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

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Message-ID: <CAEyhmHTRVfDmwwH=_hPFUnFS-_ffALNUEVrSKiO9vEznfrUSSw@mail.gmail.com>
Date: Mon, 29 Dec 2025 22:20:22 +0800
From: Hengqi Chen <hengqi.chen@...il.com>
To: George Guo <dongtai.guo@...ux.dev>
Cc: Xi Ruoyao <xry111@...111.site>, 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, Dec 29, 2025 at 3:06 PM George Guo <dongtai.guo@...ux.dev> wrote:
>
> On Fri, 19 Dec 2025 17:33:17 +0800
> Xi Ruoyao <xry111@...111.site> wrote:
>
> > 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?
> >
> Hi Ruoyao,
> I tried, it's not easy to do that.
> It's better merge this patch, then consider next step.
>

The test_bpf.ko test failed, so probably this is the wrong fix.

> Thanks!