Re: parisc: fix mmap(MAP_FIXED|MAP_SHARED) to already mmapped address

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Sun, 2015-02-22 at 19:07 +0100, Helge Deller wrote:
> >>>> SHMLBA is 4096 /* (1 << PGSHIFT) */ on hpux.
> >>>>
> >>>> The following is in <asm-generic/shmparam.h>:
> >>>> #define SHMLBA PAGE_SIZE	 /* attach addr a multiple of this */
> >>>>
> >>>> Shared mappings are handled with
> >>>> asm/shmparam.h:#define SHM_COLOUR 0x00400000	/* shared mappings colouring */
> >>>
> >>> So how is the sys-v ipc problem fixed?  There the user is told to select
> >>> an address which is a multiple of SHMLBA.  Programs that do this today
> >>> will start to break on writeable mappings if we set SHMLBA to PAGE_SIZE
> >>> because the colour will be wrong.
> >>
> >>
> >> The code returns -EINVAL.  See arch_get_unmapped_area.
> >
> > But that's not a solution.  Let me try to illustrate: I have an existing
> > application, it uses sys-v ipc and selects a shmat address based on the
> > multiple of SHMLBA for a writeable mapping.  Today it works.
> 
> It will work as well with SHMBLA=4096, if you just use SHM_RND too
> (and most applications do have SHM_RND).
> man shmat says:
>   * If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach occurs at the address equal to shmaddr rounded down to the nearest multiple of SHMLBA.
> * Otherwise, shmaddr must be a page-aligned address at which the attach occurs.
> 
> So, even here shmaddr is mentioned to be page-aligned (4k), not
> SHMLBA-aligned (4M in your case).

I think that part is x86.  All the other VI architectures impose their
VI colour constrainst in SHMLBA.  We're the odd one out because we have
a huge stride (everyone else is small multiples of pages).

But agree if no applications are affected, we can make the ABI change.

> >Tomorrow
> > when you make this change, it fails with -EINVAL.  That's breaking an
> > existing application because chances are the app will just report the
> > failure and exit.
> 
> Tomorrow?  This change is *already* implemented in eglibc since a year or
> so and I don't see any applications which break because of SHMBLA=4096...

Is eglibc a big enough sample to make the claim that no applications
will be broken?

James