Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

Anthony Yznaga
2020-07-28T17:28:07Z

On 7/28/20 4:34 AM, Kirill Tkhai wrote:
> On 27.07.2020 20:11, Anthony Yznaga wrote:
>> This patchset adds support for preserving an anonymous memory range across
>> exec(3) using a new madvise MADV_DOEXEC argument. The primary benefit for
>> sharing memory in this manner, as opposed to re-attaching to a named shared
>> memory segment, is to ensure it is mapped at the same virtual address in
>> the new process as it was in the old one. An intended use for this is to
>> preserve guest memory for guests using vfio while qemu exec's an updated
>> version of itself. By ensuring the memory is preserved at a fixed address,
> So, the goal is an update of QEMU binary without a stopping of virtual machine?
Essentially, yes.  The VM is paused very briefly.

Anthony
>
>> vfio mappings and their associated kernel data structures can remain valid.
>> In addition, for the qemu use case, qemu instances that back guest RAM with
>> anonymous memory can be updated.
>>
>> Patches 1 and 2 ensure that loading of ELF load segments does not silently
>> clobber existing VMAS, and remove assumptions that the stack is the only
>> VMA in the mm when the stack is set up. Patch 1 re-introduces the use of
>> MAP_FIXED_NOREPLACE to load ELF binaries that addresses the previous issues
>> and could be considered on its own.
>>
>> Patches 3, 4, and 5 introduce the feature and an opt-in method for its use
>> using an ELF note.
>>
>> Anthony Yznaga (5):
>> elf: reintroduce using MAP_FIXED_NOREPLACE for elf executable mappings
>> mm: do not assume only the stack vma exists in setup_arg_pages()
>> mm: introduce VM_EXEC_KEEP
>> exec, elf: require opt-in for accepting preserved mem
>> mm: introduce MADV_DOEXEC
>>
>> arch/x86/Kconfig | 1 +
>> fs/binfmt_elf.c | 196 +++++++++++++++++++++++++--------
>> fs/exec.c | 33 +++++-
>> include/linux/binfmts.h | 7 +-
>> include/linux/mm.h | 5 +
>> include/uapi/asm-generic/mman-common.h | 3 +
>> kernel/fork.c | 2 +-
>> mm/madvise.c | 25 +++++
>> mm/mmap.c | 47 ++++++++
>> 9 files changed, 266 insertions(+), 53 deletions(-)
>>