Messages in this thread Patch in this message | | | Date | Wed, 20 Oct 2021 09:56:55 -0700 | From | Josh Poimboeuf <> | Subject | Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE* |
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On Wed, Oct 20, 2021 at 01:09:51PM +0200, Peter Zijlstra wrote: > Alexei; could the above not be further improved with something like the > below? > > Despite several hours trying and Song helping, I can't seem to run > anything bpf, that stuff is cursed. So I've no idea if the below > actually works, but it seems reasonable.
The below fix gets it to run with test_verififer.
I do like it, it does seem less fiddly and more robust against future changes, though it probably needs a comment for 'out_label' clarifying it only works because this function is always called at least twice for a given bpf_tail_call emission.
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diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index 08f32c9fceaa..c9230c5bbca5 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -457,7 +457,7 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used, EMIT3(0x39, 0x56, /* cmp dword ptr [rsi + 16], edx */ offsetof(struct bpf_array, map.max_entries)); - offset = out_label - (prog - start) + 2; + offset = out_label - (prog - start + 2); EMIT2(X86_JBE, offset); /* jbe out */ /* @@ -467,7 +467,7 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used, EMIT2_off32(0x8B, 0x85, tcc_off); /* mov eax, dword ptr [rbp - tcc_off] */ EMIT3(0x83, 0xF8, MAX_TAIL_CALL_CNT); /* cmp eax, MAX_TAIL_CALL_CNT */ - offset = out_label - (prog - start) + 2; + offset = out_label - (prog - start + 2); EMIT2(X86_JA, offset); /* ja out */ EMIT3(0x83, 0xC0, 0x01); /* add eax, 1 */ EMIT2_off32(0x89, 0x85, tcc_off); /* mov dword ptr [rbp - tcc_off], eax */ @@ -482,7 +482,7 @@ static void emit_bpf_tail_call_indirect(u8 **pprog, bool *callee_regs_used, */ EMIT3(0x48, 0x85, 0xC9); /* test rcx,rcx */ - offset = out_label - (prog - start) + 2; + offset = out_label - (prog - start + 2); EMIT2(X86_JE, offset); /* je out */ *pprog = prog;
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