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Subject[patch v3] vfs: fix sys_getcwd for detached mounts
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Date
I tested this one in various situations, and it appears to do what
it's supposed to.

A somewhat analogous issue is if we have cwd (or fd) outside
current->root. Should we add a prefix for that case as well? Perhaps
a double slash?

--
From: Miklos Szeredi <mszeredi@suse.cz>

Currently getcwd(2) on a detached mount will give a garbled result:

> mkdir /mnt/foo
> mount --bind /etc /mnt/foo
> cd /mnt/foo/skel
> umount -l /mnt/foo
> /bin/pwd
etcskel

After the patch the result will be "detached:skel".

Or if the root was mounted:

> mount --bind / /mnt/foo
> cd /mnt/foo/etc
> umount -l /mnt/foo
> /bin/pwd
/etc

After the patch the result will be "detached:etc".

Thanks to John Johansen for pointing out this bug.

v2: comment from Al Viro: use "detached:" prefix to differentiate it from
attached paths.

v3: fix case when mounted dentry is root of the filesystem.

Reported-by: John Johansen <jjohansen@suse.de>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
---
fs/dcache.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c 2008-06-23 19:34:21.000000000 +0200
+++ linux-2.6/fs/dcache.c 2008-06-23 20:15:09.000000000 +0200
@@ -1762,6 +1762,12 @@ static int prepend_name(char **buffer, i
return prepend(buffer, buflen, name->name, name->len);
}

+static bool is_pseudo_root(struct dentry *dentry)
+{
+ return IS_ROOT(dentry) &&
+ (dentry->d_name.len != 1 || dentry->d_name.name[0] != '/');
+}
+
/**
* __d_path - return the path of a dentry
* @path: the dentry/vfsmount to report
@@ -1828,8 +1834,16 @@ char *__d_path(const struct path *path,

global_root:
retval += 1; /* hit the slash */
- if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
- goto Elong;
+
+ if (vfsmnt->mnt_ns != NULL || is_pseudo_root(dentry)) {
+ /* Attached, or pseudo filesystem with "foo:" prefix */
+ if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
+ goto Elong;
+ } else {
+ /* Detached */
+ if (prepend(&retval, &buflen, "detached:", 9) != 0)
+ goto Elong;
+ }
root->mnt = vfsmnt;
root->dentry = dentry;
return retval;

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