[Devel] [PATCH RH9] fs/namei: allow hardlinking between mounts with the same superblock

[Pavel Tikhomirov]

On 12.11.2021 15:34, Andrey Zhadchenko wrote:
> When we dump deleted files in criu, we are re-linking them back to fs, so
> we don't need to copy them. But if the mount where this file resides is
> overmounted we need to uncover it first. Uncovering it in original mount
> namespace is very difficult since we may need to unfreeze processes back
> if we failed somewhere later or even doing snapshot. So we copy mount
> namespace and uncover this mount there. Unfortunately we can't hardlink
> open fd to this uncovered mount since it is not the same mount where the
> file was.
> Technically there is no problem to create hardlink if superblock is
> the same for both mounts. Allow this for CAP_SYS_ADMIN and ve_pseudosper.
> 
> https://jira.sw.ru/browse/PSBM-127834

I do not see another way to solve problem with dumping an fd with 
unlinked path but existing another hardlink, when fd is residing on 
overmounted or detached mount.

Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
>   fs/namei.c | 8 +++++++-
>   1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 38f52c1..6361f52 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -39,6 +39,7 @@
>   #include <linux/bitops.h>
>   #include <linux/init_task.h>
>   #include <linux/uaccess.h>
> +#include <linux/ve.h>
>   
>   #include "internal.h"
>   #include "mount.h"
> @@ -4370,7 +4371,12 @@ static int do_linkat(int olddfd, const char __user *oldname, int newdfd,
>   
>   	error = -EXDEV;
>   	if (old_path.mnt != new_path.mnt)
> -		goto out_dput;
> +#ifdef CONFIG_VE
> +		if (!((old_path.mnt->mnt_sb == new_path.mnt->mnt_sb) &&
> +		      (capable(CAP_SYS_ADMIN) || get_exec_env()->is_pseudosuper)))
> +#endif
> +			goto out_dput;
> +
>   	mnt_userns = mnt_user_ns(new_path.mnt);
>   	error = may_linkat(mnt_userns, &old_path);
>   	if (unlikely(error))
> 

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.