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

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Message-ID: <ba37a96a-ae09-227b-92d3-f510b9c64b15@nod.at>
Date:   Thu, 5 Jan 2017 12:03:47 +0100
From:   Richard Weinberger <richard@....at>
To:     Michal Hocko <mhocko@...nel.org>
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

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.
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.

Thanks,
//richard