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    SubjectRe: [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP
    On Wed, Mar 07, 2012 at 02:30:28PM -0800, Andrew Morton wrote:
    > On Wed, 7 Mar 2012 12:00:46 -0500
    > Jason Baron <jbaron@redhat.com> wrote:
    >
    > > Hi,
    > >
    > > The motivation for this change was that I was looking at a way for a qemu-kvm
    > > process, to exclude the guest memory from its core dump, which can be quite
    > > large. There are already a number of filter flags in
    > > /proc/<pid>/coredump_filter, however, these allow one to specify 'types' of
    > > kernel memory, not specific address ranges (which is needed in this case).
    > >
    > > Since there are no more vma flags available, the first patch eliminates the
    > > need for the 'VM_ALWAYSDUMP' flag. The flag is used internally by the kernel to
    > > mark vdso and vsyscall pages. However, it is simple enough to check if a vma
    > > covers a vdso or vsyscall page without the need for this flag.
    >
    > Gee, we ran out?
    >
    > That makes it pretty inevitable that we will grow the vma by four
    > bytes. Once we have done that, your always_dump_vma() trickery becomes
    > unneeded and undesirable, yes? If so, we may as well recognise reality
    > and grow the vma now.
    >

    We could. The reason I didn't propose increasing them was because I saw
    in the archives that there was resistance when this was tried before:
    http://lkml.indiana.edu/hypermail/linux/kernel/1111.1/02053.html

    Also, the current use of 'VM_ALWAYSDUMP' is for vdso and gate pages
    exclusively. That means that VM_ALWAYSDUMP flag is 0 the majority of the
    time, and thus it probably makes sense to re-purpose it, now that we are
    tight on flags.

    > > The second patch then replaces the 'VM_ALWAYSDUMP' flag with a new
    > > 'VM_DONTDUMP' flag, which can be set by userspace using new madvise flags:
    > > 'MADV_DONTDUMP', and unset via 'MADV_DUMP'. The core dump filters continue to
    > > work the same as before unless 'MADV_DONTDUMP' is set on the region.
    > >
    > > The qemu code which implements this features is at:
    > > http://people.redhat.com/~jbaron/qemu-dump/qemu-dump.patch
    > >
    > > In my testing the qemu core dump shrunk from 383MB -> 13MB with this patch.
    > >
    > > I also believe that the 'MADV_DONTDUMP' flag might be useful for security
    > > sensitive apps, which might want to select which areas are dumped.
    > >
    >
    > Is there any way for userspace to query the state of the flag?
    >

    A place for that might be /proc/<pid>/maps, but I don't think any of
    the other vm flags are exposed.

    Thanks,

    -Jason


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