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From: NeilBrown <neilb@suse.de>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 12/20] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK_RCU.
Date: Mon, 23 Mar 2015 13:37:40 +1100
Message-ID: <20150323023739.8161.59221.stgit@notabene.brown> (raw)
In-Reply-To: <20150323023258.8161.32467.stgit@notabene.brown>

When we support ->follow_link in RCU-walk we will not want to
take a reference to the 'struct path *link' passed to follow_link,
and correspondingly will not want to drop that reference.

As link_path_walk will complete_walk() in the case of an error,
and as complete_walk() will clear LOOKUP_RCU, we cannot test
LOOKUP_RCU to determine if the path should be 'put'.

So introduce a new flag: LOOKUP_LINK_RCU.  This is set on
entry to follow_link() if appropriate and put_link() will
only call path_put() if it is clear.

Also, unlazy_walk() will fail if LOOKUP_LINK_RCU is set.
This is because there is no way for unlazy_walk to get references
on all the "struct path *link"s that are protected by that flag.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/namei.c            |   18 +++++++++++++-----
 include/linux/namei.h |    1 +
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 1a8cc0e47df6..3262c8c2e73d 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -552,6 +552,9 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
 	struct dentry *parent = nd->path.dentry;
 
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
+	if (nd->flags & LOOKUP_LINK_RCU)
+		/* Cannot unlazy in the middle of following a symlink */
+		return -ECHILD;
 
 	/*
 	 * After legitimizing the bastards, terminate_walk()
@@ -765,7 +768,8 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
 	struct inode *inode = link->dentry->d_inode;
 	if (inode->i_op->put_link)
 		inode->i_op->put_link(link->dentry, nd_get_link(nd), cookie);
-	path_put(link);
+	if (!(nd->flags & LOOKUP_LINK_RCU))
+		path_put(link);
 }
 
 int sysctl_protected_symlinks __read_mostly = 0;
@@ -892,9 +896,10 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	int error;
 	char *s;
 
-	BUG_ON(nd->flags & LOOKUP_RCU);
-
-	if (link->mnt == nd->path.mnt)
+	nd->flags &= ~LOOKUP_LINK_RCU;
+	if (nd->flags & LOOKUP_RCU)
+		nd->flags |= LOOKUP_LINK_RCU;
+	else if (link->mnt == nd->path.mnt)
 		mntget(link->mnt);
 
 	error = -ELOOP;
@@ -946,7 +951,8 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 out_put_nd_path:
 	*p = NULL;
 	terminate_walk(nd);
-	path_put(link);
+	if (!(nd->flags & LOOKUP_LINK_RCU))
+		path_put(link);
 	return error;
 }
 
@@ -1669,6 +1675,8 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
 
 	nd->link_count--;
 	nd->depth--;
+	if (!nd->depth)
+		nd->flags &= ~LOOKUP_LINK_RCU;
 	return res;
 }
 
diff --git a/include/linux/namei.h b/include/linux/namei.h
index cc8b51a47160..633101964520 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -31,6 +31,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_PARENT		0x0010
 #define LOOKUP_REVAL		0x0020
 #define LOOKUP_RCU		0x0040
+#define LOOKUP_LINK_RCU		0x0080
 
 /*
  * Intent data

  parent reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-23  2:37 [PATCH 00/20] Support follow_link in RCU-walk - V3 NeilBrown
2015-03-23  2:37 ` [PATCH 02/20] STAGING/lustre: limit follow_link recursion using stack space NeilBrown
2015-04-18  3:01   ` Al Viro
2015-04-19 20:57     ` Andreas Dilger
2015-04-19 21:33       ` Al Viro
2015-04-20  2:29         ` Al Viro
2015-03-23  2:37 ` [PATCH 03/20] VFS: replace {, total_}link_count in task_struct with pointer to nameidata NeilBrown
2015-03-23  2:37 ` [PATCH 01/20] Documentation: remove outdated information from automount-support.txt NeilBrown
2015-03-23  2:37 ` [PATCH 10/20] security: make inode_follow_link RCU-walk aware NeilBrown
2015-03-23  2:37 ` [PATCH 04/20] ovl: rearrange ovl_follow_link to it doesn't need to call ->put_link NeilBrown
2015-03-23  2:37 ` [PATCH 07/20] VFS: remove nameidata args from ->follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 06/20] SECURITY: remove nameidata arg from inode_follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 05/20] VFS: replace nameidata arg to ->put_link with a char* NeilBrown
2015-03-23  2:37 ` [PATCH 09/20] security/selinux: pass 'flags' arg to avc_audit() and avc_has_perm_flags() NeilBrown
2015-03-23  2:37 ` [PATCH 11/20] VFS/namei: use terminate_walk when symlink lookup fails NeilBrown
2015-03-23  2:37 ` [PATCH 08/20] VFS: make all ->follow_link handlers aware for LOOKUP_RCU NeilBrown
2015-03-23  2:37 ` [PATCH 13/20] VFS/namei: abort RCU-walk on symlink if atime needs updating NeilBrown
2015-03-23  2:37 ` [PATCH 14/20] VFS/namei: add 'inode' arg to put_link() NeilBrown
2015-04-17 16:25   ` Al Viro
2015-04-17 19:09     ` Al Viro
2015-04-18  8:09       ` Al Viro
2015-03-23  2:37 ` [PATCH 16/20] VFS/namei: enable RCU-walk when following symlinks NeilBrown
2015-03-23  2:37 ` [PATCH 19/20] XFS: allow follow_link to often succeed in RCU-walk NeilBrown
2015-03-23  2:37 ` [PATCH 20/20] NFS: support LOOKUP_RCU in nfs_follow_link NeilBrown
2015-03-23  2:37 ` [PATCH 18/20] xfs: use RCU to free 'struct xfs_mount' NeilBrown
2015-03-23  2:37 ` [PATCH 17/20] VFS/namei: handle LOOKUP_RCU in page_follow_link_light NeilBrown
2015-03-23  2:37 ` NeilBrown [this message]
2015-03-23  2:37 ` [PATCH 15/20] VFS/namei: enhance follow_link to support RCU-walk NeilBrown
2015-03-25 23:23 ` [PATCH 00/20] Support follow_link in RCU-walk - V3 NeilBrown

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