RE: [PATCH] kernfs: implement custom llseek method to fix userspace regression

[Dan Williams]

Valentine Sinitsyn wrote:
> Since commit 636b21b50152 ("PCI: Revoke mappings like devmem"),
> mmapable sysfs binary attributes have started receiving their
> f_mapping from the iomem pseudo filesystem, so that
> CONFIG_IO_STRICT_DEVMEM is honored in sysfs (and procfs) as well
> as in /dev/[k]mem.
> 
> This resulted in a userspace-visible regression: lseek(fd, 0, SEEK_END)
> now returns zero regardless the real sysfs attribute size which stat()
> reports. The reason is that kernfs files use generic_file_llseek()
> implementation, which relies on f_mapping->host inode to get the file
> size. As f_mapping is now redefined, f_mapping->host points to an
> anonymous zero-sized iomem inode which has nothing to do with sysfs
> attribute or kernfs file representing it. This being said, f_inode
> remains valid, so stat() which uses it works correctly.

Can you say a bit more about what userspace scenario regressed so that
others doing backports can make a judgement call on the severity?

> 
> Fixes the regression by implementing a custom llseek fop for kernfs,
> which uses an attribute's file inode to get the file size,
> just as stat() does.
> 
> Fixes: 636b21b50152 ("PCI: Revoke mappings like devmem")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Valentine Sinitsyn <valesini@xxxxxxxxxxxxxx>
> ---
>  fs/kernfs/file.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
> index 180906c36f51..6d81e0c981f3 100644
> --- a/fs/kernfs/file.c
> +++ b/fs/kernfs/file.c
> @@ -903,6 +903,21 @@ static __poll_t kernfs_fop_poll(struct file *filp, poll_table *wait)
>  	return ret;
>  }
>  
> +static loff_t kernfs_fop_llseek(struct file *file, loff_t offset, int whence)
> +{
> +	/*
> +	 * This is almost identical to generic_file_llseek() except it uses
> +	 * cached inode value instead of f_mapping->host.
> +	 * The reason is that, for PCI resources in sysfs the latter points to
> +	 * iomem_inode whose size has nothing to do with the attribute's size.
> +	 */
> +	struct inode *inode = file_inode(file);

My only concern is whether there are any scenarios where this is not
appropriate. I.e. do a bit more work to define a kernfs_ops instance
specifically for overriding lseek() in this scenario.

> +
> +	return generic_file_llseek_size(file, offset, whence,
> +					inode->i_sb->s_maxbytes,
> +					i_size_read(inode));
> +}
> +
>  static void kernfs_notify_workfn(struct work_struct *work)
>  {
>  	struct kernfs_node *kn;
> @@ -1005,7 +1020,7 @@ EXPORT_SYMBOL_GPL(kernfs_notify);
>  const struct file_operations kernfs_file_fops = {
>  	.read_iter	= kernfs_fop_read_iter,
>  	.write_iter	= kernfs_fop_write_iter,
> -	.llseek		= generic_file_llseek,
> +	.llseek		= kernfs_fop_llseek,
>  	.mmap		= kernfs_fop_mmap,
>  	.open		= kernfs_fop_open,
>  	.release	= kernfs_fop_release,
> -- 
> 2.34.1
>