From: George Guo <dongtai.guo@linux.dev>
To: Xi Ruoyao <xry111@xry111.site>
Cc: Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
John Fastabend <john.fastabend@gmail.com>,
KP Singh <kpsingh@kernel.org>,
Stanislav Fomichev <sdf@fomichev.me>,
Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
Tiezhu Yang <yangtiezhu@loongson.cn>,
Hengqi Chen <hengqi.chen@gmail.com>,
Huacai Chen <chenhuacai@kernel.org>,
WANG Xuerui <kernel@xen0n.name>,
Youling Tang <tangyouling@loongson.cn>,
bpf@vger.kernel.org, loongarch@lists.linux.dev,
linux-kernel@vger.kernel.org, George Guo <guodongtai@kylinos.cn>,
Bing Huang <huangbing@kylinos.cn>
Subject: Re: [PATCH] LoongArch: BPF: Fix sign extension for 12-bit immediates
Date: Mon, 29 Dec 2025 15:06:19 +0800 [thread overview]
Message-ID: <20251229150619.0000195f@linux.dev> (raw)
In-Reply-To: <130f896382dc8f56ead371208d9809ec06c7400c.camel@xry111.site>
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On Fri, 19 Dec 2025 17:33:17 +0800
Xi Ruoyao <xry111@xry111.site> wrote:
> On Mon, 2025-11-03 at 16:42 +0800, george wrote:
> > From: George Guo <guodongtai@kylinos.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@kylinos.cn>
> > Signed-off-by: George Guo <guodongtai@kylinos.cn>
> > ---
> > Signed-off-by: george <dongtai.guo@linux.dev>
> > ---
> > 0„2arch/loongarch/net/bpf_jit.h | 3 ++-
> > 0„21 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)) {
> > 0„2 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?
>
Hi Ruoyao,
I tried, it's not easy to do that.
It's better merge this patch, then consider next step.
Thanks!
next prev parent reply other threads:[~2025-12-29 7:06 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-11-03 8:42 [PATCH] LoongArch: BPF: Fix sign extension for 12-bit immediates george
2025-11-04 6:53 ` Hengqi Chen
2025-12-19 9:20 ` George Guo
2025-12-19 9:33 ` Xi Ruoyao
2025-12-29 7:06 ` George Guo [this message]
2025-12-29 14:20 ` Hengqi Chen
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