linux-kernel - [PATCH bpf-next] xsk: Elide base_addr comparison in xp_unaligned_validate

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Message-Id: <20230406212136.19716-1-kal.conley@dectris.com>
Date:   Thu,  6 Apr 2023 23:21:36 +0200
From:   Kal Conley <kal.conley@...tris.com>
To:     Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Kal Conley <kal.conley@...tris.com>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next] xsk: Elide base_addr comparison in xp_unaligned_validate_desc

Remove redundant (base_addr >= pool->addrs_cnt) comparison from the
conditional.

In particular, addr is computed as:

    addr = base_addr + offset

where base_addr and offset are stored as 48-bit and 16-bit unsigned
integers, respectively. The above sum cannot overflow u64 since
base_addr has a maximum value of 0x0000ffffffffffff and offset has a
maximum value of 0xffff (implying a maximum sum of 0x000100000000fffe).
Since overflow is impossible, it follows that addr >= base_addr.

Now if (base_addr >= pool->addrs_cnt), then clearly:

    addr >= base_addr
         >= pool->addrs_cnt

Thus, (base_addr >= pool->addrs_cnt) implies (addr >= pool->addrs_cnt).
Subsequently, the former comparison is unnecessary in the conditional
since for any boolean expressions A and B, (A || B) && (A -> B) is
equivalent to B.

Signed-off-by: Kal Conley <kal.conley@...tris.com>
---
 net/xdp/xsk_queue.h | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/net/xdp/xsk_queue.h b/net/xdp/xsk_queue.h
index 66c6f57c9c44..016f3753180a 100644
--- a/net/xdp/xsk_queue.h
+++ b/net/xdp/xsk_queue.h
@@ -153,16 +153,14 @@ static inline bool xp_aligned_validate_desc(struct xsk_buff_pool *pool,
 static inline bool xp_unaligned_validate_desc(struct xsk_buff_pool *pool,
 					      struct xdp_desc *desc)
 {
-	u64 addr, base_addr;
-
-	base_addr = xp_unaligned_extract_addr(desc->addr);
-	addr = xp_unaligned_add_offset_to_addr(desc->addr);
+	u64 addr;
 
 	if (desc->len > pool->chunk_size)
 		return false;
 
-	if (base_addr >= pool->addrs_cnt || addr >= pool->addrs_cnt ||
-	    addr + desc->len > pool->addrs_cnt ||
+	addr = xp_unaligned_add_offset_to_addr(desc->addr);
+
+	if (addr >= pool->addrs_cnt || addr + desc->len > pool->addrs_cnt ||
 	    xp_desc_crosses_non_contig_pg(pool, addr, desc->len))
 		return false;
 
-- 
2.39.2