Re: [PATCH bpf v3 1/2] bpf: disable private stack for sleepable programs

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 25, 2026 at 06:20:16PM -0700, Alexei Starovoitov wrote:
> On Sat Aug 22, 2026 at 3:54 PM PDT, Christian Simon wrote:
> > A JITed BPF program can use one private stack per program and CPU.
> > Sleepable programs can be preempted, allowing another task to run the
> > same program on the same CPU. The second invocation then reuses and can
> > overwrite the first invocation's private stack.
> 
> I'm confused by this. sleepable progs go through __bpf_prog_enter_sleepable_recur()
> which has per-prog recurison counter. So preemption of the prog
> doesn't break private stack.
> If the same prog attemps to execute on the same cpu it will be skipped.
> 
> syscall prog types go via bpf_prog_run_array_sleepable()
> that have per prog recursions counter.
> 
> Looks like we're not doing it for bpf_prog_run_array_uprobe().
> I'm not sure what the right trade off here.
> I feel universally checking for recursion is better
> then selectively disabling private stack for uprobe.

also during the load we can't tell if kprobe program will be attached
as kprobe or uprobe [1] so having a way to disable private stack for
a program would solve this as well

jirka


[1] https://lore.kernel.org/bpf/ao2f-rBgT0SqX6Pw@krava/

> 
> > Disable private stack for sleepable programs so they use the regular kernel
> > stack, which handles preemption correctly. This change is intentionally
> > limited to programs marked sleepable; preemptible non-sleepable dispatch
> > paths require separate protection.
> >
> > Fixes: 7d1cd70d4b16 ("bpf, x86: Support private stack in jit")
> > Fixes: 6c17a882d380 ("bpf, arm64: JIT support for private stack")
> > Fixes: 156d985123b6 ("powerpc64/bpf: Implement JIT support for private stack")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Christian Simon <simon@swine.de>
> > ---
> >  kernel/bpf/verifier.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 5e37ca75e5c4..038753ef07a9 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -5237,6 +5237,15 @@ static enum priv_stack_mode bpf_enable_priv_stack(struct bpf_prog *prog)
> >  	if (!bpf_jit_supports_private_stack())
> >  		return NO_PRIV_STACK;
> >  
> > +	/*
> > +	 * Sleepable programs can be preempted, allowing another task to run
> > +	 * the same program on the same CPU. Since private stack is per-CPU
> > +	 * and per-program, the second invocation would corrupt the first's
> > +	 * stack. Disable private stack for sleepable programs.
> > +	 */
> > +	if (prog->sleepable)
> > +		return NO_PRIV_STACK;
> 
> This is not true in genreal. sleepable progs can use priv stack.
> 
> pw-bot: cr