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Wed, 26 Aug 2026 06:11:47 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Wed, 26 Aug 2026 15:11:46 +0200 To: Alexei Starovoitov Cc: Christian Simon , bpf@vger.kernel.org, ast@kernel.org, andrii@kernel.org, daniel@iogearbox.net, martin.lau@kernel.org, tj@kernel.org, yonghong.song@linux.dev, stable@vger.kernel.org, andrii.nakryiko@gmail.com, olsajiri@gmail.com Subject: Re: [PATCH bpf v3 1/2] bpf: disable private stack for sleepable programs Message-ID: References: <20260822225444.2774461-1-simon@swine.de> <20260822225444.2774461-2-simon@swine.de> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > --- > > 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