Re: [PATCH 1/2] x86/uprobes: Keep shadow stack in sync for emulated CALLs

[Oleg Nesterov <oleg@redhat.com>]

On 06/23, Peter Zijlstra wrote:
>
> On Tue, Jun 23, 2026 at 02:52:32PM +0200, Oleg Nesterov wrote:
> > On 06/22, David Windsor wrote:
> > >
> > > --- a/arch/x86/kernel/uprobes.c
> > > +++ b/arch/x86/kernel/uprobes.c
> > > @@ -1246,8 +1246,12 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
> > >  		long correction = utask->vaddr - utask->xol_vaddr;
> > >  		regs->ip += correction;
> > >  	} else if (auprobe->defparam.fixups & UPROBE_FIX_CALL) {
> > > +		unsigned long retaddr = utask->vaddr + auprobe->defparam.ilen;
> > > +
> > >  		regs->sp += sizeof_long(regs); /* Pop incorrect return address */
> > > -		if (emulate_push_stack(regs, utask->vaddr + auprobe->defparam.ilen))
> > > +		if (emulate_push_stack(regs, retaddr))
> > > +			return -ERESTART;
> > > +		if (shstk_update_last_frame(retaddr))
> > >  			return -ERESTART;
> >
> > Well, if shstk_update_last_frame() fails after emulate_push_stack(), we should
> > probably return another error, so that the caller handle_singlestep() will kill
> > this task?
>
> Makes sense, the other user has a force_sig(SIGSEGV) on failure.

Offtopic question... both shstk_update_last_frame() and shstk_push() are only
used by arch/x86/kernel/uprobes.c. But they are not symmetric in that
shstk_update_last_frame() returns 0 if !features_enabled(ARCH_SHSTK_SHSTK),
while shstk_push() returns -ENOTSUPP in this case.

That is why the users can't just do "if (shstk_push(xxx)) ...". This is really
minor, but perhaps it makes sense to change shstk_push() to return 0 in this
case too? I don't think -ENOTSUPP is actually useful...

Oleg.