Re: [regression v2.6.38] Re: [PATCH v2] brk: fix min_brk lower boundcomputation for COMPAT_BRK

[Geert Uytterhoeven]

Hi Jiri,

On Fri, Mar 25, 2011 at 11:20, Jiri Kosina <jkosina@xxxxxxx> wrote:
> On Thu, 24 Mar 2011, Geert Uytterhoeven wrote:
>> > From: Jiri Kosina <jkosina@xxxxxxx>
>> > 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@xxxxxxx>
>> > ---
>> > Âmm/mmap.c | Â 10 +++++++++-
>> > Â1 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)
>> > Â Â Â Âdown_write(&mm->mmap_sem);
>> >
>> > Â#ifdef CONFIG_COMPAT_BRK
>> > - Â Â Â min_brk = mm->end_code;
>> > + Â Â Â /*
>> > + Â Â Â Â* CONFIG_COMPAT_BRK can still be overridden by setting
>> > + Â Â Â Â* randomize_va_space to 2, which will still make mm->start_brk
>> > + Â Â Â Â* to be arbitrarily shifted
>> > + Â Â Â Â*/
>> > + Â Â Â if (mm->start_brk > PAGE_ALIGN(mm->end_data))
>> > + Â Â Â Â Â Â Â min_brk = mm->start_brk;
>> > + Â Â Â else
>> > + Â Â Â Â Â Â Â min_brk = mm->end_data;
>> > Â#else
>> > Â Â Â Âmin_brk = mm->start_brk;
>> > Â#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.

Yeah, binaries from 1996 ;-)

> 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)
> Â Â Â Âif (mm->start_brk > PAGE_ALIGN(mm->end_data))
> Â Â Â Â Â Â Â Âmin_brk = mm->start_brk;
> Â Â Â Âelse
> - Â Â Â Â Â Â Â min_brk = mm->end_data;
> + Â Â Â Â Â Â Â min_brk = mm->end_code;
> Â#else
> Â Â Â Âmin_brk = mm->start_brk;
> Â#endif

Unfortunately not...

I added some printk()s:

mm->start_brk = 0x8000a000, PAGE_ALIGN(mm->end_data = 0x8000a000)
mm->start_brk = 0x8000a000, PAGE_ALIGN(mm->end_data = 0x8000a000)
mm->start_brk = 0x8000a000, PAGE_ALIGN(mm->end_data = 0x8000a000)
mm->start_brk = 0x8000a000, PAGE_ALIGN(mm->end_data = 0x8000a000)
mm->start_brk = 0x80006000, PAGE_ALIGN(mm->end_data = 0x80004000)

I.e. just before the failure, "mm->start_brk > PAGE_ALIGN(mm->end_data)"
became true.

Gr{oetje,eeting}s,

            Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
             Â Â -- Linus Torvalds
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