linux-kernel - Re: [PATCH bpf-next v2 3/4] bpf/crib: Add struct file related CRIB kfuncs

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Message-ID:
 <AM6PR03MB5848E2CFFC021ED762E347BE99562@AM6PR03MB5848.eurprd03.prod.outlook.com>
Date: Fri, 1 Nov 2024 20:28:09 +0000
From: Juntong Deng <juntong.deng@...look.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
 andrii@...nel.org, martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, memxor@...il.com, snorcht@...il.com,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
 Christian Brauner <brauner@...nel.org>,
 Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 3/4] bpf/crib: Add struct file related CRIB
 kfuncs

On 2024/11/1 19:08, Andrii Nakryiko wrote:
> On Tue, Oct 29, 2024 at 5:17 PM Juntong Deng <juntong.deng@...look.com> wrote:
>>
>> This patch adds struct file related CRIB kfuncs.
>>
>> bpf_fget_task() is used to get a pointer to the struct file
>> corresponding to the task file descriptor. Note that this function
>> acquires a reference to struct file.
>>
>> bpf_get_file_ops_type() is used to determine what exactly this file
>> is based on the file operations, such as socket, eventfd, timerfd,
>> pipe, etc, in order to perform different checkpoint/restore processing
>> for different file types. This function currently has only one return
>> value, FILE_OPS_UNKNOWN, but will increase with the file types that
>> CRIB supports for checkpoint/restore.
>>
>> Signed-off-by: Juntong Deng <juntong.deng@...look.com>
>> ---
>>   kernel/bpf/crib/crib.c  |  4 ++++
>>   kernel/bpf/crib/files.c | 44 +++++++++++++++++++++++++++++++++++++++++
>>   2 files changed, 48 insertions(+)
>>
> 
> Please CC Christian Brauner and fs mailing list
> (linux-fsdevel@...r.kernel.org, both cc'ed) on changes like this (this
> entire patch set)
> 

Thanks for your reply!

I will CC Christian Brauner and fs mailing list in the next
patch series.

>> diff --git a/kernel/bpf/crib/crib.c b/kernel/bpf/crib/crib.c
>> index e6536ee9a845..78ddd19d5693 100644
>> --- a/kernel/bpf/crib/crib.c
>> +++ b/kernel/bpf/crib/crib.c
>> @@ -14,6 +14,10 @@ BTF_ID_FLAGS(func, bpf_iter_task_file_next, KF_ITER_NEXT | KF_RET_NULL)
>>   BTF_ID_FLAGS(func, bpf_iter_task_file_get_fd)
>>   BTF_ID_FLAGS(func, bpf_iter_task_file_destroy, KF_ITER_DESTROY)
>>
>> +BTF_ID_FLAGS(func, bpf_fget_task, KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
>> +BTF_ID_FLAGS(func, bpf_get_file_ops_type, KF_TRUSTED_ARGS)
>> +BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
>> +
>>   BTF_KFUNCS_END(bpf_crib_kfuncs)
>>
>>   static const struct btf_kfunc_id_set bpf_crib_kfunc_set = {
>> diff --git a/kernel/bpf/crib/files.c b/kernel/bpf/crib/files.c
>> index ececf150303f..8e0e29877359 100644
>> --- a/kernel/bpf/crib/files.c
>> +++ b/kernel/bpf/crib/files.c
>> @@ -5,6 +5,14 @@
>>   #include <linux/fdtable.h>
>>   #include <linux/net.h>
>>
>> +/**
>> + * This enum will grow with the file types that CRIB supports for
>> + * checkpoint/restore.
>> + */
>> +enum {
>> +       FILE_OPS_UNKNOWN = 0
>> +};
>> +
>>   struct bpf_iter_task_file {
>>          __u64 __opaque[3];
>>   } __aligned(8);
>> @@ -102,4 +110,40 @@ __bpf_kfunc void bpf_iter_task_file_destroy(struct bpf_iter_task_file *it)
>>                  fput(kit->file);
>>   }
>>
>> +/**
>> + * bpf_fget_task() - Get a pointer to the struct file corresponding to
>> + * the task file descriptor
>> + *
>> + * Note that this function acquires a reference to struct file.
>> + *
>> + * @task: the specified struct task_struct
>> + * @fd: the file descriptor
>> + *
>> + * @returns the corresponding struct file pointer if found,
>> + * otherwise returns NULL
>> + */
>> +__bpf_kfunc struct file *bpf_fget_task(struct task_struct *task, unsigned int fd)
>> +{
>> +       struct file *file;
>> +
>> +       file = fget_task(task, fd);
>> +       return file;
>> +}
>> +
>> +/**
>> + * bpf_get_file_ops_type() - Determine what exactly this file is based on
>> + * the file operations, such as socket, eventfd, timerfd, pipe, etc
>> + *
>> + * This function will grow with the file types that CRIB supports for
>> + * checkpoint/restore.
>> + *
>> + * @file: a pointer to the struct file
>> + *
>> + * @returns the file operations type
>> + */
>> +__bpf_kfunc unsigned int bpf_get_file_ops_type(struct file *file)
>> +{
>> +       return FILE_OPS_UNKNOWN;
>> +}
>> +
> 
> this is not very supportable, users can do the same by accessing
> file->f_op and comparing it to a set of known struct file_operations
> references.
> 

Yes, users can access file->f_op, but there seems to be no way for
users to get references to struct file_operations for the various file
types? For example, how does a user get a reference to socket_file_ops?

Also, currently the struct file_operations for most of the file types
are static, and I cannot even get a reference to them in
crib/files.c directly.

My future plan is to add functions like is_socket_file_ops to the
corresponding files (e.g. net/socket.c).

>>   __bpf_kfunc_end_defs();
>> --
>> 2.39.5
>>