Re: [PATCH bpf v2 2/2] selftests/bpf: Test r0 bounds after BPF to BPF call with abnormal return

[Eduard Zingerman <eddyz87@gmail.com>]

On Mon, 2024-12-16 at 10:05 -0800, Alexei Starovoitov wrote:

[...]

> > Thanks for the review! Good point, I'll try to write them in C.
> > 
> > It might not be possible to do them both entirely: clang also doesn't
> > know that bpf_tail_call() can return, so it assumes the callee() will
> > return a constant r0. It sometimes optimizes branches / loads out
> > because of this.
>
> I wonder whether we should tell llvm that it's similar to longjmp()
> with __attribute__((noreturn)) or some other attribute.

GCC documents it as follows [1]:

  > The noreturn keyword tells the compiler to assume that fatal
  > cannot reaturn. It can then optimize without regard to what would
  > happen if fatal ever did return. This makes slightly better code.
  > More importantly, it helps avoid spurious warnings of
  > uninitialized variables.

But the bpf_tail_call could return if MAX_TAIL_CALL_CNT limit is exceeded,
or programs map index is out of bounds.

[1] https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/Common-Function-Attributes.html#index-noreturn-function-attribute