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rcuwalk: recheck mount_lock after mountpoint crossing attempts

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We can get false negative from __lookup_mnt() if an unrelated vfsmount
gets moved.  In that case legitimize_mnt() is guaranteed to fail,
and we will fall back to non-RCU walk... unless we end up running
into a hard error on a filesystem object we wouldn't have reached
if not for that false negative.  IOW, delaying that check until
the end of pathname resolution is wrong - we should recheck right
after we attempt to cross the mountpoint.  We don't need to recheck
unless we see d_mountpoint() being true - in that case even if
we have just raced with mount/umount, we can simply go on as if
we'd come at the moment when the sucker wasn't a mountpoint; if we
run into a hard error as the result, it was a legitimate outcome.
__lookup_mnt() returning NULL is different in that respect, since
it might've happened due to operation on completely unrelated
mountpoint.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2014-03-20 15:18:22 -04:00
parent e825196d48
commit b37199e626
1 changed files with 13 additions and 16 deletions
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29
fs/namei.c
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@ -1109,7 +1109,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
return false;
if (!d_mountpoint(path->dentry))
break;
return true;
mounted = __lookup_mnt(path->mnt, path->dentry);
if (!mounted)
@ -1125,20 +1125,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
*/
*inode = path->dentry->d_inode;
}
return true;
}
static void follow_mount_rcu(struct nameidata *nd)
{
while (d_mountpoint(nd->path.dentry)) {
struct mount *mounted;
mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
if (!mounted)
break;
nd->path.mnt = &mounted->mnt;
nd->path.dentry = mounted->mnt.mnt_root;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
}
return read_seqretry(&mount_lock, nd->m_seq);
}
static int follow_dotdot_rcu(struct nameidata *nd)
@ -1166,7 +1153,17 @@ static int follow_dotdot_rcu(struct nameidata *nd)
break;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
}
follow_mount_rcu(nd);
while (d_mountpoint(nd->path.dentry)) {
struct mount *mounted;
mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
if (!mounted)
break;
nd->path.mnt = &mounted->mnt;
nd->path.dentry = mounted->mnt.mnt_root;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
if (!read_seqretry(&mount_lock, nd->m_seq))
goto failed;
}
nd->inode = nd->path.dentry->d_inode;
return 0;