From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: <mikeba@sentinelone.com>, <bpf@vger.kernel.org>,
"Daniel Borkmann" <daniel@iogearbox.net>,
"Andrii Nakryiko" <andrii@kernel.org>
Cc: "Martin KaFai Lau" <martin.lau@linux.dev>,
"Eduard Zingerman" <eddyz87@gmail.com>,
"Kumar Kartikeya Dwivedi" <memxor@gmail.com>,
"Song Liu" <song@kernel.org>,
"Yonghong Song" <yonghong.song@linux.dev>,
"Jiri Olsa" <jolsa@kernel.org>,
"John Fastabend" <john.fastabend@gmail.com>,
"Emil Tsalapatis" <emil@etsalapatis.com>,
"Ihor Solodrai" <ihor.solodrai@linux.dev>,
"Steven Rostedt" <rostedt@goodmis.org>,
"Masami Hiramatsu" <mhiramat@kernel.org>,
"Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>,
"JP Kobryn" <jp.kobryn@linux.dev>,
<linux-trace-kernel@vger.kernel.org>,
<linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/5] bpf: Check program recursion per task via bpf_ctx
Date: Thu, 12 Nov 2026 01:04:30 +0000 [thread overview]
Message-ID: <DMSTTWR4MAET.1VEV3R66S7ZFY@gmail.com> (raw)
In-Reply-To: <20261111-bpf-preempt-v2-r2-v2-2-9d8f6b952ad2@sentinelone.com>
On Wed, Nov 11, 2026 at 11:39 AM Mike Bazov via B4 Relay <devnull+mikeba.sentinelone.com@kernel.org> wrote:
> As a result, the same program can now run in two tasks at once on one
> CPU. Programs that use a per-CPU map as scratch space across helper
> calls may see another run's data, where the run was dropped before.
The kernel has such scratch space too: bpf_raw_tp_regs.
bpf_get_stack(), bpf_get_stackid() and bpf_perf_event_output() of fentry,
raw_tp and tp_btf progs take a slot of it with get_bpf_raw_tp_regs().
It doesn't disable preemption. Three tasks on one cpu:
task A task B task C
level 1, regs[0]
<preempted>
level 2, regs[1]
<preempted>
put: level 1
level 2, regs[1]
uses the regs of C
B sends a sample or a stack trace with the regs of C.
This is an existing bug. Today it takes two or more progs.
With this patch A, B and C can be in one prog.
Doesn't hold the series, but I had to mention.
> Fixes: ca06f55b9002 ("bpf: Add per-program recursion prevention mechanism")
> Fixes: a46023d5616e ("tracing: Guard __DECLARE_TRACE() use of __DO_TRACE_CALL() with SRCU-fast")
> Fixes: 439ebd5b5708 ("bpf: Add sleepable support for raw tracepoint programs")
I don't think we will be sending them to stable soon.
This is a big behavior change.
Backports only after next merge window.
next prev parent reply other threads:[~2026-11-12 1:04 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-11 19:39 [PATCH bpf-next v2 0/5] bpf: Check recursion per task for preemptible programs Mike Bazov via B4 Relay
2026-11-11 19:39 ` [PATCH bpf-next v2 1/5] bpf: Avoid per-CPU private stack in preemptible tracing progs Mike Bazov via B4 Relay
2026-11-11 20:10 ` sashiko-bot
2026-11-11 21:07 ` bot+bpf-ci
2026-11-11 19:39 ` [PATCH bpf-next v2 2/5] bpf: Check program recursion per task via bpf_ctx Mike Bazov via B4 Relay
2026-11-12 1:04 ` Alexei Starovoitov [this message]
2026-11-11 19:39 ` [PATCH bpf-next v2 3/5] bpf: Check recursion per task for programs on syscall tracepoints Mike Bazov via B4 Relay
2026-11-11 19:39 ` [PATCH bpf-next v2 4/5] selftests/bpf: Add trampoline and raw_tp preemption recursion test Mike Bazov via B4 Relay
2026-11-11 20:08 ` sashiko-bot
2026-11-11 19:39 ` [PATCH bpf-next v2 5/5] selftests/bpf: Test syscall tracepoint programs under preemption Mike Bazov via B4 Relay
2026-11-12 1:18 ` [PATCH bpf-next v2 0/5] bpf: Check recursion per task for preemptible programs Alexei Starovoitov
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