linux-kernel - [PATCH AUTOSEL 5.17 45/49] x86,bpf: Avoid IBT objtool warning

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Message-Id: <20220412004411.349427-45-sashal@kernel.org>
Date:   Mon, 11 Apr 2022 20:44:03 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Sasha Levin <sashal@...nel.org>, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, daniel@...earbox.net,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, andrii@...nel.org,
        nathan@...nel.org, ndesaulniers@...gle.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH AUTOSEL 5.17 45/49] x86,bpf: Avoid IBT objtool warning

From: Peter Zijlstra <peterz@...radead.org>

[ Upstream commit be8a096521ca1a252bf078b347f96ce94582612e ]

Clang can inline emit_indirect_jump() and then folds constants, which
results in:

  | vmlinux.o: warning: objtool: emit_bpf_dispatcher()+0x6a4: relocation to !ENDBR: .text.__x86.indirect_thunk+0x40
  | vmlinux.o: warning: objtool: emit_bpf_dispatcher()+0x67d: relocation to !ENDBR: .text.__x86.indirect_thunk+0x40
  | vmlinux.o: warning: objtool: emit_bpf_tail_call_indirect()+0x386: relocation to !ENDBR: .text.__x86.indirect_thunk+0x20
  | vmlinux.o: warning: objtool: emit_bpf_tail_call_indirect()+0x35d: relocation to !ENDBR: .text.__x86.indirect_thunk+0x20

Suppress the optimization such that it must emit a code reference to
the __x86_indirect_thunk_array[] base.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Alexei Starovoitov <ast@...nel.org>
Link: https://lkml.kernel.org/r/20220405075531.GB30877@worktop.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/net/bpf_jit_comp.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 0ecb140864b2..b272e963388c 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -398,6 +398,7 @@ static void emit_indirect_jump(u8 **pprog, int reg, u8 *ip)
 		EMIT_LFENCE();
 		EMIT2(0xFF, 0xE0 + reg);
 	} else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
+		OPTIMIZER_HIDE_VAR(reg);
 		emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
 	} else
 #endif
-- 
2.35.1