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    SubjectRe: [PATCH v2 0/5] Fix ELF / FDPIC ELF core dumping, and use mmap_sem properly in there
    On Wed, 29 Apr 2020, Russell King - ARM Linux admin wrote:

    > On Wed, Apr 29, 2020 at 11:49:49PM +0200, Jann Horn wrote:
    > > At the moment, we have that rather ugly mmget_still_valid() helper to
    > > work around <https://crbug.com/project-zero/1790>: ELF core dumping
    > > doesn't take the mmap_sem while traversing the task's VMAs, and if
    > > anything (like userfaultfd) then remotely messes with the VMA tree,
    > > fireworks ensue. So at the moment we use mmget_still_valid() to bail
    > > out in any writers that might be operating on a remote mm's VMAs.
    > >
    > > With this series, I'm trying to get rid of the need for that as
    > > cleanly as possible.
    > > In particular, I want to avoid holding the mmap_sem across unbounded
    > > sleeps.
    > >
    > >
    > > Patches 1, 2 and 3 are relatively unrelated cleanups in the core
    > > dumping code.
    > >
    > > Patches 4 and 5 implement the main change: Instead of repeatedly
    > > accessing the VMA list with sleeps in between, we snapshot it at the
    > > start with proper locking, and then later we just use our copy of
    > > the VMA list. This ensures that the kernel won't crash, that VMA
    > > metadata in the coredump is consistent even in the presence of
    > > concurrent modifications, and that any virtual addresses that aren't
    > > being concurrently modified have their contents show up in the core
    > > dump properly.
    > >
    > > The disadvantage of this approach is that we need a bit more memory
    > > during core dumping for storing metadata about all VMAs.
    > >
    > > After this series has landed, we should be able to rip out
    > > mmget_still_valid().
    > >
    > >
    > > Testing done so far:
    > >
    > > - Creating a simple core dump on X86-64 still works.
    > > - The created coredump on X86-64 opens in GDB, and both the stack and the
    > > exectutable look vaguely plausible.
    > > - 32-bit ARM compiles with FDPIC support, both with MMU and !MMU config.
    > >
    > > I'm CCing some folks from the architectures that use FDPIC in case
    > > anyone wants to give this a spin.
    >
    > I've never had any reason to use FDPIC, and I don't have any binaries
    > that would use it. Nicolas Pitre added ARM support, so I guess he
    > would be the one to talk to about it. (Added Nicolas.)

    It's been a while since I worked with it. However Christophe Lyon (in
    CC) added support for ARM FDPIC to mainline gcc recently, so hopefully
    he might still be set up and able to help.


    Nicolas

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