runtime: handle nil pointer in sysFreeOS on sbrk platforms On sbrk platforms (Plan 9 and Wasm), sysReserveOS returns nil when it can't reserve memory at the requested hint address. When the caller passes that nil result to sysUnreserve, sysFreeOS reaches memFree(nil), which clears the region with memclrNoHeapPointers, writing to address 0. On Plan 9, this faults the program with a "sys: trap: fault write" note. It is reproducible on plan9/amd64 with TestArenaCollision, which exhausts the arena hints and unreserves the failed reservations. On mmap-based platforms, sysUnreserve(nil) is a harmless munmap(nil). This change makes sysFreeOS return early for a nil pointer, so freeing or unreserving nil is a no-op on sbrk platforms too. Change-Id: I2e3057a5bb34254d600d05b5fa128c77960d4e9a Reviewed-on: https://go-review.googlesource.com/c/go/+/793400 Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Richard Miller <millerresearch@gmail.com>
diff --git a/src/runtime/mem_sbrk.go b/src/runtime/mem_sbrk.go index 9e752df..b8a2a75 100644 --- a/src/runtime/mem_sbrk.go +++ b/src/runtime/mem_sbrk.go
@@ -206,6 +206,10 @@ } func sysFreeOS(v unsafe.Pointer, n uintptr) { + if v == nil { + // A failed sysReserveOS returns nil, so freeing it is a no-op. + return + } systemstack(func() { lock(&memlock) if uintptr(v)+n == bloc {