Re: [PATCH 6/7] mm: kill vma flag VM_EXECUTABLE

[Konstantin Khlebnikov]

Matt Helsley wrote:
> On Tue, Apr 03, 2012 at 11:32:04PM +0400, Cyrill Gorcunov wrote:
> > On Tue, Apr 03, 2012 at 11:16:31AM -0700, Matt Helsley wrote:
> > > On Tue, Apr 03, 2012 at 09:10:20AM +0400, Konstantin Khlebnikov wrote:
> > > > Matt Helsley wrote:
> > > > > On Sat, Mar 31, 2012 at 10:13:24PM +0200, Oleg Nesterov wrote:
> > > > > > On 03/31, Konstantin Khlebnikov wrote:
> > > > > > > comment from v2.6.25-6245-g925d1c4 ("procfs task exe symlink"),
> > > > > > > where all this stuff was introduced:
> > > > > > >
> > > > > > > > ...
> > > > > > > > This avoids pinning the mounted filesystem.
> > > > > > >
> > > > > > > So, this logic is hooked into every file mmap/unmmap and vma split/merge just to
> > > > > > > fix some hypothetical pinning fs from umounting by mm which already unmapped all
> > > > > > > its executable files, but still alive. Does anyone know any real world example?
> > > > > >
> > > > > > This is the question to Matt.
> > > > >
> > > > > This is where I got the scenario:
> > > > >
> > > > > https://lkml.org/lkml/2007/7/12/398
> > > >
> > > > Cyrill Gogcunov's patch "c/r: prctl: add ability to set new mm_struct::exe_file"
> > > > gives userspace ability to unpin vfsmount explicitly.
> > >
> > > Doesn't that break the semantics of the kernel ABI?
> >
> > Which one? exe_file can be changed iif there is no MAP_EXECUTABLE left.
> > Still, once assigned (via this prctl) the mm_struct::exe_file can't be changed
> > again, until program exit.
>
> The prctl() interface itself is fine as it stands now.
>
> As far as I can tell Konstantin is proposing that we remove the unusual
> counter that tracks the number of mappings of the exe_file and require
> userspace use the prctl() to drop the last reference. That's what I think
> will break the ABI because after that change you *must* change userspace
> code to use the prctl(). It's an ABI change because the same sequence of
> system calls with the same input bits produces different behavior.

But common software does not require this at all. I did not found real examples,
only hypothesis by Al Viro: https://lkml.org/lkml/2007/7/12/398
libhugetlbfs isn't good example too, the man proc says: /proc/[pid]/exe is alive until
main thread is alive, but in case libhugetlbfs /proc/[pid]/exe disappears too early.
Also I would not call it ABI, this corner-case isn't documented, I'm afraid only few
people in the world knows about it =)
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