runtime: portable access to sigev_notify_thread_id

Previously, libgo relied on the _sigev_un implementation-specific
field in struct sigevent, which is only available on glibc.
This patch uses the sigev_notify_thread_id macro instead which is
mandated by timer_create(2). In theory, this should work with any libc
implementation for Linux. Unfortunately, there is an open glibc bug
as glibc does not define this macro. For this reason, a glibc-specific
workaround is required. Other libcs (such as musl) define the macro
and don't require the workaround.

See https://sourceware.org/bugzilla/show_bug.cgi?id=27417

This makes libgo compatible with musl libc.

Based on patch by Sören Tempel.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/434755

2 files changed, 29 insertions, 1 deletions

diff --git a/libgo/go/runtime/os_linux.go b/libgo/go/runtime/os_linux.go
index 96fb178..2b2d827 100644
--- a/libgo/go/runtime/os_linux.go
+++ b/libgo/go/runtime/os_linux.go
@@ -22,6 +22,12 @@ type mOS struct {
 	profileTimerValid uint32
 }
 
+// setSigeventTID is written in C to set the sigev_notify_thread_id
+// field of a sigevent struct.
+//
+//go:noescape
+func setSigeventTID(*_sigevent, int32)
+
 func getProcID() uint64 {
 	return uint64(gettid())
 }
@@ -52,9 +58,12 @@ const (
 )
 
 // Atomically,
+//
 //	if(*addr == val) sleep
+//
 // Might be woken up spuriously; that's allowed.
 // Don't sleep longer than ns; ns < 0 means forever.
+//
 //go:nosplit
 func futexsleep(addr *uint32, val uint32, ns int64) {
 	// Some Linux kernels have a bug where futex of
@@ -73,6 +82,7 @@ func futexsleep(addr *uint32, val uint32, ns int64) {
 }
 
 // If any procs are sleeping on addr, wake up at most cnt.
+//
 //go:nosplit
 func futexwakeup(addr *uint32, cnt uint32) {
 	ret := futex(unsafe.Pointer(addr), _FUTEX_WAKE_PRIVATE, cnt, nil, nil, 0)
@@ -365,7 +375,7 @@ func setThreadCPUProfiler(hz int32) {
 	var sevp _sigevent
 	sevp.sigev_notify = _SIGEV_THREAD_ID
 	sevp.sigev_signo = _SIGPROF
-	*((*int32)(unsafe.Pointer(&sevp._sigev_un))) = int32(mp.procid)
+	setSigeventTID(&sevp, int32(mp.procid))
 	ret := timer_create(_CLOCK_THREAD_CPUTIME_ID, &sevp, &timerid)
 	if ret != 0 {
 		// If we cannot create a timer for this M, leave profileTimerValid false
diff --git a/libgo/runtime/go-signal.c b/libgo/runtime/go-signal.c
index 528d9b6..aa1b630 100644
--- a/libgo/runtime/go-signal.c
+++ b/libgo/runtime/go-signal.c
@@ -183,6 +183,24 @@ setSigactionHandler(struct sigaction* sa, uintptr handler)
 	sa->sa_sigaction = (void*)(handler);
 }
 
+#ifdef __linux__
+
+// Workaround for https://sourceware.org/bugzilla/show_bug.cgi?id=27417
+#ifndef sigev_notify_thread_id
+  #define sigev_notify_thread_id _sigev_un._tid
+#endif
+
+void setSigeventTID(struct sigevent*, int32_t)
+	__asm__ (GOSYM_PREFIX "runtime.setSigeventTID");
+
+void
+setSigeventTID(struct sigevent *sev, int32_t v)
+{
+	sev->sigev_notify_thread_id = v;
+}
+
+#endif // defined(__linux__)
+
 // C code to fetch values from the siginfo_t and ucontext_t pointers
 // passed to a signal handler.