Re: [PATCH bpf v2 2/3] bpf: Repeat check_max_stack_depth for async callbacks

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Jul 17, 2023 at 9:15 AM Kumar Kartikeya Dwivedi
<memxor@gmail.com> wrote:
>
> While the check_max_stack_depth function explores call chains emanating
> from the main prog, which is typically enough to cover all possible call
> chains, it doesn't explore those rooted at async callbacks unless the
> async callback will have been directly called, since unlike non-async
> callbacks it skips their instruction exploration as they don't
> contribute to stack depth.
>
> It could be the case that the async callback leads to a callchain which
> exceeds the stack depth, but this is never reachable while only
> exploring the entry point from main subprog. Hence, repeat the check for
> the main subprog *and* all async callbacks marked by the symbolic
> execution pass of the verifier, as execution of the program may begin at
> any of them.
>
> Consider functions with following stack depths:
> main: 256
> async: 256
> foo: 256
>
> main:
>     rX = async
>     bpf_timer_set_callback(...)
>
> async:
>     foo()
>
> Here, async is not descended as it does not contribute to stack depth of
> main (since it is referenced using bpf_pseudo_func and not
> bpf_pseudo_call). However, when async is invoked asynchronously, it will
> end up breaching the MAX_BPF_STACK limit by calling foo.
>
> Hence, in addition to main, we also need to explore call chains
> beginning at all async callback subprogs in a program.
>
> Fixes: 7ddc80a476c2 ("bpf: Teach stack depth check about async callbacks.")
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Applied. Thanks