Re: [PATCH bpf-next] selftests/bpf: Account for zero extensions in patch list expectations

[Alexei Starovoitov]

On Tue, Nov 03, 2026 at 01:53 PM Hungyu Lin <dennylin0707@xxxxxxxxx> wrote:
> patch_list_insert_in_front() checks the translated instructions around
> the call of an inlined bpf_loop() and its callback. On s390, the verifier
> adds zero extensions after the two 32-bit register writes in the loop.

Thanks a bunch. Applied.

But could you follow up to
remove the two 32-bit writes from inline_bpf_loop().
'w7 = 0' and 'r7 = 0' do the same thing.
'w0 = -E2BIG' is an old bug. bpf_loop() returns long.
The helper returns 0xfffffffffffffff9.
The inlined code returns 0xfffffff9.
So 'long ret = bpf_loop(...); if (ret < 0)' misses the error.
The "over max limit" check in prog_tests/bpf_loop.c doesn't see it.
The prog keeps the result in an int.

No other insn of this prog writes 32 bits.
With 64-bit writes the xlated code is the same on all five archs.
Then no need for the #if and the second copy of the expectations.

Something like this (compile tested only):

diff --git a/kernel/bpf/fixups.c b/kernel/bpf/fixups.c
index 6b719a2..321624a 100644
--- a/kernel/bpf/fixups.c
+++ b/kernel/bpf/fixups.c
@@ -2783,7 +2783,7 @@ static int inline_bpf_loop(struct bpf_verifier_env *env, int position,
 	 * expected number of iterations is too big.
 	 */
 	insn_buf[cnt++] = BPF_JMP_IMM(BPF_JLE, BPF_REG_1, BPF_MAX_LOOPS, 2);
-	insn_buf[cnt++] = BPF_MOV32_IMM(BPF_REG_0, -E2BIG);
+	insn_buf[cnt++] = BPF_MOV64_IMM(BPF_REG_0, -E2BIG);
 	insn_buf[cnt++] = BPF_JMP_IMM(BPF_JA, 0, 0, 16);
 	/* spill R6, R7, R8 to use these as loop vars */
 	insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_6, r6_offset);
@@ -2791,7 +2791,7 @@ static int inline_bpf_loop(struct bpf_verifier_env *env, int position,
 	insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_8, r8_offset);
 	/* initialize loop vars */
 	insn_buf[cnt++] = BPF_MOV64_REG(reg_loop_max, BPF_REG_1);
-	insn_buf[cnt++] = BPF_MOV32_IMM(reg_loop_cnt, 0);
+	insn_buf[cnt++] = BPF_MOV64_IMM(reg_loop_cnt, 0);
 	insn_buf[cnt++] = BPF_MOV64_REG(reg_loop_ctx, BPF_REG_3);
 	/* loop header,
 	 * if reg_loop_cnt >= reg_loop_max skip the loop body