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SubjectRe: [PATCH] force dentry revalidation after namespace change
Glauber Costa <glommer@parallels.com> writes:

> 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.

Equalivalently to remounting you can do
echo 3 > /proc/sys/vm/drop_caches.

> 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.

Overall good bug spotting, and good spotting of where the fix should
live.

Your summary should have said:
[PATCH] fail dentry revalidation after namespace change

And you have the test slightly wrong below.

> Signed-off-by: Glauber Costa <glommer@parallels.com>
> CC: Tejun Heo <tj@kernel.org>
> CC: Eric W. Biederman <ebiederm@xmission.com>
> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> fs/sysfs/dir.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
> index e6bb9b2..c24bdd9 100644
> --- 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;
> @@ -314,6 +315,10 @@ static int sysfs_dentry_revalidate(struct dentry *dentry, struct nameidata *nd)
> sd = dentry->d_fsdata;
> mutex_lock(&sysfs_mutex);
>
> + type = sysfs_ns_type(sd);
> + if (sd->s_ns && (sysfs_info(dentry->d_sb)->ns[type] != sd->s_ns))
> + goto out_bad;
> +

First this check should be down below with after the other rename
checks.

Second the test should be:
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))
goto out_bad;

The important difference there it is the directory that the dirent is
in that the type comes from. Not the dirent itself.

> /* The sysfs dirent has been deleted */
> if (sd->s_flags & SYSFS_FLAG_REMOVED)
> goto out_bad;

Glauber. Do you think you can fix your patch and resubmit.
Eric


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