On Mon, 17 Apr 2023 15:47:36 +0000
Yafang Shao <laoar.shao@xxxxxxxxx> 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();
> }
