[PATCH] libgo: Access to glibc-specific field in struct sigevent

["Sören Tempel" <soeren@soeren-tempel.net>]

Hi Ian,

Thanks for your input!

I am not familiar with Go runtime internals at all, but the patch below
at least compiles for me. Let me know if this works for you too / if
there is a better way to do this. The untyped uintptr_t seems to be
necessary as otherwise I encountered errors about &sevp escaping to the
heap.

Sincerely,
Sören

diff --git a/libgo/go/runtime/os_linux.go b/libgo/go/runtime/os_linux.go
index 96fb1788..6653d85e 100644
--- a/libgo/go/runtime/os_linux.go
+++ b/libgo/go/runtime/os_linux.go
@@ -22,6 +22,8 @@ type mOS struct {
        profileTimerValid uint32
 }

+func setProcID(uintptr, int32)
+
 func getProcID() uint64 {
        return uint64(gettid())
 }
@@ -365,7 +367,7 @@ 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)
+       setProcID(uintptr(unsafe.Pointer(&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 528d9b6d..347b24e2 100644
--- a/libgo/runtime/go-signal.c
+++ b/libgo/runtime/go-signal.c
@@ -183,6 +183,16 @@ setSigactionHandler(struct sigaction* sa, uintptr handler)
        sa->sa_sigaction = (void*)(handler);
 }

+void setProcID(uintptr_t, int32_t)
+       __asm__ (GOSYM_PREFIX "runtime.setProcID");
+
+void
+setProcID(uintptr_t ptr, int32_t v)
+{
+       struct sigevent *s = (void *)ptr;
+       s->sigev_notify_thread_id = v;
+}
+
 // C code to fetch values from the siginfo_t and ucontext_t pointers
 // passed to a signal handler.

Ian Lance Taylor <iant@golang.org> wrote:
> On Fri, Aug 12, 2022 at 10:21 AM Sören Tempel <soeren@soeren-tempel.net> wrote:
> >
> > This is one of the few remaining issues regarding musl compatibility for
> > gofrontend. Unfortunately, I am not sure how to best fix this one. Hence
> > reporting it here.
> >
> > The setThreadCPUProfiler implementation in libgo/go/runtime/os_linux.go
> > contains the following code:
> >
> >         var sevp _sigevent
> >         sevp.sigev_notify = _SIGEV_THREAD_ID
> >         sevp.sigev_signo = _SIGPROF
> >         *((*int32)(unsafe.Pointer(&sevp._sigev_un))) = int32(mp.procid)
> >
> > I am not entirely sure, but I assume the last line is supposed to access
> > the thread ID in struct sigevent. Unfortunately, it does so via the
> > glibc-specific _sigev_un member which doesn't work on musl libc (the
> > union is named __sev_fields on musl and hence causes a build failure).
> >
> > A portable way to access the thread ID in struct sigevent is via
> > sigev_notify_thread_id which is documented in timer_create(2). This is a
> > macro provided in siginfo.h from linux-headers for glibc [1] and in
> > signal.h for musl [2]. However, since it is macro it probably requires a
> > C wrapper function in order to be usable in libgo. Would be possible to
> > add that somehow to libgo or is there an alternative feasible solution
> > for this issue?
> 
> I think you're right that a tiny C function is the way to go here.
> 
> Ian