Re: [Valgrind-developers] [PATCH] Linux: Inhibit tail calls in cancellable system calls

[Mark W. <ma...@kl...>]

Hi all,

On Fri, Apr 18, 2025 at 05:46:54PM +0200, Mark Wielaard wrote:
> On Mon, 2025-04-14 at 14:06 +0200, Mark Wielaard wrote:
> > On Sun, Apr 06, 2025 at 03:23:54PM +0200, Mark Wielaard wrote:
> > > On Mon, Mar 31, 2025 at 11:29:41AM +0200, Mark Wielaard wrote:
> > > > On Fri, Mar 28, 2025 at 07:02:28PM +0100, Mark Wielaard wrote:
> > > > > On Fri, 2025-03-21 at 14:01 +0100, Florian Weimer wrote:
> > > > > > Without this change, the system call wrapper function is not visible
> > > > > > on the stack at the time of the system call, which causes problems
> > > > > > for interception tools such as valgrind.
> > > > > > 
> > > > > > Enhances commit 89b53077d2a58f00e7debdfe58afabe953dac60d ("nptl: Fix
> > > > > > Race conditions in pthread cancellation [BZ#12683]").
> > > > > > 
> > > > > > Tested on i686-linux-gnu, powerpc64le-linux-gnu, x86_64-linux-gnu.
> > > > > > (We're still discussing if valgrind needs this, but if it does, here's a
> > > > > > patch.)
> > > > > 
> > > > > I implemented the valgrind part of skipping the syscall_cancel frames
> > > > > here: https://bugs.kde.org/show_bug.cgi?id=502126#c2
> > > > > And there is a valgrind package build for fedora rawhide:
> > > > > https://koji.fedoraproject.org/koji/buildinfo?buildID=2687393
> > > > > 
> > > > > For ppc64le, s390x and x86_64 that patch seems enough.
> > > > > 
> > > > > For i686 and aarch64 there does seem to be an issue with missing the
> > > > > glibc calling function because of a tail call.
> > > > > 
> > > > > Also on i686 there is another extra frame on top __libc_do_syscall.
> > > > 
> > > > I extended the patch to cover some extra sycall wrapper function
> > > > symbols on i386 and armhf and pushed it to valgrind trunk and
> > > > VALGRIND_3_24_BRANCH. There are builds for fedora rawhide and
> > > > f42. This does seem to show that only on arm64 the tail calls
> > > > obscure observing the full call stack.
> > > 
> > > This has now landed in fedora rawhide and f42. Test results look good,
> > > except for some if the arm64 tests where the tail calls obscure
> > > observing the full call stack. Please let me know if you need any more
> > > input from us to get this fix in glibc.
> > 
> > Please let me know. Valgrind test results for syscall backtraces on
> > anything except arm64 look good. We are working on valgrind 3.25.0
> > now, to be released around April 24.
> 
> valgrind 3.25.0-RC1 has been released and test results look good on
> most arches. arm64 does show the issue described above where the tail
> calls obscure observing the full call stack when doing system calls.

valgrind 3.25.0 have been released and is now in Fedora rawhide and
Fedora 42 with the new glibc syscall_cancel frames. The tail calls on
aarch64 still seem to be a problem for observability of the syscall
call stack.

> Let me know what would be needed to get the above patch reviewed.

Thanks,

Mark