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

[Helge Deller <deller@gmx.de>]

On 22.02.2015 20:13, James Bottomley wrote:
> 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?

Try yourself:
https://parisc.wiki.kernel.org/index.php/Debian_Ports_Installation
Just install debian 8.0 (aka unstable for hppa). KDE, Xfce, libreoffice,
and a all others packages just work out of the box.

Helge