[PATCH 1/3] namei: add LOOKUP_DFD_ROOT to use dfd as root

[Andrey Vagin]

On Fri, Jul 1, 2016 at 5:55 PM, Omar Sandoval <osandov at osandov.com> wrote:
> On Tue, Jun 28, 2016 at 10:38:28AM -0700, Andrey Vagin wrote:
>> The problem is that a pathname can contain absolute symlinks and now
>> they are resolved relative to the current root.
>>
>> If we want to open a file in another mount namespaces and we have a file
>> descriptor to its root directory, we probably want to resolve pathname
>> in the target mount namespace. For this we add this new flag.
>>
>> If LOOKUP_DFD_ROOT is set, path_init() initializes nd->root and nd->path
>> to the same value.
>>
>> Signed-off-by: Andrey Vagin <avagin at openvz.org>
>
> Hi, Andrey,
>
> Seems like a useful feature. Make sure to cc linux-api at vger.kernel.org
> for new userspace interfaces. One comment on the implementation below.

Hi Omar,

Thank you for the patch. I have sent a new version with your changes.
If there will not be other comments in a few days, I will resent a
whole seires with linux-api at vger.kernel.org in CC.

Thanks,
Andrew

>
>> ---
>>  fs/namei.c            | 12 +++++++++++-
>>  include/linux/namei.h |  2 ++
>>  2 files changed, 13 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 70580ab..5f08b69 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -2148,7 +2148,7 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>>       nd->path.dentry = NULL;
>>
>>       nd->m_seq = read_seqbegin(&mount_lock);
>> -     if (*s == '/') {
>> +     if (*s == '/' && !(flags & LOOKUP_DFD_ROOT)) {
>>               if (flags & LOOKUP_RCU)
>>                       rcu_read_lock();
>>               set_root(nd);
>> @@ -2174,6 +2174,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>>                       get_fs_pwd(current->fs, &nd->path);
>>                       nd->inode = nd->path.dentry->d_inode;
>>               }
>> +             if (flags & LOOKUP_DFD_ROOT) {
>> +                     nd->root = nd->path;
>> +                     if (!(flags & LOOKUP_RCU))
>> +                             path_get(&nd->root);
>
> You're not initializing nd->root_seq here. That means that if we end up
> going through unlazy_walk(), we're going to call legitimize_path() (and
> thus read_seqcount_retry()) with stack garbage, get a spurious ECHILD,
> and do an unnecessary restart of the path lookup instead of dropping
> into ref-walk mode.
>
>> +             }
>>               return s;
>>       } else {
>>               /* Caller must check execute permissions on the starting path component */
>> @@ -2202,6 +2207,11 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>>                       nd->inode = nd->path.dentry->d_inode;
>>               }
>>               fdput(f);
>> +             if (flags & LOOKUP_DFD_ROOT) {
>> +                     nd->root = nd->path;
>> +                     if (!(flags & LOOKUP_RCU))
>> +                             path_get(&nd->root);
>> +             }
>
> Same here.
>
> The following should do the trick:
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 9958b605e822..101d1fb8d3cb 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2176,7 +2176,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>                 }
>                 if (flags & LOOKUP_DFD_ROOT) {
>                         nd->root = nd->path;
> -                       if (!(flags & LOOKUP_RCU))
> +                       if (flags & LOOKUP_RCU)
> +                               nd->root_seq = nd->seq;
> +                       else
>                                 path_get(&nd->root);
>                 }
>                 return s;
> @@ -2209,7 +2211,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
>                 fdput(f);
>                 if (flags & LOOKUP_DFD_ROOT) {
>                         nd->root = nd->path;
> -                       if (!(flags & LOOKUP_RCU))
> +                       if (flags & LOOKUP_RCU)
> +                               nd->root_seq = nd->seq;
> +                       else
>                                 path_get(&nd->root);
>                 }
>                 return s;
>
> --
> Omar
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