Re: [PATCH 03/25] vfs: Allow fsinfo() to query what's in an fs_context [ver #14]

[Christian Brauner]

On Mon, Jun 24, 2019 at 03:09:14PM +0100, David Howells wrote:
> Allow fsinfo() to be used to query the filesystem attached to an fs_context
> once a superblock has been created or if it comes from fspick().
> 
> The caller must specify AT_FSINFO_FROM_FSOPEN in the parameters and must

Yeah, I like that better than how it was before.

> supply the fd from fsopen() as dfd and must set filename to NULL.
> 
> This is done with something like:
> 
> 	fd = fsopen("ext4", 0);
> 	...
> 	struct fsinfo_params params = {
> 		.at_flags = AT_FSINFO_FROM_FSOPEN;
> 		...
> 	};
> 	fsinfo(fd, NULL, &params, ...);
> 
> Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
> ---
> 
>  fs/fsinfo.c                |   46 +++++++++++++++++++++++++++++++++++++++++++-
>  fs/statfs.c                |    2 +-
>  include/uapi/linux/fcntl.h |    2 ++
>  3 files changed, 48 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/fsinfo.c b/fs/fsinfo.c
> index 49b46f96dda3..c24701f994d1 100644
> --- a/fs/fsinfo.c
> +++ b/fs/fsinfo.c
> @@ -8,6 +8,7 @@
>  #include <linux/security.h>
>  #include <linux/uaccess.h>
>  #include <linux/fsinfo.h>
> +#include <linux/fs_context.h>
>  #include <uapi/linux/mount.h>
>  #include "internal.h"
>  
> @@ -340,6 +341,42 @@ static int vfs_fsinfo_fd(unsigned int fd, struct fsinfo_kparams *params)
>  	return ret;
>  }
>  
> +/*
> + * Allow access to an fs_context object as created by fsopen() or fspick().
> + */
> +static int vfs_fsinfo_fscontext(int fd, struct fsinfo_kparams *params)
> +{
> +	struct fs_context *fc;
> +	struct fd f = fdget(fd);
> +	int ret;
> +
> +	if (!f.file)
> +		return -EBADF;
> +
> +	ret = -EINVAL;
> +	if (f.file->f_op == &fscontext_fops)

Don't you mean != ?

if (f.file->f_op != &fscontext_fops)

> +		goto out_f;
> +	ret = -EOPNOTSUPP;
> +	if (fc->ops == &legacy_fs_context_ops)
> +		goto out_f;
> +
> +	ret = mutex_lock_interruptible(&fc->uapi_mutex);
> +	if (ret == 0) {
> +		ret = -EIO;

Why EIO when there's no root dentry?

> +		if (fc->root) {
> +			struct path path = { .dentry = fc->root };
> +
> +			ret = vfs_fsinfo(&path, params);
> +		}
> +
> +		mutex_unlock(&fc->uapi_mutex);
> +	}
> +
> +out_f:
> +	fdput(f);
> +	return ret;
> +}
> +
>  /*
>   * Return buffer information by requestable attribute.
>   *
> @@ -445,6 +482,9 @@ SYSCALL_DEFINE5(fsinfo,
>  		params.request = user_params.request;
>  		params.Nth = user_params.Nth;
>  		params.Mth = user_params.Mth;
> +

[1]:

> +		if ((params.at_flags & AT_FSINFO_FROM_FSOPEN) && filename)
> +			return -EINVAL;
>  	} else {
>  		params.request = FSINFO_ATTR_STATFS;
>  	}
> @@ -453,6 +493,8 @@ SYSCALL_DEFINE5(fsinfo,
>  		user_buf_size = 0;
>  		user_buffer = NULL;
>  	}
> +	if ((params.at_flags & AT_FSINFO_FROM_FSOPEN) && filename)
> +		return -EINVAL;

Sorry, why is this checked twice (see [1])? Or is the diff just
misleading here?

>  
>  	/* Allocate an appropriately-sized buffer.  We will truncate the
>  	 * contents when we write the contents back to userspace.
> @@ -500,7 +542,9 @@ SYSCALL_DEFINE5(fsinfo,
>  	if (!params.buffer)
>  		goto error_scratch;
>  
> -	if (filename)
> +	if (params.at_flags & AT_FSINFO_FROM_FSOPEN)
> +		ret = vfs_fsinfo_fscontext(dfd, &params);
> +	else if (filename)
>  		ret = vfs_fsinfo_path(dfd, filename, &params);
>  	else
>  		ret = vfs_fsinfo_fd(dfd, &params);
> diff --git a/fs/statfs.c b/fs/statfs.c
> index eea7af6f2f22..b9b63d9f4f24 100644
> --- a/fs/statfs.c
> +++ b/fs/statfs.c
> @@ -86,7 +86,7 @@ int vfs_statfs(const struct path *path, struct kstatfs *buf)
>  	int error;
>  
>  	error = statfs_by_dentry(path->dentry, buf);
> -	if (!error)
> +	if (!error && path->mnt)
>  		buf->f_flags = calculate_f_flags(path->mnt);
>  	return error;
>  }
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 1d338357df8a..6a2402a8fa30 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -91,6 +91,8 @@
>  #define AT_STATX_FORCE_SYNC	0x2000	/* - Force the attributes to be sync'd with the server */
>  #define AT_STATX_DONT_SYNC	0x4000	/* - Don't sync attributes with the server */
>  
> +#define AT_FSINFO_FROM_FSOPEN	0x2000	/* Examine the fs_context attached to dfd by fsopen() */
> +
>  #define AT_RECURSIVE		0x8000	/* Apply to the entire subtree */
>  
>  
>