Re: [PATCH 1/2 v2] fs/proc: show correct device and inode numbers in /proc/pid/maps

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Dec 14, 2023 at 8:44 AM Andrei Vagin <avagin@google.com> wrote:
>
> /proc/pid/maps shows device and inode numbers of vma->vm_file-s. Here is
> an issue. If a mapped file is on a stackable file system (e.g.,
> overlayfs), vma->vm_file is a backing file whose f_inode is on the
> underlying filesystem. To show correct numbers, we need to get a user
> file and shows its numbers. The same trick is used to show file paths in
> /proc/pid/maps.
>
> Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
> Suggested-by: Amir Goldstein <amir73il@gmail.com>
> Signed-off-by: Andrei Vagin <avagin@google.com>
> ---
> v2: Amir explained that vfs_getattr isn't needed, because
> file_user_inode(vma->vm_file).i_ino always matches an inode number
> returned by statx.

At least i_ino *should* always match st_ino for overlayfs non-dirs.
If it doesn't, it is a bug.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>

Thanks,
Amir.

>
>  fs/proc/task_mmu.c |  3 ++-
>  include/linux/fs.h | 18 +++++++++++++-----
>  2 files changed, 15 insertions(+), 6 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 435b61054b5b..1801e409a061 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -273,7 +273,8 @@ show_map_vma(struct seq_file *m, struct vm_area_struct *vma)
>         const char *name = NULL;
>
>         if (file) {
> -               struct inode *inode = file_inode(vma->vm_file);
> +               const struct inode *inode = file_user_inode(vma->vm_file);
> +
>                 dev = inode->i_sb->s_dev;
>                 ino = inode->i_ino;
>                 pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 98b7a7a8c42e..838ccfc63323 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2523,20 +2523,28 @@ struct file *backing_file_open(const struct path *user_path, int flags,
>  struct path *backing_file_user_path(struct file *f);
>
>  /*
> - * file_user_path - get the path to display for memory mapped file
> - *
>   * When mmapping a file on a stackable filesystem (e.g., overlayfs), the file
>   * stored in ->vm_file is a backing file whose f_inode is on the underlying
> - * filesystem.  When the mapped file path is displayed to user (e.g. via
> - * /proc/<pid>/maps), this helper should be used to get the path to display
> - * to the user, which is the path of the fd that user has requested to map.
> + * filesystem.  When the mapped file path and inode number are displayed to
> + * user (e.g. via /proc/<pid>/maps), these helpers should be used to get the
> + * path and inode number to display to the user, which is the path of the fd
> + * that user has requested to map and the inode number that would be returned
> + * by fstat() on that same fd.
>   */
> +/* Get the path to display in /proc/<pid>/maps */
>  static inline const struct path *file_user_path(struct file *f)
>  {
>         if (unlikely(f->f_mode & FMODE_BACKING))
>                 return backing_file_user_path(f);
>         return &f->f_path;
>  }
> +/* Get the inode whose inode number to display in /proc/<pid>/maps */
> +static inline const struct inode *file_user_inode(struct file *f)
> +{
> +       if (unlikely(f->f_mode & FMODE_BACKING))
> +               return d_inode(backing_file_user_path(f)->dentry);
> +       return file_inode(f);
> +}
>
>  static inline struct file *file_clone_open(struct file *file)
>  {
> --
> 2.43.0.472.g3155946c3a-goog
>