linux-kernel - Re: [PATCH 1/2] bpf: Add BPF_MAP_TYPE_CRED

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Message-ID: <CAADnVQLuUGaWaThSb94nv8Bb_qgA0cyr9=YmZgxuEtLaQLWzKw@mail.gmail.com>
Date: Sun, 14 Sep 2025 18:09:51 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: David Windsor <dwindsor@...il.com>
Cc: Song Liu <song@...nel.org>, bpf <bpf@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Eduard <eddyz87@...il.com>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH 1/2] bpf: Add BPF_MAP_TYPE_CRED_STORAGE map type and kfuncs

On Sat, Sep 13, 2025 at 3:27 PM David Windsor <dwindsor@...il.com> wrote:
>
>
>
> On Sat, Sep 13, 2025 at 5:58 PM Song Liu <song@...nel.org> wrote:
>>
>> On Fri, Sep 12, 2025 at 5:27 PM David Windsor <dwindsor@...il.com> wrote:
>> [...]
>> > >
>> > > Maybe I missed something, but I think you haven't addressed Alexei's
>> > > question in v1: why this is needed and why hash map is not sufficient.
>> > >
>> > > Other local storage types (task, inode, sk storage) may get a large
>> > > number of entries in a system, and thus would benefit from object
>> > > local storage. I don't think we expect too many creds in a system.
>> > > hash map of a smallish size should be good in most cases, and be
>> > > faster than cred local storage.
>> > >
>> > > Did I get this right?
>> > >
>> > > Thanks,
>> > > Song
>> > >
>> >
>> > Yes I think I addressed in the cover letter of -v2:
>> >
>> > "Like other local storage types (task, inode, sk), this provides automatic
>> > lifecycle management and is useful for LSM programs tracking credential
>> > state across LSM calls. Lifetime management is necessary for detecting
>> > credential leaks and enforcing time-based security policies."
>> >
>> > You're right it's faster and there aren't many creds, but I feel like
>> > in this case, it'll be a nightmare to manual cleanup with hashmaps. I
>> > think the correctness we get with lifetime management is worth it in
>> > this case, but could be convinced otherwise. Many cred usage patterns
>> > are short lived and a hash map could quickly become stale...
>>
>> We can clean up the hashmap in hook cred_free, no? The following
>> check in security_cred_free() seems problematic:
>>
>>         if (unlikely(cred->security == NULL))
>>                 return;
>>
>> But as far as I can tell, it is not really useful, and can be removed.
>> With this removed, hash map will work just as well. Did I miss
>> something?
>
>
> No I think actually this is easier.
>
> I will prepare a patch for the race in cleanup I stumbled on earlier which is still there and could affect other users.
>
> That said, is there any use case for local storage for these structs:
>
> - struct file
> - struct msg_msg
> - struct ipc
>
> I can off the top of my head think of some security use cases for these but not sure if hashmaps are needed, perhaps struct file

Sorry, no. This is not a copy paste territory.
The existing local storage maps were added because
performance was critical for those use cases,
but we made a few mistakes. There is a performance
cliff that has to be fixed before we adopt it to
other kernel objects.
Please use hash map and consider wrapping rhashtable
as a new bpf map type if fixed max_entries is problematic.

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