From 6a8787093b04057d855822094d63d04a2506444a Mon Sep 17 00:00:00 2001
From: Ilya Leoshkevich <iii@linux.ibm.com>
Date: Tue, 7 Sep 2021 13:41:16 +0200
Subject: [PATCH] s390/bpf: Fix 64-bit subtraction of the -0x80000000 constant
Git-commit: 6e61dc9da0b7a0d91d57c2e20b5ea4fd2d4e7e53
Patch-mainline: v5.15-rc3
References: stable-5.14.7

commit 6e61dc9da0b7a0d91d57c2e20b5ea4fd2d4e7e53 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 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 a7c9e4046f5e..59ed11f1660f 100644
--- a/arch/s390/net/bpf_jit_comp.c
+++ b/arch/s390/net/bpf_jit_comp.c
@@ -795,8 +795,13 @@ 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
-- 
2.26.2