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

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Message-ID: <CAEyhmHQoLF9dcZ2CaasrpeH7RMiaQKyo0pFTrr7Nt1T64+dhuw@mail.gmail.com>
Date: Tue, 4 Nov 2025 14:53:04 +0800
From: Hengqi Chen <hengqi.chen@...il.com>
To: george <dongtai.guo@...ux.dev>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Tiezhu Yang <yangtiezhu@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, 
	Youling Tang <tangyouling@...ngson.cn>, bpf@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, George Guo <guodongtai@...inos.cn>, 
	Bing Huang <huangbing@...inos.cn>
Subject: Re: [PATCH] LoongArch: BPF: Fix sign extension for 12-bit immediates

On Mon, Nov 3, 2025 at 4:42 PM george <dongtai.guo@...ux.dev> wrote:
>
> From: George Guo <guodongtai@...inos.cn>
>
> 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@...inos.cn>
> Signed-off-by: George Guo <guodongtai@...inos.cn>
> ---
> Signed-off-by: george <dongtai.guo@...ux.dev>
> ---
>  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@...ux.dev>
>