linux-kernel - Re: [PATCH 6.4 00/28] 6.4.1-rc1 review

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Message-ID: <b886c0ee-2ef6-5ec9-17e0-eb66c63a61d4@roeck-us.net>
Date:   Mon, 3 Jul 2023 06:06:47 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Helge Deller <deller@....de>, stable@...r.kernel.org,
        linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        linux-parisc <linux-parisc@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        John David Anglin <dave.anglin@...l.net>
Subject: Re: [PATCH 6.4 00/28] 6.4.1-rc1 review - hppa argument list too long

On 7/3/23 05:59, Guenter Roeck wrote:
> On 7/2/23 23:20, Linus Torvalds wrote:
>> On Sun, 2 Jul 2023 at 22:33, Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>> Here you are:
>>>
>>> [   31.188688] stack expand failed: ffeff000-fff00000 (ffefeff2)
>>
>> Ahhah!
>>
>> I think the problem is actually ridiculously simple.
>>
>> The thing is, the parisc stack expands upwards. That's obvious. I've
>> mentioned it several times in just this thread as being the thing that
>> makes parisc special.
>>
>> But it's *so* obvious that I didn't even think about what it really implies.
>>
>> And part of all the changes was this part in expand_downwards():
>>
>>          if (!(vma->vm_flags & VM_GROWSDOWN))
>>                  return -EFAULT;
>>
>> and that will *always* fail on parisc, because - as said multiple
>> times - the parisc stack expands upwards. It doesn't have VM_GROWSDOWN
>> set.
>>
>> What a dum-dum I am.
>>
>> And I did it that way because the *normal* stack expansion obviously
>> wants it that way and putting the check there not only made sense, but
>> simplified other code.
>>
>> But fs/execve.c is special - and only special for parisc - in that it
>> really wants to  expand a normally upwards-growing stack downwards
>> unconditionally.
>>
>> Anyway, I think that new check in expand_downwards() is the right
>> thing to do, and the real fix here is to simply make vm_flags reflect
>> reality.
>>
>> Because during execve, that stack that will _eventually_ grow upwards,
>> does in fact grow downwards.  Let's make it reflect that.
>>
>> We already do magical extra setup for the stack flags during setup
>> (VM_STACK_INCOMPLETE_SETUP), so extending that logic to contain
>> VM_GROWSDOWN seems sane and the right thing to do.
>>
>> IOW, I think a patch like the attached will fix the problem for real.
>>
>> It needs a good commit log and maybe a code comment or two, but before
>> I bother to do that, let's verify that yes, it does actually fix
>> things.
>>
> 
> Yes, it does. I'll run a complete qemu test with it applied to be sure
> there is no impact on other architectures (yes, I know, that should not
> be the case, but better safe than sorry). I'll even apply
> https://lore.kernel.org/all/20230609075528.9390-12-bhe@redhat.com/raw
> to be able to test sh4.
> 

Meh, should have figured. That fixes one problem with sh4 builds
and creates another. Should have figured.

Guenter