linux-kernel - Re: [regression v2.6.38] Re: [PATCH v2] brk: fix min_brk lower bound computation for COMPAT

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Date:	Fri, 25 Mar 2011 11:20:23 +0100 (CET)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Linux/m68k <linux-m68k@...r.kernel.org>
Subject: Re: [regression v2.6.38] Re: [PATCH v2] brk: fix min_brk lower bound
 computation for COMPAT_BRK

On Thu, 24 Mar 2011, Geert Uytterhoeven wrote:

> > From: Jiri Kosina <jkosina@...e.cz>
> > Subject: [PATCH] brk: fix min_brk lower bound computation for COMPAT_BRK
> >
> > Even if CONFIG_COMPAT_BRK is set in the kernel configuration, it can still
> > be overriden by randomize_va_space sysctl.
> >
> > If this is the case, the min_brk computation in sys_brk() implementation
> > is wrong, as it solely takes into account COMPAT_BRK setting, assuming
> > that brk start is not randomized. But that might not be the case if
> > randomize_va_space sysctl has been set to '2' at the time the binary has
> > been loaded from disk.
> >
> > In such case, the check has to be done in a same way as in
> > !CONFIG_COMPAT_BRK case.
> >
> > In addition to that, the check for the COMPAT_BRK case introduced back in
> > a5b4592c ("brk: make sys_brk() honor COMPAT_BRK when computing lower
> > bound") is slightly wrong -- the lower bound shouldn't be mm->end_code,
> > but mm->end_data instead, as that's where the legacy applications expect
> > brk section to start (i.e. immediately after last global variable).
> >
> > Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> > ---
> >  mm/mmap.c |   10 +++++++++-
> >  1 files changed, 9 insertions(+), 1 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 50a4aa0..ca2f164 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -253,7 +253,15 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
> >        down_write(&mm->mmap_sem);
> >
> >  #ifdef CONFIG_COMPAT_BRK
> > -       min_brk = mm->end_code;
> > +       /*
> > +        * CONFIG_COMPAT_BRK can still be overridden by setting
> > +        * randomize_va_space to 2, which will still make mm->start_brk
> > +        * to be arbitrarily shifted
> > +        */
> > +       if (mm->start_brk > PAGE_ALIGN(mm->end_data))
> > +               min_brk = mm->start_brk;
> > +       else
> > +               min_brk = mm->end_data;
> >  #else
> >        min_brk = mm->start_brk;
> >  #endif
> > --
> > 1.7.3.1
> 
> Sorry for chiming in this late, but I've just bisected a problem in
> 2.6.38 to commit
> 5520e89485252c759ee60d313e9422447659947b ("brk: fix min_brk lower bound
> computation for COMPAT_BRK").
> 
> When booting my very old test ramdisk on Amiga/m68k, it fails like this:
> 
> | RAMDISK: gzip image found at block 0
> | VFS: Mounted root (ext2 filesystem) readonly on device 1:0.
> | warning: process `update' used the obsolete bdflush system call
> | Fix your initscripts?
> | init: cannot open inittab
> | Kernel panic - not syncing: Attempted to kill init!
> 
> Sorry for not noticing earlier, I usually boot full Debians under ARAnyM,
> instead of booting old ramdisks with libc5-based binaries that once were
> considered new.

Oh well, one has to love the libc5-based binaries indeed.

Is the patch below fixing the issue you are seeing on your Amiga/m68k? 
Thanks.


diff --git a/mm/mmap.c b/mm/mmap.c
index 2ec8eb5..0a02531 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -262,7 +262,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
 	if (mm->start_brk > PAGE_ALIGN(mm->end_data))
 		min_brk = mm->start_brk;
 	else
-		min_brk = mm->end_data;
+		min_brk = mm->end_code;
 #else
 	min_brk = mm->start_brk;
 #endif




-- 
Jiri Kosina
SUSE Labs, Novell Inc.
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