Re: [PATCH] LoongArch: BPF: Fix sign extension for 12-bit immediates

[Hengqi Chen]

On Mon, Nov 3, 2025 at 4:42 PM george <dongtai.guo@xxxxxxxxx> wrote:
>
> From: George Guo <guodongtai@xxxxxxxxxx>
>
> When loading immediate values that fit within 12-bit signed range,
> the move_imm function incorrectly used zero extension instead of
> sign extension.
>
> The bug was exposed when scx_simple scheduler failed with -EINVAL
> in ops.init() after passing node = -1 to scx_bpf_create_dsq().
> Due to incorrect sign extension, `node >= (int)nr_node_ids`
> evaluated to true instead of false, causing BPF program failure.
>

Which bpf prog are you referring to?

> Verified by testing with the scx_simple scheduler (located in
> tools/sched_ext/). After building with `make` and running
> ./tools/sched_ext/build/bin/scx_simple, the scheduler now
> initializes successfully with this fix.
>
> Fix this by using sign extension (sext) instead of zero extension
> for signed immediate values in move_imm.
>
> Fixes: 5dc615520c4d ("LoongArch: Add BPF JIT support")
> Reported-by: Bing Huang <huangbing@xxxxxxxxxx>
> Signed-off-by: George Guo <guodongtai@xxxxxxxxxx>
> ---
> Signed-off-by: george <dongtai.guo@xxxxxxxxx>
> ---
>  arch/loongarch/net/bpf_jit.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/net/bpf_jit.h b/arch/loongarch/net/bpf_jit.h
> index 5697158fd1645fdc3d83f598b00a9e20dfaa8f6d..f1398eb135b69ae61a27ed81f80b4bb0788cf0a0 100644
> --- a/arch/loongarch/net/bpf_jit.h
> +++ b/arch/loongarch/net/bpf_jit.h
> @@ -122,7 +122,8 @@ static inline void move_imm(struct jit_ctx *ctx, enum loongarch_gpr rd, long imm
>         /* addiw rd, $zero, imm_11_0 */
>         if (is_signed_imm12(imm)) {
>                 emit_insn(ctx, addiw, rd, LOONGARCH_GPR_ZERO, imm);
> -               goto zext;
> +               emit_sext_32(ctx, rd, is32);
> +               return;
>         }

This causes kernel panic on existing bpf selftests.

>
>         /* ori rd, $zero, imm_11_0 */
>
> ---
> base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
> change-id: 20251103-1-96faa240e8f4
>
> Best regards,
> --
> george <dongtai.guo@xxxxxxxxx>
>