syscall: add //go:norace to rawSyscall on darwin

On darwin, rawSyscall, rawSyscall6, and rawSyscall9 are Go
functions (not assembly as on Linux) that get race-detector
instrumentation. When forkAndExecInChild calls rawSyscall in
the forked child process, the TSan ThreadState pointer is
invalid, causing SIGSEGV in TraceSwitchPartImpl.

Add //go:norace to these functions so the race detector does
not instrument them. Also update the stale comment in
exec_libc2.go that referred to rawSyscall as assembly.

Fixes #79804

Author: SebTardif

src/syscall/exec_libc2.go

@@ -48,8 +48,9 @@ func runtime_AfterForkInChild()
 // they might have been locked at the time of the fork. This means
 // no rescheduling, no malloc calls, and no new stack segments.
 // For the same reason compiler does not race instrument it.
-// The calls to rawSyscall are okay because they are assembly
-// functions that do not grow the stack.
+// The calls to rawSyscall are okay because they are nosplit
+// functions that do not grow the stack and are not race
+// instrumented (go:norace).
 //
 //go:norace
 func forkAndExecInChild(argv0 *byte, argv, envv []*byte, chroot, dir *byte, attr *ProcAttr, sys *SysProcAttr, pipe int) (pid int, err1 Errno) {

src/syscall/syscall_darwin.go

@@ -409,6 +409,7 @@ func syscall9(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, e
 
 //go:linkname rawSyscall
 //go:nosplit
+//go:norace
 //go:uintptrkeepalive
 func rawSyscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno) {
 	r1, r2, err = rawsyscalln(fn, a1, a2, a3)
@@ -417,6 +418,7 @@ func rawSyscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno) {
 
 //go:linkname rawSyscall6
 //go:nosplit
+//go:norace
 //go:uintptrkeepalive
 func rawSyscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno) {
 	r1, r2, err = rawsyscalln(fn, a1, a2, a3, a4, a5, a6)
@@ -425,6 +427,7 @@ func rawSyscall6(fn, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err Errno)
 
 //go:linkname rawSyscall9
 //go:nosplit
+//go:norace
 //go:uintptrkeepalive
 func rawSyscall9(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr, err Errno) {
 	r1, r2, err = rawsyscalln(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9)