linux-kernel - Re: [PATCH] proc: Fix integer overflow of VmLib

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Date:   Thu, 5 Jan 2017 12:49:50 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Richard Weinberger <richard@....at>
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        vbabka@...e.cz, kirill.shutemov@...ux.intel.com,
        jmarchan@...hat.com, gerald.schaefer@...ibm.com,
        hannes@...xchg.org, luto@...nel.org
Subject: Re: [PATCH] proc: Fix integer overflow of VmLib

On Thu 05-01-17 12:03:47, Richard Weinberger wrote:
> Michal,
> 
> Am 05.01.2017 um 11:53 schrieb Michal Hocko:
> > I guess you meant s@...rflow@...erflow@ right?
> 
> Yep, of course.
> 
> > On Thu 05-01-17 00:29:18, Richard Weinberger wrote:
> >> /proc/<pid>/status can report extremely high VmLib values which
> >> will confuse monitoring tools.
> >> VmLib is mm->exec_vm minus text size, where exec_vm is the number of
> >> bytes backed by an executable memory mapping and text size is
> >> mm->end_code - mm->start_code as set up by binfmt.
> >>
> >> For the vast majority of all programs text size is smaller than exec_vm.
> >> But if a program interprets binaries on its own the calculation result
> >> can be negative.
> >> UserModeLinux is such an example. It installs and removes lots of PROT_EXEC
> >> mappings but mm->start_code and mm->start_code remain and VmLib turns
> >> negative.
> >>
> >> Fix this by detecting the overflow and just return 0.
> >> For interpreting the value reported by VmLib is anyway useless but
> >> returning 0 does at least not confuse userspace.
> > 
> > Is really 0 what the userspace expects? Why shouldn't we just report
> > exec_vm unconditionally? Btw. we used to do something that many years
> > back https://lkml.org/lkml/2004/8/24/47. We are exporting the text size
> > so the calculation can be done by the userspace.
> 
> Strictly speaking both values, 0 and exec_vm are wrong.
> Userspace expects VmLib to be 0 when an application has no libs loaded,
> i.e. for statically linked binaries.
> 
> So, either we report 0 as "I don't know" or exec_vm, which is also wrong.

Yes unfortunately.

> I thought 0 is the better choice since it will not lead to wrong results
> when userspace tools compute the sum of values reported by /proc/<pid>/status.

Dunno. If somebody translates 0 to statically linked library then it
could be wrong.

That being said, the underflow is _clearly_ wrong. I am not sure what
the right way is to fix this but whatever we do it might just break
somebody's usecase. Sad...
-- 
Michal Hocko
SUSE Labs