linux-kernel - Re: [Uclinux-dist-devel] [PATCH] NOMMU: Add '[stack]' label to /proc/pid/maps output

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Date:	Fri, 4 Jun 2010 19:00:19 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>, uclinux-dev@...inux.org,
	linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	torvalds@...ux-foundation.org
Subject: Re: [Uclinux-dist-devel] [PATCH] NOMMU: Add '[stack]' label to 
	/proc/pid/maps output

On Fri, Jun 4, 2010 at 16:48, Andrew Morton wrote:
> On Tue, 01 Jun 2010 19:59:37 +0100 David Howells wrote:
>> From: Mike Frysinger <vapier@...too.org>
>>
>> Add support to the NOMMU /proc/pid/maps file to show which mapping is the stack
>> of the original thread after execve.  This is largely based on the MMU code.
>> Subsidiary thread stacks are not indicated.
>>
>> For FDPIC, we now get:
>>
>>       root:/> cat /proc/self/maps
>>       02064000-02067ccc rw-p 0004d000 00:01 22         /bin/busybox
>>       0206e000-0206f35c rw-p 00006000 00:01 295        /lib/ld-uClibc.so.0
>>       025f0000-025f6f0c r-xs 00000000 00:01 295        /lib/ld-uClibc.so.0
>>       02680000-026ba6b0 r-xs 00000000 00:01 297        /lib/libc.so.0
>>       02700000-0274d384 r-xs 00000000 00:01 22         /bin/busybox
>>       02816000-02817000 rw-p 00000000 00:00 0
>>       02848000-0284c0d8 rw-p 00000000 00:00 0
>>       02860000-02880000 rw-p 00000000 00:00 0          [stack]
>>
>> The semi-downside here is that for FLAT, we get:
>>
>>       root:/> cat /proc/155/maps
>>       029f0000-029f9000 rwxp 00000000 00:00 0          [stack]
>>
>> The reason being that FLAT combines a whole lot of stuff into one map
>> (including the stack).  But this isn't any worse than the current output (which
>> is nothing), so screw it.
>
> So it's a non-back-compatible change which can a) break nommu-only
> userspace and b) unbreak mmu-tested userspace which gets run on nommu.

i dont see how it breaks anything really.  any code that parses the
maps file has to account for an optional label in the maps output.

i guess if someone wrote some code that does something special with
"[stack]", then behavior would change with FLAT files, but that's
pretty unlikely i would think.  plus, FLAT is not ELF and as such, its
layout in memory is highly unlike any existing ELF layout.  so someone
would already have to be writing a custom handler for FLAT files which
means they account for the "one blob is everything" layout.
-mike
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