[GIT PULL] configfs fix for Linux 5.14

[GIT PULL] configfs fix for Linux 5.14

[Christoph Hellwig]

The following changes since commit 36a21d51725af2ce0700c6ebcb6b9594aac658a6:

  Linux 5.14-rc5 (2021-08-08 13:49:31 -0700)

are available in the Git repository at:

  git://git.infradead.org/users/hch/configfs.git tags/configfs-5.14

for you to fetch changes up to 769f52676756b8c5feb302d2d95af59577fc69ec:

  configfs: restore the kernel v5.13 text attribute write behavior (2021-08-09 16:56:00 +0200)

----------------------------------------------------------------
configfs fix for Linux 5.14

 - fix to revert to the historic write behavior (Bart Van Assche)

----------------------------------------------------------------
Bart Van Assche (1):
      configfs: restore the kernel v5.13 text attribute write behavior

 fs/configfs/file.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

Re: [GIT PULL] configfs fix for Linux 5.14

[Linus Torvalds]

On Fri, Aug 13, 2021 at 9:00 PM Christoph Hellwig <hch@infradead.org> wrote:
>
> configfs fix for Linux 5.14
>
>  - fix to revert to the historic write behavior (Bart Van Assche)

It would have been lovely to see what the problem was, but the commit
doesn't actually explain that.

I suspect it's this

    https://lkml.org/lkml/2021/7/26/581

but there might have been more.

            Linus

Re: [GIT PULL] configfs fix for Linux 5.14

[Bart Van Assche]

On 8/14/21 9:27 AM, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 9:00 PM Christoph Hellwig <hch@infradead.org> wrote:
>>
>> configfs fix for Linux 5.14
>>
>>  - fix to revert to the historic write behavior (Bart Van Assche)
> 
> It would have been lovely to see what the problem was, but the commit
> doesn't actually explain that.
> 
> I suspect it's this
> 
>     https://lkml.org/lkml/2021/7/26/581
> 
> but there might have been more.

Hi Linus,

Bodo explained to me via a private email that the historic behavior is
the behavior he needs for a patch that he is still working on. I'm not
aware of any existing user space software that relies on the historic
(non-POSIX) behavior of configfs writes.

Bart.

Re: [GIT PULL] configfs fix for Linux 5.14

[Bodo Stroesser]

On 14.08.21 18:27, Linus Torvalds wrote:
> On Fri, Aug 13, 2021 at 9:00 PM Christoph Hellwig <hch@infradead.org> wrote:
>>
>> configfs fix for Linux 5.14
>>
>>   - fix to revert to the historic write behavior (Bart Van Assche)
> 
> It would have been lovely to see what the problem was, but the commit
> doesn't actually explain that.
> 
> I suspect it's this
> 
>      https://lkml.org/lkml/2021/7/26/581

Yes. I hope I was able to describe the changed behavior sufficiently in
that mail.

Thank you,
Bodo

> 
> but there might have been more.
> 
>              Linus
> 

Re: [GIT PULL] configfs fix for Linux 5.14

[pr-tracker-bot]

The pull request you sent on Sat, 14 Aug 2021 08:59:39 +0200:

> git://git.infradead.org/users/hch/configfs.git tags/configfs-5.14

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/118516e2127722e46c5c029010df4e8743bc9722

Thank you!

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[GIT PULL] configfs fix for Linux 5.14

[Christoph Hellwig]

The following changes since commit 7fef2edf7cc753b51f7ccc74993971b0a9c81eca:

  sd: don't mess with SD_MINORS for CONFIG_DEBUG_BLOCK_EXT_DEVT (2021-07-12 12:25:37 -0700)

are available in the Git repository at:

  git://git.infradead.org/users/hch/configfs.git tags/configfs-5.13-1

for you to fetch changes up to 420405ecde061fde76d67bd3a67577a563ea758e:

  configfs: fix the read and write iterators (2021-07-13 20:56:24 +0200)

----------------------------------------------------------------
configfs fix for Linux 5.14

 - fix the read and write iterators (Bart Van Assche)

----------------------------------------------------------------
Bart Van Assche (1):
      configfs: fix the read and write iterators

 fs/configfs/file.c | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

Re: [GIT PULL] configfs fix for Linux 5.14

[Linus Torvalds]

On Wed, Jul 14, 2021 at 9:33 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> configfs fix for Linux 5.14
>
>  - fix the read and write iterators (Bart Van Assche)

I've pulled this, but I'm somewhat disgusted by it.

The overflow "protection" is just wrong:

+       to_copy = SIMPLE_ATTR_SIZE - 1 - pos;
+       if (to_copy <= 0)
+               return 0;

because if users control "pos", then that "to_copy" could be a huge
positive value even after overflow protection.

I hope/think that we always end up checking 'pos' in the VFS layer so
that this isn't a bug in practice, but people - the above is just
fundamentally bad code.

It's simply not the correct way to check limits. It does it badly, and
it's hard to read (*).

If you want to check limits, then do it (a) the obvious way and (b) right.

Something like

        if (pos < 0 || pos >= SIMPLE_ATTR_SIZE - 1)
                return 0;
        to_copy = SIMPLE_ATTR_SIZE - 1 - pos;

would have been a hell of a lot more obvious, would have been CORRECT,
and a compiler would likely be able to equally good code for it.

Doing a "x <0 || x > C" test is actually nice and cheap, and compilers
should all be smart enough to turn it into a single (unsigned)
comparison.

Possibly it even generates better code, since "to_copy" could then -
and should - no longer be a 64-bit loff_t, since it's pointless. We've
just checked the range of the values, so it can be the natural size
for the machine.

Although from a small test, gcc does seem to be too simple to take
advantage of that, and on 32-bit x86 it does the range check using
64-bit arithmetic even when unnecessary (it should just check "are the
upper 32 bits zero" rather than play around with doing a 64-bit
sub/sbb - I'm surprised, because I thought gcc already knew about
this, but maybe compiler people are starting to forget about 32-bit
stuff too).

But even if the compiler doesn't figure it out, the simple "just check
the limits" is a lot more readable for humans, and avoids the whole
overflow issue. And maybe some compilers will do better at it.

            Linus

(*) Ok, it's easy to read if you ignore the overflow possibility. IOW,
it's easy to read WRONG.

Re: [GIT PULL] configfs fix for Linux 5.14

[Linus Torvalds]

On Wed, Jul 14, 2021 at 1:05 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I hope/think that we always end up checking 'pos' in the VFS layer so
> that this isn't a bug in practice

Yeah, we seem to make sure everything is fine in rw_verify_area().

We do allow negative 'pos' things, but only for files marked with
FMODE_UNSIGNED_OFFSET, which is basically just for variations of
/dev/mem and /proc/<pid>/mem that need the whole 64-bit range.

So it _shouldn't_ be an issue here, but the points about just doing
the legible and safe version stands.

               Linus

Re: [GIT PULL] configfs fix for Linux 5.14

[pr-tracker-bot]

The pull request you sent on Wed, 14 Jul 2021 18:33:07 +0200:

> git://git.infradead.org/users/hch/configfs.git tags/configfs-5.13-1

has been merged into torvalds/linux.git:
https://git.kernel.org/torvalds/c/1013d4add290c460b816fc4b1db5174f88b71760

Thank you!

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