Messages in this thread | | | Date | Tue, 19 Jul 2022 10:34:08 +0200 | From | Peter Zijlstra <> | Subject | Re: [patch 37/38] x86/bpf: Emit call depth accounting if required |
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On Mon, Jul 18, 2022 at 10:30:01PM -0700, Alexei Starovoitov wrote: > On Sat, Jul 16, 2022 at 4:18 PM Thomas Gleixner <tglx@linutronix.de> wrote: > > @@ -1431,19 +1437,26 @@ st: if (is_imm8(insn->off)) > > break; > > > > /* call */ > > - case BPF_JMP | BPF_CALL: > > + case BPF_JMP | BPF_CALL: { > > + int offs; > > + > > func = (u8 *) __bpf_call_base + imm32; > > if (tail_call_reachable) { > > /* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */ > > EMIT3_off32(0x48, 0x8B, 0x85, > > -round_up(bpf_prog->aux->stack_depth, 8) - 8); > > - if (!imm32 || emit_call(&prog, func, image + addrs[i - 1] + 7)) > > + if (!imm32) > > return -EINVAL; > > + offs = 7 + x86_call_depth_emit_accounting(&prog, func); > > It's a bit hard to read all the macro magic in patches 28-30, > but I suspect the asm inside > callthunk_desc.template > that will be emitted here before the call > will do > some math on %rax > movq %rax, PER_CPU_VAR(__x86_call_depth). > > Only %rax register is scratched by the callthunk_desc, right? > If so, it's ok for all cases except this one. > See the comment few lines above > after if (tail_call_reachable) > and commit ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall > handling in JIT") > We use %rax to keep the tail_call count. > The callthunk_desc would need to preserve %rax. > I guess extra push %rax/pop %rax would do it.
The accounting template is basically:
sarq $5, PER_CPU_VAR(__x86_call_depth)
No registeres used (with debugging on it's a few more memops).
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