[PATCH RFC 1/3] fs: add open O_TMPFILE support

[Ahmad Fatoum]

On 20.11.23 09:37, Ahmad Fatoum wrote:
> barebox dentry cache is never cleared with the assumption that there
> should be enough RAM anyway to cache all lookups until boot.
> 
> When fuzzing barebox however, there is no limit to how many dentries
> are added to the cache. This is e.g. problematic when fuzzing the FIT
> parser: FIT images can have compressed payloads. Compressed payloads are
> passed to uncompress_buf_to_buf, which uses a new random file in ramfs
> as destination. A fuzzer would thus create a dentry for every iteration,
> rapidly depleting memory.
> 
> A general solution for that would be dropping the dentry cache on memory
> pressure. In the special case of uncompress_buf_to_buf, it would already
> be enough though to sidestep the dentry cache and create an anonymous
> file. Linux provides this with the O_TMPFILE option, so let's add the
> equivalent to barebox.
> 
> Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
> ---
>  fs/fs.c         | 29 +++++++++++++++++++++++++++++
>  include/fcntl.h |  3 +++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/fs/fs.c b/fs/fs.c
> index 1800d6826ddc..6bd3c2df3c31 100644
> --- a/fs/fs.c
> +++ b/fs/fs.c
> @@ -2539,6 +2539,35 @@ int open(const char *pathname, int flags, ...)
>  	const char *s;
>  	struct filename *filename;
>  
> +	if (flags & O_TMPFILE) {
> +		fsdev = get_fsdevice_by_path(pathname);
> +		if (!fsdev) {
> +			errno = ENOENT;
> +			return -errno;
> +		}
> +
> +		if (fsdrv != ramfs_driver) {
> +			errno = EOPNOTSUPP;
> +			return -errno;
> +		}

Ouch should be fsdev->driver. Will retest and resend.

> +
> +		f = get_file();
> +		if (!f) {
> +			errno = EMFILE;
> +			return -errno;
> +		}
> +
> +		f->path = NULL;
> +		f->dentry = NULL;
> +		f->f_inode = new_inode(&fsdev->sb);
> +		f->f_inode->i_mode = S_IFREG;
> +		f->flags = flags;
> +		f->size = 0;
> +		f->fsdev = fsdev;
> +
> +		return f->no;
> +	}
> +
>  	filename = getname(pathname);
>  	if (IS_ERR(filename))
>  		return PTR_ERR(filename);
> diff --git a/include/fcntl.h b/include/fcntl.h
> index 2e7c0eed3479..1b4cd8ad3783 100644
> --- a/include/fcntl.h
> +++ b/include/fcntl.h
> @@ -16,6 +16,9 @@
>  #define O_APPEND	00002000
>  #define O_DIRECTORY	00200000	/* must be a directory */
>  #define O_NOFOLLOW	00400000	/* don't follow links */
> +#define __O_TMPFILE	020000000
> +
> +#define O_TMPFILE       (__O_TMPFILE | O_DIRECTORY)
>  
>  /* barebox additional flags */
>  #define O_RWSIZE_MASK	017000000

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