Hence, for these, we just check for clearly bad runtime conditions.
//
// The panic{Index,Slice} functions are implemented in assembly and tail call
// to the goPanic{Index,Slice} functions below. This is done so we can use
// a space-minimal register calling convention.
// failures in the comparisons for s[x], 0 <= x < y (y == len(s))
//
//go:yeswritebarrierrec
func goPanicIndex(x int, y int) {
panicCheck1(getcallerpc(), "index out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsIndex})
}
//go:yeswritebarrierrec
func goPanicIndexU(x uint, y int) {
panicCheck1(getcallerpc(), "index out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsIndex})
}
// failures in the comparisons for s[:x], 0 <= x <= y (y == len(s) or cap(s))
//
//go:yeswritebarrierrec
func goPanicSliceAlen(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAlen})
}
//go:yeswritebarrierrec
func goPanicSliceAlenU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAlen})
}
//go:yeswritebarrierrec
func goPanicSliceAcap(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceAcap})
}
//go:yeswritebarrierrec
func goPanicSliceAcapU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceAcap})
}
// failures in the comparisons for s[x:y], 0 <= x <= y
//
//go:yeswritebarrierrec
func goPanicSliceB(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSliceB})
}
//go:yeswritebarrierrec
func goPanicSliceBU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSliceB})
}
// failures in the comparisons for s[::x], 0 <= x <= y (y == len(s) or cap(s))
func goPanicSlice3Alen(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Alen})
}
func goPanicSlice3AlenU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Alen})
}
func goPanicSlice3Acap(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3Acap})
}
func goPanicSlice3AcapU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3Acap})
}
// failures in the comparisons for s[:x:y], 0 <= x <= y
func goPanicSlice3B(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3B})
}
func goPanicSlice3BU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3B})
}
// failures in the comparisons for s[x:y:], 0 <= x <= y
func goPanicSlice3C(x int, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsSlice3C})
}
func goPanicSlice3CU(x uint, y int) {
panicCheck1(getcallerpc(), "slice bounds out of range")
panic(boundsError{x: int64(x), signed: false, y: y, code: boundsSlice3C})
}
// failures in the conversion ([x]T)(s) or (*[x]T)(s), 0 <= x <= y, y == len(s)
func goPanicSliceConvert(x int, y int) {
panicCheck1(getcallerpc(), "slice length too short to convert to array or pointer to array")
panic(boundsError{x: int64(x), signed: true, y: y, code: boundsConvert})
}
// Implemented in assembly, as they take arguments in registers.
// Declared here to mark them as ABIInternal.
func panicIndex(x int, y int)
func panicIndexU(x uint, y int)
func panicSliceAlen(x int, y int)
func panicSliceAlenU(x uint, y int)
func panicSliceAcap(x int, y int)
func panicSliceAcapU(x uint, y int)
func panicSliceB(x int, y int)
func panicSliceBU(x uint, y int)
func panicSlice3Alen(x int, y int)
func panicSlice3AlenU(x uint, y int)
func panicSlice3Acap(x int, y int)
func panicSlice3AcapU(x uint, y int)
func panicSlice3B(x int, y int)
func panicSlice3BU(x uint, y int)
func panicSlice3C(x int, y int)
func panicSlice3CU(x uint, y int)
func panicSliceConvert(x int, y int)
var shiftError = error(errorString("negative shift amount"))
//go:yeswritebarrierrec
func panicshift() {
panicCheck1(getcallerpc(), "negative shift amount")
panic(shiftError)
}
var divideError = error(errorString("integer divide by zero"))
//go:yeswritebarrierrec
func panicdivide() {
panicCheck2("integer divide by zero")
panic(divideError)
}
var overflowError = error(errorString("integer overflow"))
func panicoverflow() {
panicCheck2("integer overflow")
panic(overflowError)
}
var floatError = error(errorString("floating point error"))
func panicfloat() {
panicCheck2("floating point error")
panic(floatError)
}
var memoryError = error(errorString("invalid memory address or nil pointer dereference"))
func panicmem() {
panicCheck2("invalid memory address or nil pointer dereference")
panic(memoryError)
}
func panicmemAddr(addr uintptr) {
panicCheck2("invalid memory address or nil pointer dereference")
panic(errorAddressString{msg: "invalid memory address or nil pointer dereference", addr: addr})
}
// Create a new deferred function fn, which has no arguments and results.
// The compiler turns a defer statement into a call to this.
func deferproc(fn func()) {
gp := getg()
if gp.m.curg != gp {
// go code on the system stack can't defer
throw("defer on system stack")
}
d := newdefer()
d.link = gp._defer
gp._defer = d
d.fn = fn
d.pc = getcallerpc()
// We must not be preempted between calling getcallersp and
// storing it to d.sp because getcallersp's result is a
// uintptr stack pointer.
d.sp = getcallersp()
// deferproc returns 0 normally.
// a deferred func that stops a panic
// makes the deferproc return 1.
// the code the compiler generates always
// checks the return value and jumps to the
// end of the function if deferproc returns != 0.
return0()
// No code can go here - the C return register has
// been set and must not be clobbered.
}
var rangeExitError = error(errorString("range function continued iteration after exit"))
//go:noinline
func panicrangeexit() {
panic(rangeExitError)
}
// deferrangefunc is called by functions that are about to
// execute a range-over-function loop in which the loop body
// may execute a defer statement. That defer needs to add to
// the chain for the current function, not the func literal synthesized
// to represent the loop body. To do that, the original function
// calls deferrangefunc to obtain an opaque token representing
// the current frame, and then the loop body uses deferprocat
// instead of deferproc to add to that frame's defer lists.
//
// The token is an 'any' with underlying type *atomic.Pointer[_defer].
// It is the atomically-updated head of a linked list of _defer structs
// representing deferred calls. At the same time, we create a _defer
// struct on the main g._defer list with d.head set to this head pointer.
//
// The g._defer list is now a linked list of deferred calls,
// but an atomic list hanging off:
//
// g._defer => d4 -> d3 -> drangefunc -> d2 -> d1 -> nil
// | .head
// |
// +--> dY -> dX -> nil
//
// with each -> indicating a d.link pointer, and where drangefunc
// has the d.rangefunc = true bit set.
// Note that the function being ranged over may have added
// its own defers (d4 and d3), so drangefunc need not be at the
// top of the list when deferprocat is used. This is why we pass
// the atomic head explicitly.
//
// To keep misbehaving programs from crashing the runtime,
// deferprocat pushes new defers onto the .head list atomically.
// The fact that it is a separate list from the main goroutine
// defer list means that the main goroutine's defers can still
// be handled non-atomically.
//
// In the diagram, dY and dX are meant to be processed when
// drangefunc would be processed, which is to say the defer order
// should be d4, d3, dY, dX, d2, d1. To make that happen,
// when defer processing reaches a d with rangefunc=true,
// it calls deferconvert to atomically take the extras
// away from d.head and then adds them to the main list.
//
// That is, deferconvert changes this list:
//
// g._defer => drangefunc -> d2 -> d1 -> nil
// | .head
// |
// +--> dY -> dX -> nil
//
// into this list:
//
// g._defer => dY -> dX -> d2 -> d1 -> nil
//
// It also poisons *drangefunc.head so that any future
// deferprocat using that head will throw.
// (The atomic head is ordinary garbage collected memory so that
// it's not a problem if user code holds onto it beyond
// the lifetime of drangefunc.)
//
// TODO: We could arrange for the compiler to call into the
// runtime after the loop finishes normally, to do an eager
// deferconvert, which would catch calling the loop body
// and having it defer after the loop is done. If we have a
// more general catch of loop body misuse, though, this
// might not be worth worrying about in addition.
//
// See also ../cmd/compile/internal/rangefunc/rewrite.go.
func deferrangefunc() any {
gp := getg()
if gp.m.curg != gp {
// go code on the system stack can't defer
throw("defer on system stack")
}
d := newdefer()
d.link = gp._defer
gp._defer = d
d.pc = getcallerpc()
// We must not be preempted between calling getcallersp and
// storing it to d.sp because getcallersp's result is a
// uintptr stack pointer.
d.sp = getcallersp()
d.rangefunc = true
d.head = new(atomic.Pointer[_defer])
return d.head
}
// badDefer returns a fixed bad defer pointer for poisoning an atomic defer list head.
func badDefer() *_defer {
return (*_defer)(unsafe.Pointer(uintptr(1)))
}
// deferprocat is like deferproc but adds to the atomic list represented by frame.
// See the doc comment for deferrangefunc for details.
func deferprocat(fn func(), frame any) {
head := frame.(*atomic.Pointer[_defer])
if raceenabled {
racewritepc(unsafe.Pointer(head), getcallerpc(), abi.FuncPCABIInternal(deferprocat))
}
d1 := newdefer()
d1.fn = fn
for {
d1.link = head.Load()
if d1.link == badDefer() {
throw("defer after range func returned")
}
if head.CompareAndSwap(d1.link, d1) {
break
}
}
// Must be last - see deferproc above.
return0()
}
// deferconvert converts the rangefunc defer list of d0 into an ordinary list
// following d0.
// See the doc comment for deferrangefunc for details.
func deferconvert(d0 *_defer) {
head := d0.head
if raceenabled {
racereadpc(unsafe.Pointer(head), getcallerpc(), abi.FuncPCABIInternal(deferconvert))
}
tail := d0.link
d0.rangefunc = false
var d *_defer
for {
d = head.Load()
if head.CompareAndSwap(d, badDefer()) {
break
}
}
if d == nil {
return
}
for d1 := d; ; d1 = d1.link {
d1.sp = d0.sp
d1.pc = d0.pc
if d1.link == nil {
d1.link = tail
break
}
}
d0.link = d
return
}
// deferprocStack queues a new deferred function with a defer record on the stack.
// The defer record must have its fn field initialized.
// All other fields can contain junk.
// Nosplit because of the uninitialized pointer fields on the stack.
//
//go:nosplit
func deferprocStack(d *_defer) {
gp := getg()
if gp.m.curg != gp {
// go code on the system stack can't defer
throw("defer on system stack")
}
// fn is already set.
// The other fields are junk on entry to deferprocStack and
// are initialized here.
d.heap = false
d.rangefunc = false
d.sp = getcallersp()
d.pc = getcallerpc()
// The lines below implement:
// d.panic = nil
// d.fd = nil
// d.link = gp._defer
// d.head = nil
// gp._defer = d
// But without write barriers. The first three are writes to
// the stack so they don't need a write barrier, and furthermore
// are to uninitialized memory, so they must not use a write barrier.
// The fourth write does not require a write barrier because we
// explicitly mark all the defer structures, so we don't need to
// keep track of pointers to them with a write barrier.
*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
*(*uintptr)(unsafe.Pointer(&d.head)) = 0
*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))
return0()
// No code can go here - the C return register has
// been set and must not be clobbered.
}
// Each P holds a pool for defers.
// Allocate a Defer, usually using per-P pool.
// Each defer must be released with freedefer. The defer is not
// added to any defer chain yet.
func newdefer() *_defer {
var d *_defer
mp := acquirem()
pp := mp.p.ptr()
if len(pp.deferpool) == 0 && sched.deferpool != nil {
lock(&sched.deferlock)
for len(pp.deferpool) < cap(pp.deferpool)/2 && sched.deferpool != nil {
d := sched.deferpool
sched.deferpool = d.link
d.link = nil
pp.deferpool = append(pp.deferpool, d)
}
unlock(&sched.deferlock)
}
if n := len(pp.deferpool); n > 0 {
d = pp.deferpool[n-1]
pp.deferpool[n-1] = nil
pp.deferpool = pp.deferpool[:n-1]
}
releasem(mp)
mp, pp = nil, nil
if d == nil {
// Allocate new defer.
d = new(_defer)
}
d.heap = true
return d
}
// popDefer pops the head of gp's defer list and frees it.
func popDefer(gp *g) {
d := gp._defer
d.fn = nil // Can in theory point to the stack
// We must not copy the stack between the updating gp._defer and setting
// d.link to nil. Between these two steps, d is not on any defer list, so
// stack copying won't adjust stack pointers in it (namely, d.link). Hence,
// if we were to copy the stack, d could then contain a stale pointer.
gp._defer = d.link
d.link = nil
// After this point we can copy the stack.
if !d.heap {
return
}
mp := acquirem()
pp := mp.p.ptr()
if len(pp.deferpool) == cap(pp.deferpool) {
// Transfer half of local cache to the central cache.
var first, last *_defer
for len(pp.deferpool) > cap(pp.deferpool)/2 {
n := len(pp.deferpool)
d := pp.deferpool[n-1]
pp.deferpool[n-1] = nil
pp.deferpool = pp.deferpool[:n-1]
if first == nil {
first = d
} else {
last.link = d
}
last = d
}
lock(&sched.deferlock)
last.link = sched.deferpool
sched.deferpool = first
unlock(&sched.deferlock)
}
*d = _defer{}
pp.deferpool = append(pp.deferpool, d)
releasem(mp)
mp, pp = nil, nil
}
// deferreturn runs deferred functions for the caller's frame.
// The compiler inserts a call to this at the end of any
// function which calls defer.
func deferreturn() {
var p _panic
p.deferreturn = true
p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
for {
fn, ok := p.nextDefer()
if !ok {
break
}
fn()
}
}
// Goexit terminates the goroutine that calls it. No other goroutine is affected.
// Goexit runs all deferred calls before terminating the goroutine. Because Goexit
// is not a panic, any recover calls in those deferred functions will return nil.
//
// Calling Goexit from the main goroutine terminates that goroutine
// without func main returning. Since func main has not returned,
// the program continues execution of other goroutines.
// If all other goroutines exit, the program crashes.
func Goexit() {
// Create a panic object for Goexit, so we can recognize when it might be
// bypassed by a recover().
var p _panic
p.goexit = true
p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
for {
fn, ok := p.nextDefer()
if !ok {
break
}
fn()
}
goexit1()
}
// Call all Error and String methods before freezing the world.
// Used when crashing with panicking.
func preprintpanics(p *_panic) {
defer func() {
text := "panic while printing panic value"
switch r := recover().(type) {
case nil:
// nothing to do
case string:
throw(text + ": " + r)
default:
throw(text + ": type " + toRType(efaceOf(&r)._type).string())
}
}()
for p != nil {
switch v := p.arg.(type) {
case error:
p.arg = v.Error()
case stringer:
p.arg = v.String()
}
p = p.link
}
}
// Print all currently active panics. Used when crashing.
// Should only be called after preprintpanics.
func printpanics(p *_panic) {
if p.link != nil {
printpanics(p.link)
if !p.link.goexit {
print("\t")
}
}
if p.goexit {
return
}
print("panic: ")
printany(p.arg)
if p.recovered {
print(" [recovered]")
}
print("\n")
}
// readvarintUnsafe reads the uint32 in varint format starting at fd, and returns the
// uint32 and a pointer to the byte following the varint.
//
// The implementation is the same with runtime.readvarint, except that this function
// uses unsafe.Pointer for speed.
func readvarintUnsafe(fd unsafe.Pointer) (uint32, unsafe.Pointer) {
var r uint32
var shift int
for {
b := *(*uint8)(fd)
fd = add(fd, unsafe.Sizeof(b))
if b < 128 {
return r + uint32(b)<<shift, fd
}
r += uint32(b&0x7F) << (shift & 31)
shift += 7
if shift > 28 {
panic("Bad varint")
}
}
}
// A PanicNilError happens when code calls panic(nil).
//
// Before Go 1.21, programs that called panic(nil) observed recover returning nil.
// Starting in Go 1.21, programs that call panic(nil) observe recover returning a *PanicNilError.
// Programs can change back to the old behavior by setting GODEBUG=panicnil=1.
type PanicNilError struct {
// This field makes PanicNilError structurally different from
// any other struct in this package, and the _ makes it different
// from any struct in other packages too.
// This avoids any accidental conversions being possible
// between this struct and some other struct sharing the same fields,
// like happened in go.dev/issue/56603.
_ [0]*PanicNilError
}
func (*PanicNilError) Error() string { return "panic called with nil argument" }
func (*PanicNilError) RuntimeError() {}
var panicnil = &godebugInc{name: "panicnil"}
// The implementation of the predeclared function panic.
func gopanic(e any) {
if e == nil {
if debug.panicnil.Load() != 1 {
e = new(PanicNilError)
} else {
panicnil.IncNonDefault()
}
}
gp := getg()
if gp.m.curg != gp {
print("panic: ")
printany(e)
print("\n")
throw("panic on system stack")
}
if gp.m.mallocing != 0 {
print("panic: ")
printany(e)
print("\n")
throw("panic during malloc")
}
if gp.m.preemptoff != "" {
print("panic: ")
printany(e)
print("\n")
print("preempt off reason: ")
print(gp.m.preemptoff)
print("\n")
throw("panic during preemptoff")
}
if gp.m.locks != 0 {
print("panic: ")
printany(e)
print("\n")
throw("panic holding locks")
}
var p _panic
p.arg = e
runningPanicDefers.Add(1)
p.start(getcallerpc(), unsafe.Pointer(getcallersp()))
for {
fn, ok := p.nextDefer()
if !ok {
break
}
fn()
}
// ran out of deferred calls - old-school panic now
// Because it is unsafe to call arbitrary user code after freezing
// the world, we call preprintpanics to invoke all necessary Error
// and String methods to prepare the panic strings before startpanic.
preprintpanics(&p)
fatalpanic(&p) // should not return
*(*int)(nil) = 0 // not reached
}
// start initializes a panic to start unwinding the stack.
//
// If p.goexit is true, then start may return multiple times.
func (p *_panic) start(pc uintptr, sp unsafe.Pointer) {
gp := getg()
// Record the caller's PC and SP, so recovery can identify panics
// that have been recovered. Also, so that if p is from Goexit, we
// can restart its defer processing loop if a recovered panic tries
// to jump past it.
p.startPC = getcallerpc()
p.startSP = unsafe.Pointer(getcallersp())
if p.deferreturn {
p.sp = sp
if s := (*savedOpenDeferState)(gp.param); s != nil {
// recovery saved some state for us, so that we can resume
// calling open-coded defers without unwinding the stack.
gp.param = nil
p.retpc = s.retpc
p.deferBitsPtr = (*byte)(add(sp, s.deferBitsOffset))
p.slotsPtr = add(sp, s.slotsOffset)
}
return
}
p.link = gp._panic
gp._panic = (*_panic)(noescape(unsafe.Pointer(p)))
// Initialize state machine, and find the first frame with a defer.
//
// Note: We could use startPC and startSP here, but callers will
// never have defer statements themselves. By starting at their
// caller instead, we avoid needing to unwind through an extra
// frame. It also somewhat simplifies the terminating condition for
// deferreturn.
p.lr, p.fp = pc, sp
p.nextFrame()
}
// nextDefer returns the next deferred function to invoke, if any.
//
// Note: The "ok bool" result is necessary to correctly handle when
// the deferred function itself was nil (e.g., "defer (func())(nil)").
func (p *_panic) nextDefer() (func(), bool) {
gp := getg()
if !p.deferreturn {
if gp._panic != p {
throw("bad panic stack")
}
if p.recovered {
mcall(recovery) // does not return
throw("recovery failed")
}
}
// The assembler adjusts p.argp in wrapper functions that shouldn't
// be visible to recover(), so we need to restore it each iteration.
p.argp = add(p.startSP, sys.MinFrameSize)
for {
for p.deferBitsPtr != nil {
bits := *p.deferBitsPtr
// Check whether any open-coded defers are still pending.
//
// Note: We need to check this upfront (rather than after
// clearing the top bit) because it's possible that Goexit
// invokes a deferred call, and there were still more pending
// open-coded defers in the frame; but then the deferred call
// panic and invoked the remaining defers in the frame, before
// recovering and restarting the Goexit loop.
if bits == 0 {
p.deferBitsPtr = nil
break
}
// Find index of top bit set.
i := 7 - uintptr(sys.LeadingZeros8(bits))
// Clear bit and store it back.
bits &^= 1 << i
*p.deferBitsPtr = bits
return *(*func())(add(p.slotsPtr, i*goarch.PtrSize)), true
}
Recheck:
if d := gp._defer; d != nil && d.sp == uintptr(p.sp) {
if d.rangefunc {
deferconvert(d)
popDefer(gp)
goto Recheck
}
fn := d.fn
// TODO(mdempsky): Instead of having each deferproc call have
// its own "deferreturn(); return" sequence, we should just make
// them reuse the one we emit for open-coded defers.
p.retpc = d.pc
// Unlink and free.
popDefer(gp)
return fn, true
}
if !p.nextFrame() {
return nil, false
}
}
}
// nextFrame finds the next frame that contains deferred calls, if any.
func (p *_panic) nextFrame() (ok bool) {
if p.lr == 0 {
return false
}
gp := getg()
systemstack(func() {
var limit uintptr
if d := gp._defer; d != nil {
limit = d.sp
}
var u unwinder
u.initAt(p.lr, uintptr(p.fp), 0, gp, 0)
for {
if !u.valid() {
p.lr = 0
return // ok == false
}
// TODO(mdempsky): If we populate u.frame.fn.deferreturn for
// every frame containing a defer (not just open-coded defers),
// then we can simply loop until we find the next frame where
// it's non-zero.
if u.frame.sp == limit {
break // found a frame with linked defers
}
if p.initOpenCodedDefers(u.frame.fn, unsafe.Pointer(u.frame.varp)) {
break // found a frame with open-coded defers
}
u.next()
}
p.lr = u.frame.lr
p.sp = unsafe.Pointer(u.frame.sp)
p.fp = unsafe.Pointer(u.frame.fp)
ok = true
})
return
}
func (p *_panic) initOpenCodedDefers(fn funcInfo, varp unsafe.Pointer) bool {
fd := funcdata(fn, abi.FUNCDATA_OpenCodedDeferInfo)
if fd == nil {
return false
}
if fn.deferreturn == 0 {
throw("missing deferreturn")
}
deferBitsOffset, fd := readvarintUnsafe(fd)
deferBitsPtr := (*uint8)(add(varp, -uintptr(deferBitsOffset)))
if *deferBitsPtr == 0 {
return false // has open-coded defers, but none pending
}
slotsOffset, fd := readvarintUnsafe(fd)
p.retpc = fn.entry() + uintptr(fn.deferreturn)
p.deferBitsPtr = deferBitsPtr
p.slotsPtr = add(varp, -uintptr(slotsOffset))
return true
}
// The implementation of the predeclared function recover.
// Cannot split the stack because it needs to reliably
// find the stack segment of its caller.
//
// TODO(rsc): Once we commit to CopyStackAlways,
// this doesn't need to be nosplit.
//
//go:nosplit
func gorecover(argp uintptr) any {
// Must be in a function running as part of a deferred call during the panic.
// Must be called from the topmost function of the call
// (the function used in the defer statement).
// p.argp is the argument pointer of that topmost deferred function call.
// Compare against argp reported by caller.
// If they match, the caller is the one who can recover.
gp := getg()
p := gp._panic
if p != nil && !p.goexit && !p.recovered && argp == uintptr(p.argp) {
p.recovered = true
return p.arg
}
return nil
}
//go:linkname sync_throw sync.throw
func sync_throw(s string) {
throw(s)
}
//go:linkname sync_fatal sync.fatal
func sync_fatal(s string) {
fatal(s)
}
// throw triggers a fatal error that dumps a stack trace and exits.
//
// throw should be used for runtime-internal fatal errors where Go itself,
// rather than user code, may be at fault for the failure.
//
//go:nosplit
func throw(s string) {
// Everything throw does should be recursively nosplit so it
// can be called even when it's unsafe to grow the stack.
systemstack(func() {
print("fatal error: ", s, "\n")
})
fatalthrow(throwTypeRuntime)
}
// fatal triggers a fatal error that dumps a stack trace and exits.
//
// fatal is equivalent to throw, but is used when user code is expected to be
// at fault for the failure, such as racing map writes.
//
// fatal does not include runtime frames, system goroutines, or frame metadata
// (fp, sp, pc) in the stack trace unless GOTRACEBACK=system or higher.
//
//go:nosplit
func fatal(s string) {
// Everything fatal does should be recursively nosplit so it
// can be called even when it's unsafe to grow the stack.
systemstack(func() {
print("fatal error: ", s, "\n")
})
fatalthrow(throwTypeUser)
}
// runningPanicDefers is non-zero while running deferred functions for panic.
// This is used to try hard to get a panic stack trace out when exiting.
var runningPanicDefers atomic.Uint32
// panicking is non-zero when crashing the program for an unrecovered panic.
var panicking atomic.Uint32
// paniclk is held while printing the panic information and stack trace,
// so that two concurrent panics don't overlap their output.
var paniclk mutex
// Unwind the stack after a deferred function calls recover
// after a panic. Then arrange to continue running as though
// the caller of the deferred function returned normally.
//
// However, if unwinding the stack would skip over a Goexit call, we
// return into the Goexit loop instead, so it can continue processing
// defers instead.
func recovery(gp *g) {
p := gp._panic
pc, sp, fp := p.retpc, uintptr(p.sp), uintptr(p.fp)
p0, saveOpenDeferState := p, p.deferBitsPtr != nil && *p.deferBitsPtr != 0
// Unwind the panic stack.
for ; p != nil && uintptr(p.startSP) < sp; p = p.link {
// Don't allow jumping past a pending Goexit.
// Instead, have its _panic.start() call return again.
//
// TODO(mdempsky): In this case, Goexit will resume walking the
// stack where it left off, which means it will need to rewalk
// frames that we've already processed.
//
// There's a similar issue with nested panics, when the inner
// panic supercedes the outer panic. Again, we end up needing to
// walk the same stack frames.
//
// These are probably pretty rare occurrences in practice, and
// they don't seem any worse than the existing logic. But if we
// move the unwinding state into _panic, we could detect when we
// run into where the last panic started, and then just pick up
// where it left off instead.
//
// With how subtle defer handling is, this might not actually be
// worthwhile though.
if p.goexit {
pc, sp = p.startPC, uintptr(p.startSP)
saveOpenDeferState = false // goexit is unwinding the stack anyway
break
}
runningPanicDefers.Add(-1)
}
gp._panic = p
if p == nil { // must be done with signal
gp.sig = 0
}
if gp.param != nil {
throw("unexpected gp.param")
}
if saveOpenDeferState {
// If we're returning to deferreturn and there are more open-coded
// defers for it to call, save enough state for it to be able to
// pick up where p0 left off.
gp.param = unsafe.Pointer(&savedOpenDeferState{
retpc: p0.retpc,
// We need to save deferBitsPtr and slotsPtr too, but those are
// stack pointers. To avoid issues around heap objects pointing
// to the stack, save them as offsets from SP.
deferBitsOffset: uintptr(unsafe.Pointer(p0.deferBitsPtr)) - uintptr(p0.sp),
slotsOffset: uintptr(p0.slotsPtr) - uintptr(p0.sp),
})
}
// TODO(mdempsky): Currently, we rely on frames containing "defer"
// to end with "CALL deferreturn; RET". This allows deferreturn to
// finish running any pending defers in the frame.
//
// But we should be able to tell whether there are still pending
// defers here. If there aren't, we can just jump directly to the
// "RET" instruction. And if there are, we don't need an actual
// "CALL deferreturn" instruction; we can simulate it with something
// like:
//
// if usesLR {
// lr = pc
// } else {
// sp -= sizeof(pc)
// *(*uintptr)(sp) = pc
// }
// pc = funcPC(deferreturn)
//
// So that we effectively tail call into deferreturn, such that it
// then returns to the simple "RET" epilogue. That would save the
// overhead of the "deferreturn" call when there aren't actually any
// pending defers left, and shrink the TEXT size of compiled
// binaries. (Admittedly, both of these are modest savings.)
// Ensure we're recovering within the appropriate stack.
if sp != 0 && (sp < gp.stack.lo || gp.stack.hi < sp) {
print("recover: ", hex(sp), " not in [", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
throw("bad recovery")
}
// Make the deferproc for this d return again,
// this time returning 1. The calling function will
// jump to the standard return epilogue.
gp.sched.sp = sp
gp.sched.pc = pc
gp.sched.lr = 0
// Restore the bp on platforms that support frame pointers.
// N.B. It's fine to not set anything for platforms that don't
// support frame pointers, since nothing consumes them.
switch {
case goarch.IsAmd64 != 0:
// on x86, fp actually points one word higher than the top of
// the frame since the return address is saved on the stack by
// the caller
gp.sched.bp = fp - 2*goarch.PtrSize
case goarch.IsArm64 != 0:
// on arm64, the architectural bp points one word higher
// than the sp. fp is totally useless to us here, because it
// only gets us to the caller's fp.
gp.sched.bp = sp - goarch.PtrSize
}
gp.sched.ret = 1
gogo(&gp.sched)
}
// fatalthrow implements an unrecoverable runtime throw. It freezes the
// system, prints stack traces starting from its caller, and terminates the
// process.
//
//go:nosplit
func fatalthrow(t throwType) {
pc := getcallerpc()
sp := getcallersp()
gp := getg()
if gp.m.throwing == throwTypeNone {
gp.m.throwing = t
}
// Switch to the system stack to avoid any stack growth, which may make
// things worse if the runtime is in a bad state.
systemstack(func() {
if isSecureMode() {
exit(2)
}
startpanic_m()
if dopanic_m(gp, pc, sp) {
// crash uses a decent amount of nosplit stack and we're already
// low on stack in throw, so crash on the system stack (unlike
// fatalpanic).
crash()
}
exit(2)
})
*(*int)(nil) = 0 // not reached
}
// fatalpanic implements an unrecoverable panic. It is like fatalthrow, except
// that if msgs != nil, fatalpanic also prints panic messages and decrements
// runningPanicDefers once main is blocked from exiting.
//
//go:nosplit
func fatalpanic(msgs *_panic) {
pc := getcallerpc()
sp := getcallersp()
gp := getg()
var docrash bool
// Switch to the system stack to avoid any stack growth, which
// may make things worse if the runtime is in a bad state.
systemstack(func() {
if startpanic_m() && msgs != nil {
// There were panic messages and startpanic_m
// says it's okay to try to print them.
// startpanic_m set panicking, which will
// block main from exiting, so now OK to
// decrement runningPanicDefers.
runningPanicDefers.Add(-1)
printpanics(msgs)
}
docrash = dopanic_m(gp, pc, sp)
})
if docrash {
// By crashing outside the above systemstack call, debuggers
// will not be confused when generating a backtrace.
// Function crash is marked nosplit to avoid stack growth.
crash()
}
systemstack(func() {
exit(2)
})
*(*int)(nil) = 0 // not reached
}
// startpanic_m prepares for an unrecoverable panic.
//
// It returns true if panic messages should be printed, or false if
// the runtime is in bad shape and should just print stacks.
//
// It must not have write barriers even though the write barrier
// explicitly ignores writes once dying > 0. Write barriers still
// assume that g.m.p != nil, and this function may not have P
// in some contexts (e.g. a panic in a signal handler for a signal
// sent to an M with no P).
//
//go:nowritebarrierrec
func startpanic_m() bool {
gp := getg()
if mheap_.cachealloc.size == 0 { // very early
print("runtime: panic before malloc heap initialized\n")
}
// Disallow malloc during an unrecoverable panic. A panic
// could happen in a signal handler, or in a throw, or inside
// malloc itself. We want to catch if an allocation ever does
// happen (even if we're not in one of these situations).
gp.m.mallocing++
// If we're dying because of a bad lock count, set it to a
// good lock count so we don't recursively panic below.
if gp.m.locks < 0 {
gp.m.locks = 1
}
switch gp.m.dying {
case 0:
// Setting dying >0 has the side-effect of disabling this G's writebuf.
gp.m.dying = 1
panicking.Add(1)
lock(&paniclk)
if debug.schedtrace > 0 || debug.scheddetail > 0 {
schedtrace(true)
}
freezetheworld()
return true
case 1:
// Something failed while panicking.
// Just print a stack trace and exit.
gp.m.dying = 2
print("panic during panic\n")
return false
case 2:
// This is a genuine bug in the runtime, we couldn't even
// print the stack trace successfully.
gp.m.dying = 3
print("stack trace unavailable\n")
exit(4)
fallthrough
default:
// Can't even print! Just exit.
exit(5)
return false // Need to return something.
}
}
var didothers bool
var deadlock mutex
// gp is the crashing g running on this M, but may be a user G, while getg() is
// always g0.
func dopanic_m(gp *g, pc, sp uintptr) bool {
if gp.sig != 0 {
signame := signame(gp.sig)
if signame != "" {
print("[signal ", signame)
} else {
print("[signal ", hex(gp.sig))
}
print(" code=", hex(gp.sigcode0), " addr=", hex(gp.sigcode1), " pc=", hex(gp.sigpc), "]\n")
}
level, all, docrash := gotraceback()
if level > 0 {
if gp != gp.m.curg {
all = true
}
if gp != gp.m.g0 {
print("\n")
goroutineheader(gp)
traceback(pc, sp, 0, gp)
} else if level >= 2 || gp.m.throwing >= throwTypeRuntime {
print("\nruntime stack:\n")
traceback(pc, sp, 0, gp)
}
if !didothers && all {
didothers = true
tracebackothers(gp)
}
}
unlock(&paniclk)
if panicking.Add(-1) != 0 {
// Some other m is panicking too.
// Let it print what it needs to print.
// Wait forever without chewing up cpu.
// It will exit when it's done.
lock(&deadlock)
lock(&deadlock)
}
printDebugLog()
return docrash
}
// canpanic returns false if a signal should throw instead of
// panicking.
//
//go:nosplit
func canpanic() bool {
gp := getg()
mp := acquirem()
// Is it okay for gp to panic instead of crashing the program?
// Yes, as long as it is running Go code, not runtime code,
// and not stuck in a system call.
if gp != mp.curg {
releasem(mp)
return false
}
// N.B. mp.locks != 1 instead of 0 to account for acquirem.
if mp.locks != 1 || mp.mallocing != 0 || mp.throwing != throwTypeNone || mp.preemptoff != "" || mp.dying != 0 {
releasem(mp)
return false
}
status := readgstatus(gp)
if status&^_Gscan != _Grunning || gp.syscallsp != 0 {
releasem(mp)
return false
}
if GOOS == "windows" && mp.libcallsp != 0 {
releasem(mp)
return false
}
releasem(mp)
return true
}
// shouldPushSigpanic reports whether pc should be used as sigpanic's
// return PC (pushing a frame for the call). Otherwise, it should be
// left alone so that LR is used as sigpanic's return PC, effectively
// replacing the top-most frame with sigpanic. This is used by
// preparePanic.
func shouldPushSigpanic(gp *g, pc, lr uintptr) bool {
if pc == 0 {
// Probably a call to a nil func. The old LR is more
// useful in the stack trace. Not pushing the frame
// will make the trace look like a call to sigpanic
// instead. (Otherwise the trace will end at sigpanic
// and we won't get to see who faulted.)
return false
}
// If we don't recognize the PC as code, but we do recognize
// the link register as code, then this assumes the panic was
// caused by a call to non-code. In this case, we want to
// ignore this call to make unwinding show the context.
//
// If we running C code, we're not going to recognize pc as a
// Go function, so just assume it's good. Otherwise, traceback
// may try to read a stale LR that looks like a Go code
// pointer and wander into the woods.
if gp.m.incgo || findfunc(pc).valid() {
// This wasn't a bad call, so use PC as sigpanic's
// return PC.
return true
}
if findfunc(lr).valid() {
// This was a bad call, but the LR is good, so use the
// LR as sigpanic's return PC.
return false
}
// Neither the PC or LR is good. Hopefully pushing a frame
// will work.
return true
}
// isAbortPC reports whether pc is the program counter at which
// runtime.abort raises a signal.
//
// It is nosplit because it's part of the isgoexception
// implementation.
//
//go:nosplit
func isAbortPC(pc uintptr) bool {
f := findfunc(pc)
if !f.valid() {
return false
}
return f.funcID == abi.FuncID_abort
}