[SRU Impish/Unstable 2/3] s390/bpf: Fix 64-bit subtraction of the -0x80000000 constant

Thadeu Lima de Souza Cascardo cascardo at canonical.com
Tue Sep 21 18:35:24 UTC 2021


From: Ilya Leoshkevich <iii at linux.ibm.com>

BugLink: https://bugs.launchpad.net/bugs/1943960

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 at anyfinetworks.com>
Fixes: 054623105728 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Reviewed-by: Heiko Carstens <hca at linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii at linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor at linux.ibm.com>
(cherry picked from commit 6e61dc9da0b7a0d91d57c2e20b5ea4fd2d4e7e53)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 arch/s390/net/bpf_jit_comp.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/s390/net/bpf_jit_comp.c b/arch/s390/net/bpf_jit_comp.c
index c3bd630e9b43..245f98d5f690 100644
--- 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
-- 
2.30.2




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