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

[Yonghong Song <yhs@fb.com>]

On 3/3/22 10:56 AM, Hao Luo wrote:
> On Wed, Mar 2, 2022 at 12:55 PM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> 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@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)
>>
>> 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.
>>
> 
> SG. Will make this change.
> 
> Actually, I think I need to cap patthname_sz from above, to ensure
> pathname_sz isn't too big. Is that necessary? I see there are other
> helpers that don't have this type of check.

There is no need. The verifier should ensure the memory held by pathname 
will have at least size of pathname_sz.

> 
>>> + *   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
>> [...]