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From: George Guo To: Hengqi Chen Cc: Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Eduard Zingerman , Song Liu , Yonghong Song , John Fastabend , KP Singh , Stanislav Fomichev , Hao Luo , Jiri Olsa , Tiezhu Yang , Huacai Chen , WANG Xuerui , bpf@vger.kernel.org, loongarch@lists.linux.dev, linux-kernel@vger.kernel.org, George Guo Subject: Re: [PATCH] LoongArch: BPF: Fix sign extension for 12-bit immediates Message-ID: <20251219172039.00007242@linux.dev> In-Reply-To: References: <20251103-1-v1-1-20e6641a57da@linux.dev> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=GB18030 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Tue, 4 Nov 2025 14:53:04 +0800 Hengqi Chen wrote: > On Mon, Nov 3, 2025 at 4:42 PM george wrote: > > > > From: George Guo > > > > 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? this bpf prog: ./tools/sched_ext/build/bin/scx_simple > > 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 > > Signed-off-by: George Guo > > --- > > Signed-off-by: george > > --- > > 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. Hi Hengqi, I tried there would kerenl panic even without the patch in kernle 6.18. The patch is needed, please consider merging it. Thanks！ > > > > /* ori rd, $zero, imm_11_0 */ > > > > --- > > base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0 > > change-id: 20251103-1-96faa240e8f4 > > > > Best regards, > > -- > > george > >