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

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Date:   Mon, 3 Jul 2023 05:59:18 -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 7/2/23 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.

Guenter