[PATCH bpf-next 1/3] bpf: Allow 'may_goto 0' instruction

[Yonghong Song <yonghong.song@xxxxxxxxx>]

Commit 011832b97b31 ("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: Emil Tsalapatis <etsal@xxxxxxxx>
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index b8ca227c78af..edf3cc42a220 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -15899,9 +15899,8 @@ 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);
-- 
2.43.5