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List:       linux-parisc
Subject:    Re: parisc: fix mmap(MAP_FIXED|MAP_SHARED) to already mmapped address
From:       James Bottomley <James.Bottomley () HansenPartnership ! com>
Date:       2015-02-22 19:42:50
Message-ID: 1424634170.2146.119.camel () HansenPartnership ! com
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On Sun, 2015-02-22 at 20:16 +0100, Helge Deller wrote:
> 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.

I know, I do, but that's sort of expected: most modern linux apps use
mmap.  It's the older stuff that uses sys-v ipc, but perhaps for us this
doesn't matter.

James



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