Commit aefaa431 authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
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bpf: Allow 'may_goto 0' instruction in verifier 



Commit 011832b9 ("bpf: Introduce may_goto instruction") added support
for may_goto insn. The 'may_goto 0' insn is disallowed since the insn is
equivalent to a nop as both branch will go to the next insn.

But it is possible that compiler transformation may generate 'may_goto 0'
insn. Emil Tsalapatis from Meta reported such a case which caused
verification failure. For example, for the following code,
   int i, tmp[3];
   for (i = 0; i < 3 && can_loop; i++)
     tmp[i] = 0;
   ...

clang 20 may generate code like
   may_goto 2;
   may_goto 1;
   may_goto 0;
   r1 = 0; /* tmp[0] = 0; */
   r2 = 0; /* tmp[1] = 0; */
   r3 = 0; /* tmp[2] = 0; */

Let us permit 'may_goto 0' insn to avoid verification failure for codes
like the above.

Reported-by: default avatarEmil Tsalapatis <etsal@meta.com>
Acked-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Signed-off-by: default avatarYonghong Song <yonghong.song@linux.dev>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20250118192024.2124059-1-yonghong.song@linux.dev


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent d10cafc5

kernel/bpf/verifier.c

+2 −3
Original line number Diff line number Diff line
@@ -15972,9 +15972,8 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 








		if (insn->code != (BPF_JMP | BPF_JCOND) ||









		    insn->src_reg != BPF_MAY_GOTO ||









		    insn->dst_reg || insn->imm || insn->off == 0) {









			verbose(env, "invalid may_goto off %d imm %d\n",









				insn->off, insn->imm);









		    insn->dst_reg || insn->imm) {









			verbose(env, "invalid may_goto imm %d\n", insn->imm);









			return -EINVAL;









		}









		prev_st = find_prev_entry(env, cur_st->parent, idx);