Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf_syscall

[Yonghong Song]

On 2/25/22 3:43 PM, 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@xxxxxxxxxx>
> ---
>   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)

Can we make pathname_sz to be u32 instead of int? pathname_sz should 
never be negative any way.

Also, I think it is a good idea to add 'u64 flags' parameter for all
three helpers, so we have room in the future to tune for new use cases.

> + *	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),			\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
[...]