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    SubjectRe: 2.6.23 regression: accessing invalid mmap'ed memory from gdb causes unkillable spinning


    On Thu, 1 Nov 2007, Nick Piggin wrote:

    > On Wed, Oct 31, 2007 at 04:08:21PM -0700, Linus Torvalds wrote:
    > >
    > > We made much bigger changes to ptrace support when we disallowed writing
    > > to read-only shared memory areas (we used to do the magic per-page COW
    > > thing).
    >
    > Really? No, we still do that magic COW thing which creates anonymous
    > pages in MAP_SHARED vmas, don't we?

    No, we don't. I'm pretty sure. It didn't work with the VM cleanups, since
    the MAP_SHARED vma's won't be on the anonymous list any more, and cannot
    be swapped out.

    So now, if you try to write to a read-only shared page through ptrace,
    you'll get "Unable to access".

    Of course, I didn't really look closely, so maybe I just don't remember
    things right..

    > > access_vm_pages() (things like core-dumping comes to mind - although I
    > > think we don't dump pure file mappings at all, do we?) it would certainly
    > > be good to run any such tests on the current -git tree...
    >
    > We do for MAP_SHARED|MAP_ANONYMOUS, by the looks.

    Well, as we should. There's no way for a debugger to get those pages back.
    So that all looks sane.

    > - vm_flags |= VM_SHARED | VM_MAYSHARE;
    > - if (!(file->f_mode & FMODE_WRITE))
    > - vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
    > + vm_flags |= VM_MAYSHARE;
    > + if (file->f_mode & FMODE_WRITE)
    > + vm_flags |= VM_SHARED;
    > + if (!(vm_flags & VM_WRITE))
    > + vm_flags &= ~VM_MAYWRITE;

    This looks totally bogus. What was the intent of this patch?

    The VM_MAYWRITE flag is *not* supposed to track the VM_WRITE flag: that
    would defeat the whole purpose of it! The whole point of that flag is to
    say whether mprotect() could turn it into a VM_WRITE mapping, and it
    depends on the file mode, not VM_WRITE!

    > vm_flags |= VM_SHARED | VM_MAYSHARE;
    > + if (!(vm_flags & VM_WRITE))
    > + vm_flags &= ~VM_MAYWRITE;

    More apparent bogosities.

    Linus
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