LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Andreas Gruenbacher <agruen@suse.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	Al Viro <viro@ftp.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, Tony Jones <tonyj@suse.de>,
	Neil Brown <neilb@suse.de>, Christoph Hellwig <hch@lst.de>,
	Greg Kroah-Hartman <gregkh@suse.de>, walt <wa1ter@myrealbox.com>
Subject: Re: [PATCH] Fix d_path for lazy unmounts
Date: Sat, 17 Feb 2007 05:30:27 -0800	[thread overview]
Message-ID: <200702170530.28235.agruen@suse.de> (raw)
In-Reply-To: <200702141913.21944.agruen@suse.de>

[-- Attachment #1: Type: text/plain, Size: 505 bytes --]

On Wednesday 14 February 2007 19:13, Andreas Gruenbacher wrote:
> So here is how this could be implemented. See the lengthy explanations in
> the patch headers, too.

Turns out I messed up with one of Neil's review comments. Here is yet another 
update.

With this fix, everything works as expected in testing now. sys_getcwd 
returns -ENOENT for paths not connected to the chroot. /proc/mounts 
and /proc/$pid/mountstats never contain a relative path. gnome-vfs-daemon 
seems quite happy again.

Andreas

[-- Attachment #2: d_path-lazy-unmounts.diff --]
[-- Type: text/x-diff, Size: 8026 bytes --]

Fix __d_path() for lazy unmounts and make it unambiguous

First, when d_path() hits a lazily unmounted mount point, it tries to
prepend the name of the lazily unmounted dentry to the path name. It
gets this wrong, and also overwrites the slash that separates the name
from the following pathname component.

Second, it isn't always possible to tell from the __d_path result
whether the specified root and rootmnt (i.e., the chroot) was reached:
lazy unmounts of bind mounts will produce a path that does start with a
non-slash so we can tell from that, but other lazy unmounts will produce
a path that starts with a slash, just like "ordinary" paths.

Third, sys_getcwd() shouldn't return disconnected paths. The patch
checks for that, and makes it fail with -ENOENT in that case.

The attached patch cleans up __d_path() to fix the bug with overlapping
pathname components. It also adds a @fail_deleted argument, which allows
to get rid of some of the mess in sys_getcwd(). We make sure that paths
will only start with a slash if the path leads all the way up to the
root. If the resulting path would otherwise be empty, we return "."
instead so that some users of seq_path for files in /proc won't break.

The @fail_deleted argument allows sys_getcwd() to be simplified.
Grabbing the dcache_lock can be moved into __d_path().

The @fail_deleted argument could be added to d_path() as well: this would
allow callers to recognize deleted files without having to resort to the
ambiguous check for the " (deleted)" string at the end of the pathnames.
This is not currently done, but it might be worthwhile.

Signed-off-by: Andreas Gruenbacher <agruen@suse.de>
Reviewed-by: NeilBrown <neilb@suse.de>

Index: b/fs/dcache.c
===================================================================
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -1732,52 +1732,51 @@ shouldnt_be_hashed:
 }
 
 /**
- * d_path - return the path of a dentry
+ * __d_path - return the path of a dentry
  * @dentry: dentry to report
  * @vfsmnt: vfsmnt to which the dentry belongs
  * @root: root dentry
  * @rootmnt: vfsmnt to which the root dentry belongs
  * @buffer: buffer to return value in
  * @buflen: buffer length
+ * @fail_deleted: what to return for deleted files
  *
- * Convert a dentry into an ASCII path name. If the entry has been deleted
+ * Convert a dentry into an ASCII path name. If the entry has been deleted,
+ * then if @fail_deleted is true, ERR_PTR(-ENOENT) is returned. Otherwise,
  * the string " (deleted)" is appended. Note that this is ambiguous.
  *
- * Returns the buffer or an error code if the path was too long.
+ * If @dentry is not connected to @root, the path returned will be relative
+ * (i.e., it will not start with a slash).
  *
- * "buflen" should be positive. Caller holds the dcache_lock.
+ * Returns the buffer or an error code.
  */
-static char * __d_path( struct dentry *dentry, struct vfsmount *vfsmnt,
-			struct dentry *root, struct vfsmount *rootmnt,
-			char *buffer, int buflen)
-{
-	char * end = buffer+buflen;
-	char * retval;
-	int namelen;
+static char *__d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
+		      struct dentry *root, struct vfsmount *rootmnt,
+		      char *buffer, int buflen, int fail_deleted)
+{
+	int namelen, is_slash;
+
+	if (buflen < 2)
+		return ERR_PTR(-ENAMETOOLONG);
+	buffer += --buflen;
+	*buffer = '\0';
 
-	*--end = '\0';
-	buflen--;
+	spin_lock(&dcache_lock);
 	if (!IS_ROOT(dentry) && d_unhashed(dentry)) {
-		buflen -= 10;
-		end -= 10;
-		if (buflen < 0)
+		if (fail_deleted) {
+			buffer = ERR_PTR(-ENOENT);
+			goto out;
+		}
+		if (buflen < 10)
 			goto Elong;
-		memcpy(end, " (deleted)", 10);
+		buflen -= 10;
+		buffer -= 10;
+		memcpy(buffer, " (deleted)", 10);
 	}
-
-	if (buflen < 1)
-		goto Elong;
-	/* Get '/' right */
-	retval = end-1;
-	*retval = '/';
-
-	for (;;) {
+	while (dentry != root || vfsmnt != rootmnt) {
 		struct dentry * parent;
 
-		if (dentry == root && vfsmnt == rootmnt)
-			break;
 		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
-			/* Global root? */
 			spin_lock(&vfsmount_lock);
 			if (vfsmnt->mnt_parent == vfsmnt) {
 				spin_unlock(&vfsmount_lock);
@@ -1791,33 +1790,63 @@ static char * __d_path( struct dentry *d
 		parent = dentry->d_parent;
 		prefetch(parent);
 		namelen = dentry->d_name.len;
-		buflen -= namelen + 1;
-		if (buflen < 0)
+		if (buflen < namelen + 1)
 			goto Elong;
-		end -= namelen;
-		memcpy(end, dentry->d_name.name, namelen);
-		*--end = '/';
-		retval = end;
+		buflen -= namelen + 1;
+		buffer -= namelen;
+		memcpy(buffer, dentry->d_name.name, namelen);
+		*--buffer = '/';
 		dentry = parent;
 	}
+	/* Get '/' right. */
+	if (*buffer != '/')
+		*--buffer = '/';
 
-	return retval;
+out:
+	spin_unlock(&dcache_lock);
+	return buffer;
 
 global_root:
+	/*
+	 * We went past the (vfsmount, dentry) we were looking for and have
+	 * either hit a root dentry, a lazily unmounted dentry, an
+	 * unconnected dentry, or the file is on a pseudo filesystem.
+	 */
 	namelen = dentry->d_name.len;
-	buflen -= namelen;
-	if (buflen < 0)
+	is_slash = (namelen == 1 && *dentry->d_name.name == '/');
+	if (is_slash || (dentry->d_sb->s_flags & MS_NOUSER)) {
+		/*
+		 * Make sure we won't return a pathname starting with '/'.
+		 *
+		 * Historically, we also glue together the root dentry and
+		 * remaining name for pseudo filesystems like pipefs, which
+		 * have the MS_NOUSER flag set. This results in pathnames
+		 * like "pipe:[439336]".
+		 */
+		if (*buffer == '/') {
+			buffer++;
+			buflen++;
+		}
+		if (is_slash) {
+			if (*buffer == '\0')
+				*--buffer = '.';
+			goto out;
+		}
+	}
+	if (buflen < namelen)
 		goto Elong;
-	retval -= namelen-1;	/* hit the slash */
-	memcpy(retval, dentry->d_name.name, namelen);
-	return retval;
+	buffer -= namelen;
+	memcpy(buffer, dentry->d_name.name, namelen);
+	goto out;
+
 Elong:
-	return ERR_PTR(-ENAMETOOLONG);
+	buffer = ERR_PTR(-ENAMETOOLONG);
+	goto out;
 }
 
 /* write full pathname into buffer and return start of pathname */
-char * d_path(struct dentry *dentry, struct vfsmount *vfsmnt,
-				char *buf, int buflen)
+char *d_path(struct dentry *dentry, struct vfsmount *vfsmnt, char *buf,
+	     int buflen)
 {
 	char *res;
 	struct vfsmount *rootmnt;
@@ -1827,9 +1856,7 @@ char * d_path(struct dentry *dentry, str
 	rootmnt = mntget(current->fs->rootmnt);
 	root = dget(current->fs->root);
 	read_unlock(&current->fs->lock);
-	spin_lock(&dcache_lock);
-	res = __d_path(dentry, vfsmnt, root, rootmnt, buf, buflen);
-	spin_unlock(&dcache_lock);
+	res = __d_path(dentry, vfsmnt, root, rootmnt, buf, buflen, 0);
 	dput(root);
 	mntput(rootmnt);
 	return res;
@@ -1855,10 +1882,10 @@ char * d_path(struct dentry *dentry, str
  */
 asmlinkage long sys_getcwd(char __user *buf, unsigned long size)
 {
-	int error;
+	int error, len;
 	struct vfsmount *pwdmnt, *rootmnt;
 	struct dentry *pwd, *root;
-	char *page = (char *) __get_free_page(GFP_USER);
+	char *page = (char *) __get_free_page(GFP_USER), *cwd;
 
 	if (!page)
 		return -ENOMEM;
@@ -1870,29 +1897,21 @@ asmlinkage long sys_getcwd(char __user *
 	root = dget(current->fs->root);
 	read_unlock(&current->fs->lock);
 
+	cwd = __d_path(pwd, pwdmnt, root, rootmnt, page, PAGE_SIZE, 1);
+	error = PTR_ERR(cwd);
+	if (IS_ERR(cwd))
+		goto out;
 	error = -ENOENT;
-	/* Has the current directory has been unlinked? */
-	spin_lock(&dcache_lock);
-	if (pwd->d_parent == pwd || !d_unhashed(pwd)) {
-		unsigned long len;
-		char * cwd;
-
-		cwd = __d_path(pwd, pwdmnt, root, rootmnt, page, PAGE_SIZE);
-		spin_unlock(&dcache_lock);
-
-		error = PTR_ERR(cwd);
-		if (IS_ERR(cwd))
-			goto out;
+	if (*cwd != '/')
+		goto out;
 
-		error = -ERANGE;
-		len = PAGE_SIZE + page - cwd;
-		if (len <= size) {
-			error = len;
-			if (copy_to_user(buf, cwd, len))
-				error = -EFAULT;
-		}
-	} else
-		spin_unlock(&dcache_lock);
+	error = -ERANGE;
+	len = PAGE_SIZE + page - cwd;
+	if (len <= size) {
+		error = len;
+		if (copy_to_user(buf, cwd, len))
+			error = -EFAULT;
+	}
 
 out:
 	dput(pwd);

  reply	other threads:[~2007-02-17 13:30 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-03  3:23 Andreas Gruenbacher
2007-02-05  0:15 ` Neil Brown
2007-02-15  2:43   ` Andreas Gruenbacher
2007-02-05  8:32 ` Andreas Gruenbacher
2007-02-05 18:37   ` [PATCH]Add memory barrier before clear bit in unlock_buffer() Mingming Cao
2007-02-14  8:19   ` [PATCH] Fix d_path for lazy unmounts Andreas Gruenbacher
2007-02-14  8:29     ` Olaf Hering
2007-02-14  8:42       ` Andreas Gruenbacher
2007-02-14 15:37     ` Linus Torvalds
2007-02-14 19:39       ` Andreas Gruenbacher
2007-02-14 22:57         ` Andreas Gruenbacher
2007-02-15  3:13           ` Andreas Gruenbacher
2007-02-17 13:30             ` Andreas Gruenbacher [this message]
2007-02-15 12:53           ` Jan Engelhardt
2007-02-15 13:19             ` Andreas Gruenbacher

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=200702170530.28235.agruen@suse.de \
    --to=agruen@suse.de \
    --cc=akpm@linux-foundation.org \
    --cc=gregkh@suse.de \
    --cc=hch@lst.de \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=neilb@suse.de \
    --cc=tonyj@suse.de \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@ftp.linux.org.uk \
    --cc=wa1ter@myrealbox.com \
    --subject='Re: [PATCH] Fix d_path for lazy unmounts' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).