Re: [Qemu-devel] [PATCH] Fix off-by-1 error in RAM migration code

[Juan Quintela <quintela@redhat.com>]

David Gibson <dwg@au1.ibm.com> wrote:
> On Wed, Oct 31, 2012 at 01:08:16PM +0200, Orit Wasserman wrote:
>> On 10/31/2012 05:43 AM, David Gibson wrote:
>> > The code for migrating (or savevm-ing) memory pages starts off by creating
>> > a dirty bitmap and filling it with 1s.  Except, actually, because bit
>> > addresses are 0-based it fills every bit except bit 0 with 1s and puts an
>> > extra 1 beyond the end of the bitmap, potentially corrupting unrelated
>> > memory.  Oops.  This patch fixes it.
>> > 
>> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
>> > ---
>> >  arch_init.c |    2 +-
>> >  1 file changed, 1 insertion(+), 1 deletion(-)
>> > 
>> > diff --git a/arch_init.c b/arch_init.c
>> > index e6effe8..b75a4c5 100644
>> > --- a/arch_init.c
>> > +++ b/arch_init.c
>> > @@ -568,7 +568,7 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>> >      int64_t ram_pages = last_ram_offset() >> TARGET_PAGE_BITS;
>> >  
>> >      migration_bitmap = bitmap_new(ram_pages);
>> > -    bitmap_set(migration_bitmap, 1, ram_pages);
>> > +    bitmap_set(migration_bitmap, 0, ram_pages);
>> >      migration_dirty_pages = ram_pages;
>> >  
>> >      bytes_transferred = 0;
>> > 
>> You are correct, good catch.
>> Reviewed-by: Orit Wasserman <owasserm@redhat.com>
>
> Juan,
>
> Sorry, forgot to CC you on the original mailing here, which I should
> have done.  This is a serious bug in the migration code and we should
> apply to mainline ASAP.

Reviewed-by: Juan Quintela <quintela@redhat.com> 

Good catch, I missunderstood the function when fixing a different bug,
and never undrestood why it fixed it.

Thanks, Juan.