linux-kernel - [tip: x86/urgent] x86,bpf: Avoid IBT objtool warning

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Date:   Thu, 07 Apr 2022 09:46:42 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Alexei Starovoitov <ast@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/urgent] x86,bpf: Avoid IBT objtool warning

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     be8a096521ca1a252bf078b347f96ce94582612e
Gitweb:        https://git.kernel.org/tip/be8a096521ca1a252bf078b347f96ce94582612e
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Mon, 28 Mar 2022 13:13:41 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Thu, 07 Apr 2022 11:27:02 +02:00

x86,bpf: Avoid IBT objtool warning

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
---
 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 8fe35ed..16b6efa 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -412,6 +412,7 @@ 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