On Tue, Dec 12, 2023 at 1:27 AM Christian Brauner <brauner@xxxxxxxxxx> wrote: > > On Tue, Dec 12, 2023 at 07:51:31AM +0200, Amir Goldstein wrote: > > +fsdevel, +overlayfs, +brauner, +miklos > > > > On Mon, Dec 11, 2023 at 9:30 PM Andrei Vagin <avagin@xxxxxxxxxx> 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@xxxxxxxxx> > > > Cc: Alexander Mikhalitsyn <alexander@xxxxxxxxxxxxx> > > > Signed-off-by: Andrei Vagin <avagin@xxxxxxxxxx> > > > --- > > > 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. > > It's also very dicy calling into the filesystem from procfs. You might > hang the system if you end up talking to a hung NFS server or something. > What locks does show_map_vma() hold? And is it safe to call helpers that > might generate io? I had the same thoughts when I was thinking about whether it is safe to use it here or not. Then I found AT_STATX_DONT_SYNC (don't sync attributes with the server) and decided that it should be safe. Anyway, Amir explains that vfs_getattr_nosec isn't needed for overlay files. Thanks, Andrei
