lkml.org 
[lkml]   [2021]   [Oct]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [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;
\
 
 \ /
  Last update: 2021-10-20 18:58    [W:0.156 / U:0.116 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site