[PATCH v1 net] af_packet: Fix fortified memcpy() without flex array.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

Sergei Trofimovich reported a regression [0] caused by commit a0ade8404c3b
("af_packet: Fix warning of fortified memcpy() in packet_getname().").

It introduced a flex array sll_addr_flex in struct sockaddr_ll as a
union-ed member with sll_addr to work around the fortified memcpy() check.

However, a userspace program uses a struct that has struct sockaddr_ll in
the middle, where a flex array is illegal to exist.

  include/linux/if_packet.h:24:17: error: flexible array member 'sockaddr_ll::<unnamed union>::<unnamed struct>::sll_addr_flex' not at end of 'struct packet_info_t'
     24 |                 __DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
        |                 ^~~~~~~~~~~~~~~~~~~~

To fix the regression, let's go back to the first attempt [1] telling
memcpy() the actual size of the array.

Reported-by: Sergei Trofimovich <slyich@gmail.com>
Closes: https://github.com/NixOS/nixpkgs/pull/252587#issuecomment-1741733002 [0]
Link: https://lore.kernel.org/netdev/20230720004410.87588-3-kuniyu@amazon.com/ [1]
Fixes: a0ade8404c3b ("af_packet: Fix warning of fortified memcpy() in packet_getname().")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
---
 include/uapi/linux/if_packet.h | 6 +-----
 net/packet/af_packet.c         | 7 ++++++-
 2 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/if_packet.h b/include/uapi/linux/if_packet.h
index 4d0ad22f83b5..9efc42382fdb 100644
--- a/include/uapi/linux/if_packet.h
+++ b/include/uapi/linux/if_packet.h
@@ -18,11 +18,7 @@ struct sockaddr_ll {
 	unsigned short	sll_hatype;
 	unsigned char	sll_pkttype;
 	unsigned char	sll_halen;
-	union {
-		unsigned char	sll_addr[8];
-		/* Actual length is in sll_halen. */
-		__DECLARE_FLEX_ARRAY(unsigned char, sll_addr_flex);
-	};
+	unsigned char	sll_addr[8];
 };
 
 /* Packet types */
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 8f97648d652f..a84e00b5904b 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3607,7 +3607,12 @@ static int packet_getname(struct socket *sock, struct sockaddr *uaddr,
 	if (dev) {
 		sll->sll_hatype = dev->type;
 		sll->sll_halen = dev->addr_len;
-		memcpy(sll->sll_addr_flex, dev->dev_addr, dev->addr_len);
+
+		/* Let __fortify_memcpy_chk() know the actual buffer size. */
+		memcpy(((struct sockaddr_storage *)sll)->__data +
+		       offsetof(struct sockaddr_ll, sll_addr) -
+		       offsetofend(struct sockaddr_ll, sll_family),
+		       dev->dev_addr, dev->addr_len);
 	} else {
 		sll->sll_hatype = 0;	/* Bad: we have no ARPHRD_UNSPEC */
 		sll->sll_halen = 0;
-- 
2.30.2