Re: [RFC PATCH v1 07/14] bpf: Use hidden subprog trampoline for bpf_throw

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2024-02-01 at 04:21 +0000, Kumar Kartikeya Dwivedi wrote:
> When we perform a bpf_throw kfunc call, callee saved registers in BPF
> calling convention (R6-R9) may end up getting saved and clobbered by
> bpf_throw. Typically, the kernel will restore the registers before
> returning back to the BPF program, but in case of bpf_throw, the
> function will never return. Therefore, any acquired resources sitting in
> these registers will end up getting destroyed if not saved on the
> stack, without any cleanup happening for them.

Could you please paraphrase this description a bit?
It took me a while to figure out the difference between regular bpf
calls and kfunc calls. Something like:

  - For regular bpf subprogram calls jit emits code that pushes R6-R9 to stack
    before jumping into callee.
  - For kfunc calls jit emits instructions that do not guarantee that
    R6-R9 would be preserved on stack. E.g. for x86 kfunc call is translated
    as "call" instruction, which only pushes return address to stack.

--

Also, what do you think about the following hack:
- declare a hidden kfunc "bpf_throw_r(u64 r6, u64 r7, u64 r8, u64 r9)";
- replace all calls to bpf_throw() with calls to bpf_throw_r()
  (r1-r5 do not have to be preserved anyways).
Thus avoid necessity to introduce the trampoline.

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