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    SubjectRe: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3
    On Thu, Mar 08, 2012 at 11:03:03PM +0400, Cyrill Gorcunov wrote:
    > On Thu, Mar 08, 2012 at 07:26:23PM +0100, Oleg Nesterov wrote:
    > > On 03/08, Cyrill Gorcunov wrote:
    > > >
    > > > Hi Oleg, could you please take a look once you get a minute (no urgency).
    > >
    > > Add Matt. I won't touch the text below to keep the patch intact.
    >
    > Thanks for CC'ing Matt, Oleg (I forgot, sorry).
    >
    > >
    > > With this change
    > >
    > > down_write(&mm->mmap_sem);
    > > if (mm->num_exe_file_vmas) {
    > > fput(mm->exe_file);
    > > mm->exe_file = exe_file;
    > > exe_file = NULL;
    > > } else
    > > set_mm_exe_file(mm, exe_file);
    > > up_write(&mm->mmap_sem);
    > >
    > > I simply do not understand what mm->num_exe_file_vmas means after
    > > PR_SET_MM_EXE_FILE.

    I think it should fail if the num_exe_file_vmas is not 0 when
    PR_SET_MM_EXE_FILE is used. It's simple, keeps things clear, might
    catch userspace bugs (harder to accidentally leave a mapping of the original
    executable), and could avoid kernel bugs too.

    > >
    > > I think that you should do
    > >
    > > down_write(&mm->mmap_sem);
    > > if (mm->num_exe_file_vmas) {
    > > fput(mm->exe_file);
    > > mm->exe_file = exe_file;
    > > exe_file = NULL;
    > > }
    > > up_write(&mm->mmap_sem);
    > >
    > > to keep the current "mm->exe_file goes away after the final
    > > unmap(MAP_EXECUTABLE)" logic.
    > >
    > > OK, may be this doesn't work in c/r case because you are actually
    > > going to remove the old mappings? But in this case the new exe_file
    > > will go away anyway, afaics PR_SET_MM_EXE_FILE is called when you
    > > still have the old mappings.
    >
    > Yes, exactly, I need to remove old mappings first (because VMAs
    > we're about to restore may intersect with current map the host
    > program has). And yes, once they all are removed I don't have
    > /proc/pid/exe anymore. That's why I need num_exe_file_vmas == 0
    > case.
    >
    > When I setup new exe_file with num_exe_file_vmas = 0, this reference
    > to a file brings /proc/pid/exe back to live (and when process exiting
    > it'll call set_mm_exe_file(mm, NULL) and the new exe_file will be dropped,
    > so no leak here).

    Makes sense, I think.

    > > And I don't think the unconditional
    > >
    > > down_write(&mm->mmap_sem);
    > > set_mm_exe_file(mm, exe_file);
    > > up_write(&mm->mmap_sem);
    > >
    > > is 100% right, this clears ->num_exe_file_vmas. This means that
    > > (if you still have the old mapping) the new exe_file can go away
    > > after added_exe_file_vma() + removed_exe_file_vma(). Normally this
    > > should happen, but afaics this is possible. Note that even, say,
    > > mprotect() can trigger added_exe_file_vma().
    > >
    >
    > Wait, Oleg, I'm confused, in case if there *is* exitsting VM_EXECUTABLEs
    > then we jump into first banch and simply replace old exe_file.

    What happens if multiple prctl calls are made? We'll have a mix of N
    executable files that've been mapped n_i times. I think we're better off
    just returning an error in that case -- -EBUSY or something.

    > If there is no VM_EXECUTABLEs, then we simply setup new exe_file
    > and num_exe_file_vmas remains zero.

    Which is fine.

    Cheers,
    -Matt



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