On Feb 23 2024, Eduard Zingerman wrote:
On Wed, 2024-02-21 at 17:25 +0100, Benjamin Tissoires wrote:
This is still a WIP, but I think this can be dropped as we never get to this instruction. So what should we do here?
As Alexei replied in a separate sub-thread you probably want this for sleepable timers. Here is full source code block:
if (insn->imm == BPF_FUNC_timer_set_callback || is_bpf_timer_set_sleepable_cb_kfunc(insn->imm)) { ... struct bpf_insn ld_addrs[2] = { BPF_LD_IMM64(BPF_REG_3, (long)prog->aux), }; insn_buf[0] = ld_addrs[0]; insn_buf[1] = ld_addrs[1]; insn_buf[2] = *insn; cnt = 3; new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt); ... }Effectively, it sets up third function call parameter (R3) for timer_set_callback() to be prog->aux. E.g. before bpf_patch_insn_data():
r1 = ... timer ... r2 = ... callback address ... call timer_set_callback
After bpf_patch_insn_data():
r1 = ... timer ... r2 = ... callback address ... r3 = prog->aux ll call timer_set_callback
This way it won't be necessary to walk stack in search for ctx.aux in bpf_timer_set_sleepable_cb().
Hmm, I must still be missing a piece of the puzzle: if I declare bpf_timer_set_sleepable_cb() to take a third "aux" argument, given that it is declared as kfunc, I also must declare it in my bpf program, or I get the following:
# libbpf: extern (func ksym) 'bpf_timer_set_sleepable_cb': func_proto [264] incompatible with vmlinux [18151]
And if I declare it, then I don't know what to pass, given that this is purely added by the verifier:
43: (85) call bpf_timer_set_sleepable_cb#18152 arg#2 pointer type STRUCT bpf_prog_aux must point to scalar, or struct with scalar
Maybe I should teach the verifier that this kfunc only takes 2 arguments, and the third one is virtual, but that also means that when the kfunc definitions are to be included in vmlinux.h, they would also have this special case.
(I just tried with a blank u64 instead of the struct bpf_prog_aux*, but it crashes with KASAN complaining).
Cheers, Benjamin