From: Amir Goldstein <amir73il@gmail.com>
To: Andrei Vagin <avagin@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
linux-kernel@vger.kernel.org,
Alexander Mikhalitsyn <alexander@mihalicyn.com>,
Christian Brauner <brauner@kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Miklos Szeredi <miklos@szeredi.hu>,
overlayfs <linux-unionfs@vger.kernel.org>
Subject: Re: [PATCH 1/2] fs/proc: show correct device and inode numbers in /proc/pid/maps
Date: Tue, 12 Dec 2023 07:51:31 +0200 [thread overview]
Message-ID: <CAOQ4uxik0=0F-6CLRsuaOheFjwWF-B-Q5iEQ6qJbRszL52HeQQ@mail.gmail.com> (raw)
In-Reply-To: <20231211193048.580691-1-avagin@google.com>
+fsdevel, +overlayfs, +brauner, +miklos
On Mon, Dec 11, 2023 at 9:30 PM Andrei Vagin <avagin@google.com> wrote:
>
> Device and inode numbers in /proc/pid/maps have to match numbers returned by
> statx for the same files.
That statement may be true for regular files.
It is not true for block/char as far as I know.
I think that your fix will break that by displaying the ino/dev
of the block/char reference inode and not their backing rdev inode.
>
> /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.
For the *same* trick, see my patch below.
>
> But it isn't the end of this story. A file system can manipulate inode numbers
> within the getattr callback (e.g., ovl_getattr), so vfs_getattr must be used to
> get correct numbers.
This explanation is inaccurate, because it mixes two different overlayfs
traits which are unrelated.
It is true that a filesystem *can* manipulate st_dev in a way that will not
match i_ino and it is true that overlayfs may do that in some non-default
configurations (see [1]), but this is not the reason that you are seeing
mismatches ino/dev in /proc/<pid>/maps.
[1] https://docs.kernel.org/filesystems/overlayfs.html#inode-properties
The reason is that the vma->vm_file is a special internal backing file
which is not otherwise exposed to userspace.
Please see my suggested fix below.
>
> Cc: Amir Goldstein <amir73il@gmail.com>
> Cc: Alexander Mikhalitsyn <alexander@mihalicyn.com>
> Signed-off-by: Andrei Vagin <avagin@google.com>
> ---
> fs/proc/task_mmu.c | 20 +++++++++++++++++---
> 1 file changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 435b61054b5b..abbf96c091ad 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -273,9 +273,23 @@ 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);
> - dev = inode->i_sb->s_dev;
> - ino = inode->i_ino;
> + const struct path *path;
> + struct kstat stat;
> +
> + path = file_user_path(file);
> + /*
> + * A file system can manipulate inode numbers within the
> + * getattr callback (e.g. ovl_getattr).
> + */
> + if (!vfs_getattr_nosec(path, &stat, STATX_INO, AT_STATX_DONT_SYNC)) {
Should you prefer to keep this solution it should be constrained to
regular files.
> + dev = stat.dev;
> + ino = stat.ino;
> + } else {
> + struct inode *inode = d_backing_inode(path->dentry);
d_inode() please.
d_backing_inode()/d_backing_dentry() are relics of an era that never existed
(i.e. union mounts).
> +
> + dev = inode->i_sb->s_dev;
> + ino = inode->i_ino;
> + }
> pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
> }
>
Would you mind trying this alternative (untested) patch?
I think it is preferred, because it is simpler.
Thanks,
Amir.
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index ef2eb12906da..5328266be6b5 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);
+ 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 900d0cd55b50..d78412c6fd47 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2581,20 +2581,28 @@ 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 helper 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 path *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);
+}
next parent reply other threads:[~2023-12-12 5:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20231211193048.580691-1-avagin@google.com>
2023-12-12 5:51 ` Amir Goldstein [this message]
2023-12-12 9:27 ` [PATCH 1/2] fs/proc: show correct device and inode numbers in /proc/pid/maps Christian Brauner
2023-12-12 19:19 ` Andrei Vagin
2023-12-12 19:08 ` Andrei Vagin
2023-12-13 17:05 ` Amir Goldstein
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