Re: Crash when attaching uretprobes to processes running in Docker

Peter Zijlstra
2025-01-14T10:42:31Z

On Tue, Jan 14, 2025 at 10:22:20AM +0100, Jiri Olsa wrote:
> On Sat, Jan 11, 2025 at 07:40:15PM +0100, Jiri Olsa wrote:
> > On Sat, Jan 11, 2025 at 02:25:37AM +1100, Aleksa Sarai wrote:
> > > On 2025-01-10, Eyal Birger <eyal.birger@gmail.com> wrote:
> > > > Hi,
> > > >
> > > > When attaching uretprobes to processes running inside docker, the attached
> > > > process is segfaulted when encountering the retprobe. The offending commit
> > > > is:
> > > >
> > > > ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
> > > >
> > > > To my understanding, the reason is that now that uretprobe is a system call,
> > > > the default seccomp filters in docker block it as they only allow a specific
> > > > set of known syscalls.
> > >
> > > FWIW, the default seccomp profile of Docker _should_ return -ENOSYS for
> > > uretprobe (runc has a bunch of ugly logic to try to guarantee this if
> > > Docker hasn't updated their profile to include it). Though I guess that
> > > isn't sufficient for the magic that uretprobe(2) does...
> > >
> > > > This behavior can be reproduced by the below bash script, which works before
> > > > this commit.
> > > >
> > > > Reported-by: Rafael Buchbinder <rafi@rbk.io>
> >
> > hi,
> > nice ;-) thanks for the report, the problem seems to be that uretprobe syscall
> > is blocked and uretprobe trampoline does not expect that
> >
> > I think we could add code to the uretprobe trampoline to detect this and
> > execute standard int3 as fallback to process uretprobe, I'm checking on that
>
> hack below seems to fix the issue, it's using rbx to signal that uretprobe
> syscall got executed, if not, trampoline does int3 and executes uretprobe
> handler in the old way
>
> unfortunately now the uretprobe trampoline size crosses the xol slot limit so
> will need to come up with some generic/arch code solution for that, code below
> is neglecting that for now

Can't you detect the filter earlier and simply not install the
trampoline?