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

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

Hi all,

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.

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

Thanks,

Mark