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

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Message-ID: <130f896382dc8f56ead371208d9809ec06c7400c.camel@xry111.site>
Date: Fri, 19 Dec 2025 17:33:17 +0800
From: Xi Ruoyao <xry111@...111.site>
To: george <dongtai.guo@...ux.dev>, 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>, Hengqi Chen <hengqi.chen@...il.com>, Huacai Chen	
 <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>, Youling Tang	
 <tangyouling@...ngson.cn>
Cc: 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, 2025-11-03 at 16:42 +0800, george 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.
> 
> 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);

The addi.w instruction already produces the sign-extended value.  Why do
we need to sign-extend it again?

-- 
Xi Ruoyao <xry111@...111.site>