Re: [patch 8/8] x86/vdso: Implement __vdso_futex_robust_try_unlock()

[Thomas Weißschuh]

On Thu, Mar 19, 2026 at 09:53:53AM +0100, Florian Weimer wrote:
> * Thomas Weißschuh:
> 
> > On Wed, Mar 18, 2026 at 05:17:38PM +0100, Thomas Gleixner wrote:
> >> On Tue, Mar 17 2026 at 12:17, Thomas Weißschuh wrote:
> >> >> Due to my pretty limited userspace DSO knowledge that was the best I
> >> >> came up with. If you have a better idea, please let me know.
> >> >
> >> > I would have expected GDB to be able to use the separate vDSO debugging
> >> > symbols to find these symbols. So far I was not able to make it work,
> >> > but I blame my limited GDB knowledge.
> >> 
> >> I got it "working" by manually loading vdso64.so.dbg at the right
> >> offset, which only took about 10 attempts to get it right. Then you can
> >> use actual local symbols.
> >> 
> >> vdso2c picks them up correctly too.
> >
> > What also works is to have GDB look up the debug symbols through their
> > debug ids. At this point the load address of the vDSO is already known.
> >
> > $ make vdso_install INSTALL_MOD_PATH=$SOME_DIRECTORY
> > $ gdb -ex "set debug-file-directory $SOME_DIRECTORY/lib/modules/$(uname -r)/vdso" $BINARY
> >
> > Depending on the distribution the vDSO from the kernel package might already
> > be set up to be found automatically.
> >
> > Maybe we could add a helper to scripts/gdb/ which uses $(vdso-install-y)
> > to either populate a debug-file-directory automatically or hook into the GDB
> > lookup process to avoid these manual steps.
> 
> If the the /lib/modules/$(uname -r)/vdso/vdsoNN.so path is standard, we
> can use it in the link map, at the expense of an additional system call
> during process startup.  It isn't just a performance cost.  We've seen
> seccomp filters that kill the process on uname calls.

It's not a standard, only what 'make vdso_install' ends up doing and some
package managers are copying. Also the file is not guaranteed to be there,
as it is only part of the kernel debug packages, if packaged at all.

So I don't think it makes sense to integrate it into low-level system
components. For debugging, the distros can already make the file available
through /usr/lib/debug/ so it works out-of-the box. (As done by Debian)


Thomas