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Tue, 05 Aug 2025 02:07:09 -0700 (PDT) Received: from krava ([173.38.220.35]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-af91a23fec4sm871953366b.121.2025.08.05.02.07.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Aug 2025 02:07:09 -0700 (PDT) From: Jiri Olsa X-Google-Original-From: Jiri Olsa Date: Tue, 5 Aug 2025 11:07:07 +0200 To: Tao Chen Cc: Jiri Olsa , ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, song@kernel.org, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, mattbobrowski@google.com, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Subject: Re: [PATCH bpf-next] bpf: Disable migrate when kprobe_multi attach to access bpf_prog_active Message-ID: References: <20250804121615.1843956-1-chen.dylane@linux.dev> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Aug 04, 2025 at 10:15:46PM +0800, Tao Chen wrote: > 在 2025/8/4 21:02, Jiri Olsa 写道: > > On Mon, Aug 04, 2025 at 08:16:15PM +0800, Tao Chen wrote: > > > The syscall link_create not protected by bpf_disable_instrumentation, > > > accessing percpu data bpf_prog_active should use cpu local_lock when > > > kprobe_multi program attach. > > > > > > Fixes: 0dcac2725406 ("bpf: Add multi kprobe link") > > > Signed-off-by: Tao Chen > > > --- > > > kernel/trace/bpf_trace.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > > > index 3ae52978cae..f6762552e8e 100644 > > > --- a/kernel/trace/bpf_trace.c > > > +++ b/kernel/trace/bpf_trace.c > > > @@ -2728,23 +2728,23 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link, > > > struct pt_regs *regs; > > > int err; > > > + migrate_disable(); > > > if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) { > > > > this is called all the way from graph tracer, which disables preemption in > > function_graph_enter_regs, so I think we can safely use __this_cpu_inc_return > > > > > > > bpf_prog_inc_misses_counter(link->link.prog); > > > err = 1; > > > goto out; > > > } > > > - migrate_disable(); > > > > hum, but now I'm not sure why we disable migration in here then > > > > It seems that there is a cant_migrate() check in bpf_prog_run, so it should > be disabled before run. yes, but disabled preemption will take care of that I think we can do change below plus some comment that Yonghong is suggesting in the other reply jirka --- diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index 3ae52978cae6..74e8d9543c6d 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -2734,14 +2734,12 @@ kprobe_multi_link_prog_run(struct bpf_kprobe_multi_link *link, goto out; } - migrate_disable(); rcu_read_lock(); regs = ftrace_partial_regs(fregs, bpf_kprobe_multi_pt_regs_ptr()); old_run_ctx = bpf_set_run_ctx(&run_ctx.session_ctx.run_ctx); err = bpf_prog_run(link->link.prog, regs); bpf_reset_run_ctx(old_run_ctx); rcu_read_unlock(); - migrate_enable(); out: __this_cpu_dec(bpf_prog_active);