On Fri, Jul 01, 2022 at 03:53:22PM +0200, Ard Biesheuvel wrote:

The 32-bit ARM kernel implements fixups on behalf of user space when
using LDM/STM or LDRD/STRD instructions on addresses that are not 32-bit
aligned. This is not something that is supported by the architecture,
but was done anyway to increase compatibility with user space software,
which mostly targeted x86 at the time and did not care about aligned
accesses.

This feature is one of the remaining impediments to being able to switch
to 64-bit kernels on 64-bit capable hardware running 32-bit user space,
so let's implement it for the arm64 compat layer as well.

Note that the intent is to implement the exact same handling of
misaligned multi-word loads and stores as the 32-bit kernel does,
including what appears to be missing support for user space programs
that rely on SETEND to switch to a different byte order and back. Also,
like the 32-bit ARM version, we rely on the faulting address reported by
the CPU to infer the memory address, instead of decoding the instruction
fully to obtain this information.

This implementation is taken from the 32-bit ARM tree, with all pieces
removed that deal with instructions other than LDRD/STRD and LDM/STM, or
that deal with alignment exceptions taken in kernel mode.

Cc: debian-arm@lists.debian.org
Cc: Vagrant Cascadian <redacted>
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Steve McIntyre <redacted>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
Note to cc'ees: if this is something you would like to see merged,
please indicate so. This stuff is unlikely to get in if there are no
users.

v2: - drop some obsolete comments
    - emit a perf alignment-fault event for every handled instruction
    - use arm64_skip_faulting_instruction() to get the correct behavior
      wrt IT state and single step
    - use types with correct endianness annotation (instructions are
      always little endian on v7/v8+)

It looks like that's a fairly popular request from people running 32-bit
user on AArch64 kernels, so happy to queue it for 6.1 (if it still
applies cleanly). I'm not too keen on code duplication but it's a lot
more hassle to create a common decoding/emulation library to share with
arch/arm, especially as such code is not going to change in the future.

+config COMPAT_ALIGNMENT_FIXUPS
+	bool "Fix up misaligned multi-word loads and stores in user space"
+	default y

For consistency with ARMV8_DEPRECATED, I think we should keep this as
default n.

Thanks.

-- 
Catalin

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