syscall: Plan 9: use lightweight errstr in entersyscall mode
Change 231af8ac63aa (CL 7314062) made runtime.enteryscall()
set m->mcache = nil, which means that we can no longer use
syscall.errstr in syscall.Syscall and syscall.Syscall6, since it
requires a new buffer to be allocated for holding the error string.
Instead, we use pre-allocated per-M storage to hold error strings
from syscalls made while in entersyscall mode, and call
runtime.findnull to calculate the lengths.
Fixes #4994.
R=rsc, rminnich, ality, dvyukov, rminnich, r
CC=golang-dev
https://golang.org/cl/7567043
diff --git a/src/pkg/runtime/sys_plan9_amd64.s b/src/pkg/runtime/sys_plan9_amd64.s
index b34f98a..68563f3 100644
--- a/src/pkg/runtime/sys_plan9_amd64.s
+++ b/src/pkg/runtime/sys_plan9_amd64.s
@@ -206,3 +206,24 @@
MOVL AX, 0(SP)
LDMXCSR 0(SP)
RET
+
+#define ERRMAX 128 /* from os_plan9.h */
+
+// func errstr() String
+// Only used by package syscall.
+// Grab error string due to a syscall made
+// in entersyscall mode, without going
+// through the allocator (issue 4994).
+// See ../syscall/asm_plan9_386.s:/·Syscall/
+TEXT runtime·errstr(SB),7,$0
+ get_tls(AX)
+ MOVQ m(AX), BX
+ MOVQ m_errstr(BX), CX
+ MOVQ CX, 8(SP)
+ MOVQ $ERRMAX, 16(SP)
+ MOVQ $0x8000, AX
+ MOVQ $41, BP
+ SYSCALL
+ CALL runtime·findnull(SB)
+ MOVQ AX, 16(SP)
+ RET
