Messages in this thread | | | Date | Fri, 06 Apr 2012 01:04:43 +0400 | From | Konstantin Khlebnikov <> | Subject | Re: [PATCH 6/7] mm: kill vma flag VM_EXECUTABLE |
| |
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 =)
| |