Re: [RFC PATCH 0/5] madvise MADV_DOEXEC

[Steven Sistare]

On 7/8/2021 5:52 AM, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> Hi Anthony and Steven,
> 
> 在 2020/7/28 1:11, Anthony Yznaga 写道:
>> 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,
>> 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.
> 
> We have a requirement like yours, but ours seems more complex. We want to
> isolate some memory regions from the VM's memory space and the start a child
> process who will using these memory regions.
> 
> I've wrote a draft to support this feature, but I just find that my draft is
> pretty like yours.
> 
> It seems that you've already abandoned this patchset, why ?

Hi Longpeng,
  The reviewers did not like the proposal for several reasons, but the showstopper
was that they did not want to add complexity to the exec path in the kernel.  You
can read the email archive for details.

We solved part of our problem by adding new vfio interfaces: VFIO_DMA_UNMAP_FLAG_VADDR
and VFIO_DMA_MAP_FLAG_VADDR.  That solves the vfio problem for shared memory, but not
for mmap MAP_ANON memory.

- Steve

>> 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(-)
>>
>