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

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Message-Id: <20231211114306.08211860253a77c69bab2a87@linux-foundation.org>
Date:   Mon, 11 Dec 2023 11:43:06 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andrei Vagin <avagin@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Amir Goldstein <amir73il@...il.com>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 1/2] fs/proc: show correct device and inode numbers in
 /proc/pid/maps

On Mon, 11 Dec 2023 11:30:47 -0800 Andrei Vagin <avagin@...gle.com> wrote:

> Device and inode numbers in /proc/pid/maps have to match numbers returned by
> statx for the same files.
> 
> /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.
> 
> 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.

Al, could you please comment on this?

Thanks.

> --- 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)) {
> +			dev = stat.dev;
> +			ino = stat.ino;
> +		} else {
> +			struct inode *inode = d_backing_inode(path->dentry);
> +
> +			dev = inode->i_sb->s_dev;
> +			ino = inode->i_ino;
> +		}
>  		pgoff = ((loff_t)vma->vm_pgoff) << PAGE_SHIFT;
>  	}
>  
> -- 
> 2.43.0.472.g3155946c3a-goog