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

[george]

From: George Guo <guodongtai@xxxxxxxxxx>

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@xxxxxxxxxx>
Signed-off-by: George Guo <guodongtai@xxxxxxxxxx>
---
Signed-off-by: george <dongtai.guo@xxxxxxxxx>
---
 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;
 	}
 
 	/* ori rd, $zero, imm_11_0 */

---
base-commit: 6146a0f1dfae5d37442a9ddcba012add260bceb0
change-id: 20251103-1-96faa240e8f4

Best regards,
-- 
george <dongtai.guo@xxxxxxxxx>