[PATCH v2] LoongArch: BPF: Fix off-by-one error for insn_is_cast_user

[Tiezhu Yang]

In the LoongArch BPF JIT code, the branch offset represents the number
of instructions. An offset of 1 means the target of the "beq" is the
current PC plus 1 instruction (PC + 4 bytes). This matches the exact
same path as the sequential non-branch execution, the "or" instruction
is always executed for the cast_user JIT arm in build_insn().

If the pointer is not NULL, there is no side effect. But if the pointer
is NULL, it is incorrectly combined with the base address and turns into
a non-zero address, meaning a zero arena offset no longer casts to NULL.

Fix this by changing the branch offset from 1 to 2, which properly skips
the "or" instruction and jumps directly to the "move_reg" instruction if
the pointer is NULL, ensuring the destination register is safely cleared
to 0.

Fixes: 4fdb5dd8aeba ("LoongArch: BPF: Implement bpf_addr_space_cast instruction")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
v2: Update the commit message, no code changes.

 arch/loongarch/net/bpf_jit.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 1eb588e443c9..4da278900938 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -717,7 +717,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
 			move_reg(ctx, t1, src);
 			emit_zext_32(ctx, t1, true);
 			move_imm(ctx, dst, (ctx->user_vm_start >> 32) << 32, false);
-			emit_insn(ctx, beq, t1, LOONGARCH_GPR_ZERO, 1);
+			emit_insn(ctx, beq, t1, LOONGARCH_GPR_ZERO, 2);
 			emit_insn(ctx, or, t1, dst, t1);
 			move_reg(ctx, dst, t1);
 			break;
-- 
2.42.0