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Date: Thu, 3 Mar 2022 11:13:47 -0800
From: Yonghong Song <yhs@...com>
To: Hao Luo <haoluo@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Martin KaFai Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
KP Singh <kpsingh@...nel.org>,
Shakeel Butt <shakeelb@...gle.com>,
Joe Burton <jevburton.kernel@...il.com>,
Tejun Heo <tj@...nel.org>, joshdon@...gle.com, sdf@...gle.com,
bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls
for prog_bpf_syscall
On 3/3/22 10:56 AM, Hao Luo wrote:
> On Wed, Mar 2, 2022 at 12:55 PM Yonghong Song <yhs@...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@...gle.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
>> [...]
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