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

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

Hi,

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.

Cheers,

Mark