linux-kernel - Re: [PATCH v3 08/10] selftests/mm: Skip gup

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <95a15856-5c47-4040-9291-5b972786efeb@redhat.com>
Date: Fri, 14 Mar 2025 13:17:05 +0100
From: David Hildenbrand <david@...hat.com>
To: Brendan Jackman <jackmanb@...gle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>, Shuah Khan <shuah@...nel.org>,
 Dev Jain <dev.jain@....com>, linux-mm@...ck.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 08/10] selftests/mm: Skip gup_longerm tests on weird
 filesystems

On 14.03.25 13:10, David Hildenbrand wrote:
> On 12.03.25 09:34, Brendan Jackman wrote:
>> On Tue, Mar 11, 2025 at 08:53:02PM +0100, David Hildenbrand wrote:
>>>> 2. 9pfs seems to pass the f_type through from the host. So you can't
>>>> detect it this way anyway.
>>>>
>>>> [3. I guess overlayfs & friends would also be an issue here although
>>>> that doesn't affect my usecase.]
>>>>
>>>> Anyway, I think we would have to scrape /proc/mounts to do this :(
>>>>
>>>
>>> The question I am asking myself: is this a 9pfs design bug or is it a 9pfs
>>> hypervisor bug. Because we shouldn't try too hard to work around hypervisor
>>> bugs.
>>>
>>> Which 9pfs implementation are you using in the hypervisor?
>>
>> I'm using QEMU via virtme-ng. IIUC virtme-ng knows how to use viortfs
>> for the rootfs, but for individually-mounted directories with
>> --rwdir/--rodir it uses 9pfs unconditionally.
> 
> Ah okay, that makes sense.
> 
>>
>> Even if it's a bug in QEMU, I think it is worth working around this
>> one way or another. QEMU by far the most practical way to run these
>> tests, and virtme-ng is probably the most popular/practical way to do
>> that.
> 
> I'm afraid yes. Although allocating temp files form 9pfs is rather ...
> weird. :) One would assume that /tmp is usually backed by tmpfs. But
> well, a disto can do what it wants.
> 
>> I think even if we are confident it's just a bunch of broken
>> code that isn't even in Linux, it's pragmatic to spend a certain
>> amount of energy on having green tests there.
>>
> 
> Yeah, we're trying ...
> 
>> (Also, this f_type thing might be totally intentional specified
>> filesystem behaviour, I don't know).
> 
> I assume it's broken in various ways to mimic that you are a file system
> which you are not.
> 
> Your approach is likely the easiest approach to deal with this 9pfs crap.
> 
> Can you document in the code+description better what we learned, and why
> we cannot even trust f_type with crappy 9pfs?

Staring a bit at that code, it's mostly 9p specific I think.

t14s: ~/git/linux s390x-file-thp2 $ git grep "= NFS_SUPER_MAGIC"
fs/nfs/super.c: buf->f_type = NFS_SUPER_MAGIC;
fs/nfs/super.c: sb->s_magic = NFS_SUPER_MAGIC;

t14s: ~/git/linux s390x-file-thp2 $ git grep "= V9FS_MAGIC"
fs/9p/vfs_super.c:      sb->s_magic = V9FS_MAGIC;
  $ git grep "f_type" | grep 9p
fs/9p/vfs_super.c:                      buf->f_type = rs.type;


-- 
Cheers,

David / dhildenb