s390/bpf: Fix 64-bit subtraction of the -0x80000000 constant

The JIT uses agfi for subtracting constants, but -(-0x80000000) cannot
be represented as a 32-bit signed binary integer. Fix by using algfi in
this particular case.

Reported-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Fixes: 054623105728 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index c3bd630e9b43ab88533b8cb89c7bf6e09860854d..245f98d5f6902f3321946baffcebc77351cae770 100644 (file)
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -794,8 +794,13 @@ static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp,
        case BPF_ALU64 | BPF_SUB | BPF_K: /* dst = dst - imm */
                if (!imm)
                        break;
-               /* agfi %dst,-imm */
-               EMIT6_IMM(0xc2080000, dst_reg, -imm);
+               if (imm == -0x80000000) {
+                       /* algfi %dst,0x80000000 */
+                       EMIT6_IMM(0xc20a0000, dst_reg, 0x80000000);
+               } else {
+                       /* agfi %dst,-imm */
+                       EMIT6_IMM(0xc2080000, dst_reg, -imm);
+               }
                break;
        /*
         * BPF_MUL