[RFC PATCH 0/5] madvise MADV_DOEXEC

Anthony Yznaga
2020-07-27T17:11:22Z

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.

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(-)

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1.8.3.1