From: Eric Dumazet <edumazet@google.com>
Date: Thu, 25 Sep 2025 22:49:14 +0000 (+0000)
Subject: scm: use masked_user_access_begin() in put_cmsg()
X-Git-Tag: v6.18-rc1~132^2~30
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=2b235765e9d4426cf56d7fd1a331f81a4dbbd85a;p=thirdparty%2Flinux.git

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@google.com>
Link: https://patch.msgid.link/20250925224914.3590290-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---

diff --git a/net/core/scm.c b/net/core/scm.c
index 072d5742440ad..66eaee783e8be 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);