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

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

Hello Ian,

Have you had the chance to look at this yet?

Let me know if there is a better way to do this.

Greetings,
Sören

Sören Tempel <soeren@soeren-tempel.net> wrote:
> 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