Re: [PATCH v2] fail dentry revalidation after namespace change

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  6 Jul 2012 13:09:07 +0400
Glauber Costa <glommer@parallels.com> wrote:

> When we change the namespace tag of a sysfs entry, the associated dentry
> is still kept around. readdir() will work correctly and not display the
> old entries, but open() will still succeed, so will reads and writes.
> 
> This will no longer happen if sysfs is remounted, hinting that this is a
> cache-related problem.
> 
> I am using the following sequence to demonstrate that:
> 
> shell1:
> ip link add type veth
> unshare -nm
> 
> shell2:
> ip link set veth1 <pid_of_shell_1>
> cat /sys/devices/virtual/net/veth1/ifindex
> 
> Before that patch, this will succeed (fail to fail). After it, it will
> correctly return an error. Differently from a normal rename, which we
> handle fine, changing the object namespace will keep it's path intact.
> So this check seems necessary as well.
> 
> ...
>
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -307,6 +307,7 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  {
>  	struct sysfs_dirent *sd;
>  	int is_dir;
> +	int type;
>  
>  	if (nd->flags & LOOKUP_RCU)
>  		return -ECHILD;
> @@ -326,6 +327,13 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
>  	if (strcmp(dentry->d_name.name, sd->s_name) != 0)
>  		goto out_bad;
>  
> +	/* The sysfs dirent has been moved to a different namespace */
> +	type = KOBJ_NS_TYPE_NONE;
> +	if (sd->s_parent)
> +		type = sysfs_ns_type(sd->s_parent);
> +	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))

eww, the code is assuming that KOBJ_NS_TYPE_NONE has a value of zero. 
Don't do that; it smells bad.

I renamed my version of this patch to "sysfs: fail dentry revalidation
after namespace change", as carefully explained in section 15 of the
excellent Documentation/SubmittingPatches, then queued this:


From: Andrew Morton <akpm@linux-foundation.org>
Subject: sysfs-fail-dentry-revalidation-after-namespace-change-fix

don't assume that KOBJ_NS_TYPE_NONE==0.  Also save a test-n-branch.

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Glauber Costa <glommer@parallels.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/sysfs/dir.c |    8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff -puN fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix fs/sysfs/dir.c
--- a/fs/sysfs/dir.c~sysfs-fail-dentry-revalidation-after-namespace-change-fix
+++ a/fs/sysfs/dir.c
@@ -329,10 +329,12 @@ static int sysfs_dentry_revalidate(struc
 
 	/* The sysfs dirent has been moved to a different namespace */
 	type = KOBJ_NS_TYPE_NONE;
-	if (sd->s_parent)
+	if (sd->s_parent) {
 		type = sysfs_ns_type(sd->s_parent);
-	if (type && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
-		goto out_bad;
+		if (type != KOBJ_NS_TYPE_NONE &&
+				sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns)
+			goto out_bad;
+	}
 
 	mutex_unlock(&sysfs_mutex);
 out_valid:
_