Re: [Qemu-devel] [PATCH 0/2] core dump: re-purpose VM_ALWAYSDUMP to user controlled VM_DONTDUMP

[Andrew Morton]

On Wed, 7 Mar 2012 12:00:46 -0500
Jason Baron <address@hidden> 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.

> 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?