linux-kernel - Re: WARNING: CPU: 0 PID: 1 at mm/mmap.c:1110 vma

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Date:   Fri, 17 Mar 2017 23:51:43 +0100
From:   Richard Weinberger <richard@....at>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     adityam@...rosoft.com, Rik van Riel <riel@...hat.com>,
        hughd@...gle.com, mgorman@...hsingularity.net,
        janvorli@...rosoft.com, akpm@...ux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Anton Ivanov <anton.ivanov@...-begemot.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        user-mode-linux-devel <user-mode-linux-devel@...ts.sourceforge.net>
Subject: Re: WARNING: CPU: 0 PID: 1 at mm/mmap.c:1110 vma_merge+0x9a/0x2c8

[re-sending with mailinglists]

Andrea,

Am 17.03.2017 um 19:36 schrieb Andrea Arcangeli:
> I think the problem is in setup_arg_pages that is making an assumption
> non true in UML (and only UML) case, i.e. that there are no vmas below
> bprm->vma by the time mprotect_fixup is executed. That is not the case
> for UML as you likely installed vmas below bprm->vma before calling
> mprotect_fixup in setup_arg_pages. The fix should be just to change
> setup_arg_pages to find the correct "prev" vma, then this code will
> pick "area" as prev->vm_next which is bprm->vma as expected:
> 
> 	if (prev)
> 		next = prev->vm_next;
> 	else
> 		next = mm->mmap;
> 	area = next;
> 
> *prev == NULL tells vma_merge that the only vma that there is is
> mm->mmap so it uses that as "area" but for you mm->mmap is prev or
> prev->prev or something like that, not the "area". To make it find the
> right area you need to pass the right *prev to mprotect_fixup if there
> are real "prev" vmas not just bprm->vma instantiated.

I think you are right, thanks for the great explanation!

UML installs this vma very early and as of now I'm not sure why we can't
install it a bit later just like we do for the vDSO.
So far it did not explode. :)
This solution would be better than finding a pre-existing vma before
calling mprotect_fixup().

Thanks,
//richard