netdev - [PATCH bpf-next] bpf: Avoid unnecessary instruction in conver_bpf_ld

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Message-Id: <20181126.134241.1241912335032719237.davem@redhat.com>
Date:   Mon, 26 Nov 2018 13:42:41 -0800 (PST)
From:   David Miller <davem@...hat.com>
To:     daniel@...earbox.net
CC:     ast@...nel.org, netdev@...r.kernel.org
Subject: [PATCH bpf-next] bpf: Avoid unnecessary instruction in
 conver_bpf_ld_abs()


'offset' is constant and if it is zero, no need to subtract it
from BPF_REG_TMP.

Signed-off-by: David S. Miller <davem@...emloft.net>

diff --git a/net/core/filter.c b/net/core/filter.c
index aa274679965d..f50ea971f7a9 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -463,7 +463,8 @@ static bool convert_bpf_ld_abs(struct sock_filter *fp, struct bpf_insn **insnp)
 		bool ldx_off_ok = offset <= S16_MAX;
 
 		*insn++ = BPF_MOV64_REG(BPF_REG_TMP, BPF_REG_H);
-		*insn++ = BPF_ALU64_IMM(BPF_SUB, BPF_REG_TMP, offset);
+		if (offset)
+			*insn++ = BPF_ALU64_IMM(BPF_SUB, BPF_REG_TMP, offset);
 		*insn++ = BPF_JMP_IMM(BPF_JSLT, BPF_REG_TMP,
 				      size, 2 + endian + (!ldx_off_ok * 2));
 		if (ldx_off_ok) {