Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[Anthony Yznaga <anthony.yznaga@oracle.com>]

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(-)
>>