linux-kernel - Re: [PATCH bpf-next v8 5/7] bpf: Implement bpf_local

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Date:   Tue, 18 Aug 2020 17:33:29 +0200
From:   KP Singh <kpsingh@...omium.org>
To:     Martin KaFai Lau <kafai@...com>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>
Subject: Re: [PATCH bpf-next v8 5/7] bpf: Implement bpf_local_storage for
 inodes



On 8/18/20 5:23 PM, Martin KaFai Lau wrote:
> On Tue, Aug 18, 2020 at 05:10:34PM +0200, KP Singh wrote:
>>
>>
>> On 8/18/20 3:27 AM, Martin KaFai Lau wrote:>>> On Mon, Aug 03, 2020 at 06:46:53PM +0200, KP Singh wrote:

[...]

a get_file
>> rather fcheck followed by get_file_rcu:
>>
>> #define get_file_rcu_many(x, cnt)	\
>> 	atomic_long_add_unless(&(x)->f_count, (cnt), 0)
>> #define get_file_rcu(x) get_file_rcu_many((x), 1)
>> #define file_count(x)	atomic_long_read(&(x)->f_count)
>>
>> But there is an easier way than all of this and this is to use 
>> fget_raw which also calls get_file_rcu_many 
>> and ensures a non-zero count before getting a reference.
> ic. Make sense.
> 
> There are fdget() and fdput() also which are used in bpf/syscall.c.

Yeah we could use fdget_raw but we don't really need the struct fd but just the 
struct file.

he non-raw versions masks away FMODE_PATH (O_PATH) files, we should still be able to
access blobs on the O_PATH files, thus the _raw version here.

- KP

>