syscall: define Syscall in terms of RawSyscall on linux

This is a re-do of CL 388477, fixing #52472.

It is unsafe to call syscall.RawSyscall from syscall.Syscall with
-coverpkg=all and -race. This is because:

1. Coverage adds a sync/atomic call in RawSyscall to increment the
   coverage counter.
2. Race mode instruments sync/atomic calls with TSAN runtime calls. TSAN
   eventually calls runtime.racecallbackfunc, which expects
   getg().m.p != 0, which is no longer true after entersyscall().

cmd/go actually avoids adding coverage instrumention to package runtime
in race mode entirely to avoid these kinds of problems. Rather than also
excluding all of syscall for this one function, work around by calling
RawSyscall6 instead, which avoids coverage instrumention both by being
written in assembly and in package runtime/*.

For #51087
Fixes #52472

Change-Id: Iaffd27df03753020c4716059a455d6ca7b62f347
Reviewed-on: https://go-review.googlesource.com/c/go/+/401654
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 4535f62053..b281d9f3b8 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -3658,7 +3658,7 @@ func reentersyscall(pc, sp uintptr) {
 
 // Standard syscall entry used by the go syscall library and normal cgo calls.
 //
-// This is exported via linkname to assembly in the syscall package.
+// This is exported via linkname to assembly in the syscall package and x/sys.
 //
 //go:nosplit
 //go:linkname entersyscall