Messages in this thread Patch in this message | | | Date | Thu, 27 Apr 2023 09:26:28 -0400 | From | Steven Rostedt <> | Subject | Re: [PATCH bpf-next 5/6] bpf: Improve tracing recursion prevention mechanism |
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On Mon, 17 Apr 2023 15:47:36 +0000 Yafang Shao <laoar.shao@gmail.com> wrote:
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c > index f61d513..3df39a5 100644 > --- a/kernel/bpf/trampoline.c > +++ b/kernel/bpf/trampoline.c > @@ -842,15 +842,21 @@ static __always_inline u64 notrace bpf_prog_start_time(void) > static u64 notrace __bpf_prog_enter_recur(struct bpf_prog *prog, struct bpf_tramp_run_ctx *run_ctx) > __acquires(RCU)
Because __bpf_prog_enter_recur() and __bpf_prog_exit_recur() can legitimately nest (as you pointed out later in the thread), I think my original plan is the way to go.
> { > - rcu_read_lock(); > - migrate_disable(); > - > - run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx); > + int bit; > > - if (unlikely(this_cpu_inc_return(*(prog->active)) != 1)) { > + rcu_read_lock(); > + bit = test_recursion_try_acquire(_THIS_IP_, _RET_IP_); > + run_ctx->recursion_bit = bit; > + if (bit < 0) { > + preempt_disable_notrace(); > bpf_prog_inc_misses_counter(prog); > + preempt_enable_notrace(); > return 0; > } > + > + migrate_disable();
Just encompass the migrate_disable/enable() with the recursion protection.
That is, here add:
test_recursion_release(recursion_bit);
No need to save it in the run_ctx, as you can use a local variable.
As I mentioned, if it passes when checking migrate_disable() it will also pass when checking around migrate_enable() so the two will still be paired properly, even if only the migrate_enable() starts recursing.
bit = test_recursion_try_acquire() // OK if (bit < 0) return; migrate_disable(); test_recursion_release(bit); [..]
bit = test_recursion_try_acquire() // OK migrate_enable() // traced and recurses... bit = test_recursion_try_acquire() // fails if (bit < 0) return; // returns here migrate_disable() // does not get called. The recursion around migrate_disable/enable() is needed because it's done before other checks. You can't attach the test_recursion logic to the __bpf_prog_enter/exit() routines, because those can legitimately recurse.
-- Steve
> + > + run_ctx->saved_run_ctx = bpf_set_run_ctx(&run_ctx->run_ctx); > return bpf_prog_start_time(); > }
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