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objtool: Fix segfault on unknown alternatives 

So 'objtool --link -d vmlinux.o' gets surprised by this endbr64+endbr64 pattern
in ___bpf_prog_run():

	___bpf_prog_run:
	1e7680:  ___bpf_prog_run+0x0                                                     push   %r12
	1e7682:  ___bpf_prog_run+0x2                                                     mov    %rdi,%r12
	1e7685:  ___bpf_prog_run+0x5                                                     push   %rbp
	1e7686:  ___bpf_prog_run+0x6                                                     xor    %ebp,%ebp
	1e7688:  ___bpf_prog_run+0x8                                                     push   %rbx
	1e7689:  ___bpf_prog_run+0x9                                                     mov    %rsi,%rbx
	1e768c:  ___bpf_prog_run+0xc                                                     movzbl (%rbx),%esi
	1e768f:  ___bpf_prog_run+0xf                                                     movzbl %sil,%edx
	1e7693:  ___bpf_prog_run+0x13                                                    mov    %esi,%eax
	1e7695:  ___bpf_prog_run+0x15                                                    mov    0x0(,%rdx,8),%rdx
	1e769d:  ___bpf_prog_run+0x1d                                                    jmp    0x1e76a2 <__x86_indirect_thunk_rdx>
	1e76a2:  ___bpf_prog_run+0x22                                                    endbr64
	1e76a6:  ___bpf_prog_run+0x26                                                    endbr64
	1e76aa:  ___bpf_prog_run+0x2a                                                    mov    0x4(%rbx),%edx

And crashes due to blindly dereferencing alt->insn->alt_group.

Bail out on NULL ->alt_group, which produces this warning and continues
with the disassembly, instead of a segfault:

  .git/O/vmlinux.o: warning: objtool: <alternative.1e769d>: failed to disassemble alternative

Cc: Alexandre Chartre <alexandre.chartre@oracle.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2025-12-01 10:42:27 +01:00
parent 59bfa64082
commit 6ec33db1aa
1 changed files with 3 additions and 0 deletions
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3
tools/objtool/disas.c
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@ -684,6 +684,9 @@ char *disas_alt_name(struct alternative *alt)
* '?' unknown flag * '?' unknown flag
*/ */
if (!alt->insn->alt_group)
return NULL;
feature = alt->insn->alt_group->feature; feature = alt->insn->alt_group->feature;
num = alt_feature(feature); num = alt_feature(feature);
flags = alt_flags(feature); flags = alt_flags(feature);