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Date:   Sun, 27 Feb 2022 10:48:21 +0530
From:   Kumar Kartikeya Dwivedi <memxor@gmail.com>
To:     Hao Luo <haoluo@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Joe Burton <jevburton.kernel@gmail.com>,
        Tejun Heo <tj@kernel.org>, joshdon@google.com, sdf@google.com,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls
 for prog_bpf_syscall
Message-ID: <20220227051821.fwrmeu7r6bab6tio@apollo.legion>
References: <20220225234339.2386398-1-haoluo@google.com>
 <20220225234339.2386398-2-haoluo@google.com>
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On Sat, Feb 26, 2022 at 05:13:31AM IST, Hao Luo wrote:
> This patch allows bpf_syscall prog to perform some basic filesystem
> operations: create, remove directories and unlink files. Three bpf
> helpers are added for this purpose. When combined with the following
> patches that allow pinning and getting bpf objects from bpf prog,
> this feature can be used to create directory hierarchy in bpffs that
> help manage bpf objects purely using bpf progs.
>
> The added helpers subject to the same permission checks as their syscall
> version. For example, one can not write to a read-only file system;
> The identity of the current process is checked to see whether it has
> sufficient permission to perform the operations.
>
> Only directories and files in bpffs can be created or removed by these
> helpers. But it won't be too hard to allow these helpers to operate
> on files in other filesystems, if we want.
>
> Signed-off-by: Hao Luo <haoluo@google.com>
> ---
>  include/linux/bpf.h            |   1 +
>  include/uapi/linux/bpf.h       |  26 +++++
>  kernel/bpf/inode.c             |   9 +-
>  kernel/bpf/syscall.c           | 177 +++++++++++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  26 +++++
>  5 files changed, 236 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f19abc59b6cd..fce5e26179f5 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1584,6 +1584,7 @@ int bpf_link_new_fd(struct bpf_link *link);
>  struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
>  struct bpf_link *bpf_link_get_from_fd(u32 ufd);
>
> +bool bpf_path_is_bpf_dir(const struct path *path);
>  int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
>  int bpf_obj_get_user(const char __user *pathname, int flags);
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index afe3d0d7f5f2..a5dbc794403d 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -5086,6 +5086,29 @@ union bpf_attr {
>   *	Return
>   *		0 on success, or a negative error in case of failure. On error
>   *		*dst* buffer is zeroed out.
> + *
> + * long bpf_mkdir(const char *pathname, int pathname_sz, u32 mode)
> + *	Description
> + *		Attempts to create a directory name *pathname*. The argument
> + *		*pathname_sz* specifies the length of the string *pathname*.
> + *		The argument *mode* specifies the mode for the new directory. It
> + *		is modified by the process's umask. It has the same semantic as
> + *		the syscall mkdir(2).
> + *	Return
> + *		0 on success, or a negative error in case of failure.
> + *
> + * long bpf_rmdir(const char *pathname, int pathname_sz)
> + *	Description
> + *		Deletes a directory, which must be empty.
> + *	Return
> + *		0 on sucess, or a negative error in case of failure.
> + *
> + * long bpf_unlink(const char *pathname, int pathname_sz)
> + *	Description
> + *		Deletes a name and possibly the file it refers to. It has the
> + *		same semantic as the syscall unlink(2).
> + *	Return
> + *		0 on success, or a negative error in case of failure.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -5280,6 +5303,9 @@ union bpf_attr {
>  	FN(xdp_load_bytes),		\
>  	FN(xdp_store_bytes),		\
>  	FN(copy_from_user_task),	\
> +	FN(mkdir),			\
> +	FN(rmdir),			\
> +	FN(unlink),			\
>  	/* */
>

How about only introducing bpf_sys_mkdirat and bpf_sys_unlinkat? That would be
more useful for other cases in future, and when AT_FDCWD is passed, has the same
functionality as these, but when openat/fget is supported, it would work
relative to other dirfds as well. It can also allow using dirfd of the process
calling read for a iterator (e.g. if it sets the fd number using skel->bss).
unlinkat's AT_REMOVEDIR flag also removes the need for a bpf_rmdir.

WDYT?

> [...]

--
Kartikeya