netdev - [PATCH net-next] scm: use masked_user_access_begin() in put

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Message-ID: <20250925224914.3590290-1-edumazet@google.com>
Date: Thu, 25 Sep 2025 22:49:14 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH net-next] scm: use masked_user_access_begin() in put_cmsg()

Use the greatest and latest uaccess construct to get an optimal code.

Before :

	lea    (%r9,%rcx,1),%r10
	movabs $<USER_PTR_MAX>,%r11
	mov    $0xfffffff2,%eax
	cmp    %rcx,%r10
	jb     ffffffff81cdc312 <put_cmsg+0x152>
	cmp    %r11,%r10
	ja     ffffffff81cdc312 <put_cmsg+0x152>
	stac
	lfence
	mov    %r9,(%rcx)

After:

	movabs $<USER_PTR_MAX>,%r9
	cmp    %r9,%rax
	cmova  %r9,%rax
	stac
	mov    %rcx,(%rax)

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/scm.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/scm.c b/net/core/scm.c
index 072d5742440adae36757592c102657a9a93391be..66eaee783e8be8ed650a1679a15d0a41309f9f0d 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -273,7 +273,9 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 
 		check_object_size(data, cmlen - sizeof(*cm), true);
 
-		if (!user_write_access_begin(cm, cmlen))
+		if (can_do_masked_user_access())
+			cm = masked_user_access_begin(cm);
+		else if (!user_write_access_begin(cm, cmlen))
 			goto efault;
 
 		unsafe_put_user(cmlen, &cm->cmsg_len, efault_end);
-- 
2.51.0.536.g15c5d4f767-goog