linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Date:   Wed, 20 Oct 2021 09:56:55 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

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.

---

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;