linux-kernel - Re: [PATCH] bpf: use FS_USERNS

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Message-ID: <174025c0-e13c-49a1-8835-5d971c024bb5@iogearbox.net>
Date: Thu, 5 Feb 2026 13:38:21 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Alexander Mikhalitsyn <alexander@...alicyn.com>, bot+bpf-ci@...nel.org
Cc: ast@...nel.org, andrii@...nel.org, martin.lau@...ux.dev,
 eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev,
 john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, jlayton@...nel.org, brauner@...nel.org,
 bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, aleksandr.mikhalitsyn@...urfusion.io,
 martin.lau@...nel.org, clm@...a.com, ihor.solodrai@...ux.dev
Subject: Re: [PATCH] bpf: use FS_USERNS_DELEGATABLE for bpffs

Hi Alexander,

On 2/5/26 12:27 PM, Alexander Mikhalitsyn wrote:
> Am Do., 5. Feb. 2026 um 12:20 Uhr schrieb <bot+bpf-ci@...nel.org>:
>>
>>> diff --git a/kernel/bpf/inode.c b/kernel/bpf/inode.c
>>> index 005ea3a2c..c350857b2 100644
>>
>> [ ... ]
>>
>>> @@ -1077,7 +1073,7 @@ static struct file_system_type bpf_fs_type = {
>>>        .init_fs_context = bpf_init_fs_context,
>>>        .parameters     = bpf_fs_parameters,
>>>        .kill_sb        = bpf_kill_super,
>>> -     .fs_flags       = FS_USERNS_MOUNT,
>>> +     .fs_flags       = FS_USERNS_DELEGATABLE,
>>>   };
>>
>> FS_USERNS_DELEGATABLE does not appear to be defined anywhere in the kernel
>> tree. A search of include/linux/fs.h shows only FS_USERNS_MOUNT is defined
>> (at line 2268), not FS_USERNS_DELEGATABLE.
>>
>> The commit message states this flag was "recently introduced", but it is not
>> present in this codebase. Will this cause a build failure due to an undefined
>> identifier?
> 
> Yeah, this should be applied on top of
> https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/commit/?h=vfs-7.0.misc&id=269c46e936f3b5f2b6b567ca124d5f5ea07a371c
> in vfs/vfs-7.0.misc I think.
If this goes via Christian's vfs tree, it would make sense at least to open a
test PR against https://github.com/kernel-patches/bpf to run this through the
BPF CI with the vfs branch + your patch on top to make sure the tests don't
break.

Thanks,
Daniel