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

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

[Sören Tempel]

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

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

[Sören Tempel]

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

[PATCH v2] libgo: Portable access to thread ID in struct sigevent

[soeren]

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

Tested on x86_64 Arch Linux (glibc) and Alpine Linux (musl libc).

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.

This makes go_signal compatible with musl libc.

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

Signed-off-by: Sören Tempel <soeren@soeren-tempel.net>
---
Changes since v1: Add workaround for glibc.

 libgo/go/runtime/os_linux.go |  4 +++-
 libgo/runtime/go-signal.c    | 15 +++++++++++++++
 2 files changed, 18 insertions(+), 1 deletion(-)

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..c56350cc 100644
--- a/libgo/runtime/go-signal.c
+++ b/libgo/runtime/go-signal.c
@@ -16,6 +16,11 @@
   #define SA_RESTART 0
 #endif
 
+// Workaround for https://sourceware.org/bugzilla/show_bug.cgi?id=27417
+#if __linux__ && !defined(sigev_notify_thread_id)
+  #define sigev_notify_thread_id _sigev_un._tid
+#endif
+
 #ifdef USING_SPLIT_STACK
 
 extern void __splitstack_getcontext(void *context[10]);
@@ -183,6 +188,16 @@ 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.
 

Re: [PATCH v2] libgo: Portable access to thread ID in struct sigevent

[Ian Lance Taylor]

[-- Attachment #1: Type: text/plain, Size: 927 bytes --]

On Fri, Sep 23, 2022 at 6:59 AM <soeren@soeren-tempel.net> wrote:
>
> From: Sören Tempel <soeren@soeren-tempel.net>
>
> Tested on x86_64 Arch Linux (glibc) and Alpine Linux (musl libc).
>
> 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.
>
> This makes go_signal compatible with musl libc.
>
> See: https://sourceware.org/bugzilla/show_bug.cgi?id=27417

Thanks.  Committed with some changes, as appended.

Sorry for the delay.

Ian

[-- Attachment #2: patch.txt --]
[-- Type: text/plain, Size: 2699 bytes --]

e73d9fcafbd07bc3714fbaf8a82db71d50015c92
diff --git a/gcc/go/gofrontend/MERGE b/gcc/go/gofrontend/MERGE
index 73aa712dbdf..4793c821eba 100644
--- a/gcc/go/gofrontend/MERGE
+++ b/gcc/go/gofrontend/MERGE
@@ -1,4 +1,4 @@
-0140cca9bc0fad1108c7ed369376ac71cc4bfecf
+8f1a91aeff400d572857895b7f5e863ec5a4d93e
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.
diff --git a/libgo/go/runtime/os_linux.go b/libgo/go/runtime/os_linux.go
index 96fb178870e..2b2d827cee8 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 528d9b6d9fe..aa1b6305ad0 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.