[PATCH v4 0/8] arm64: Allow 64-bit tasks to invoke compat syscalls

From: Amanieu d'Antras
Date: Fri Apr 30 2021 - 16:28:49 EST

This series allows AArch64 tasks to perform 32-bit syscalls by setting
the top bit of x8 and using AArch32 compat syscall numbers:

syscall(0x80000000 | __ARM_NR_write, 1, "foo\n", 4);

Internally, setting this bit does the following:
- The remainder of x8 is treated as a compat syscall number and is used
to index the compat syscall table.
- in_compat_syscall will return true for the duration of the syscall.
- VM allocations performed by the syscall will be located in the lower
4G of the address space. A separate compat_mmap_base is used so that
these allocations are still properly randomized.
- Interrupted compat syscalls are properly restarted as compat syscalls.
- Seccomp will treats the syscall as having AUDIT_ARCH_ARM instead of
AUDIT_ARCH_AARCH64. This affects the arch value seen by seccomp
filters and reported by SIGSYS.
- PTRACE_GET_SYSCALL_INFO also treats the syscall as having
AUDIT_ARCH_ARM. Recent versions of strace will correctly report the
syscall name and parameters when an AArch64 task mixes 32-bit and
64-bit syscalls.

This feature is intended for use in software compatibility layers which
emulate a 32-bit program on AArch64. This patch has been tested on two
such emulators:
- Tango [1], which enables AArch32 binaries to run on AArch64 CPUs which
do not have hardware support for AArch32. Tango is used to run virtual
Android devices on AArch64 servers.
- FEX [2], an emulator for running x86 and x86_64 binaries on AArch64.
FEX can already run many x86_64 programs including 3D games, but
requires kernel support for running 32-bit x86 binaries.

Both FEX and Tango have previously attempted to translate 32-bit
syscalls purely in user mode like QEMU does for its user mode
emulation. While this works for simple programs, there are many
limitations which cannot be solved without kernel support, for example:
- There are a huge number of ioctls which behave differently in 32-bit
mode. It is impractical and error prone to manually emulate them all
in user mode. Specifically, the kernel already has a well-tested and
reliable compatibility layer and it makes sense to reuse this. QEMU
supports emulating some ioctls in userspace but this still does not
cover devices like GPUs which are needed for accelerated rendering.
- The 64-bit set_robust_list is not compatible with the 32-bit ABI. The
compat version of set_robust_list must be used. Emulating this in user
mode is not reliable since SIGKILL cannot be caught.
- io_uring uses iovec structures as part of its API, which have
different sizes on 32-bit and 64-bit.
- ext4 represents positions in directories as 64-bit hashes, which break
if they are truncated to 32 bits. There is special support for 32-bit
off_t in the ext4 driver but this is only used when in_compat_syscall
is true: https://bugzilla.kernel.org/show_bug.cgi?id=205957
- The io_setup syscall allocates a VM area for the AIO context and
returns it. But there is no way to control where this context is
allocated so it will almost always end up above the 4GB limit.
- Some ioctls will also perform VM allocations, with the same issues as
io_setup. Search for "vm_mmap" in drivers/.
- Some file descriptors have alignment requirements which are not known
to userspace. For example, a hugetlbfs file can only be mmaped at a
huge page alignment but there is no way for userspace to know this
when it needs to manually select an address below 4GB for the mapping.

All of these issues are solved in FEX and Tango by invoking compat
syscalls directly. In the case of FEX, there remain some differences
between the arm and x86 ABIs due to alignment issues, but these are few
enough to be individually handled in userspace.

There is a precedent for exposing this functionality to userspace:
x86_64 has 2 ways to invoke 32-bit syscalls. The first is to use int
0x80 with a 32-bit syscall number and the second is to use
__X32_SYSCALL_BIT with a 64-bit syscall number. As such, the generic
kernel code is already able to properly handle tasks that invoke both
32-bit and 64-bit syscalls.

[1] https://www.amanieusystems.com/
[2] https://github.com/FEX-Emu/FEX

Changelog since v3:
- Renamed aarch64_compat_syscall to use_compat_syscall and enable it
permanently for AArch32 tasks.

Changelog since v2:
- Complete rewrite, based on the patch that was previously posted as:
[PATCH v2] [RFC] arm64: Exposes support for 32-bit syscalls

Amanieu d'Antras (8):
mm: Add arch_get_mmap_base_topdown macro
hugetlbfs: Use arch_get_mmap_* macros
mm: Support mmap_compat_base with the generic layout
arm64: Separate in_compat_syscall from is_compat_task
arm64: Add a compat syscall flag to thread_info
arm64: Forbid calling compat sigreturn from 64-bit tasks
arm64: Allow 64-bit tasks to invoke compat syscalls

arch/arm64/Kconfig | 1 +
arch/arm64/include/asm/compat.h | 24 ++++++++++++---
arch/arm64/include/asm/elf.h | 21 ++++++++++---
arch/arm64/include/asm/ftrace.h | 2 +-
arch/arm64/include/asm/processor.h | 32 +++++++++++--------
arch/arm64/include/asm/syscall.h | 6 ++--
arch/arm64/include/asm/thread_info.h | 6 ++++
arch/arm64/include/uapi/asm/unistd.h | 2 ++
arch/arm64/kernel/ptrace.c | 2 +-
arch/arm64/kernel/signal.c | 5 +++
arch/arm64/kernel/signal32.c | 8 +++++
arch/arm64/kernel/syscall.c | 23 ++++++++++++--
arch/arm64/mm/mmap.c | 33 ++++++++++++++++++++
fs/hugetlbfs/inode.c | 22 ++++++++++---
mm/mmap.c | 14 ++++++---
mm/util.c | 46 +++++++++++++++++++++++-----
16 files changed, 202 insertions(+), 45 deletions(-)