arm64: compat: Work around uninitialized variable warning

Dan reports that smatch complains about a potential uninitialized
variable being used in the compat alignment fixup code.

The logic is not wrong per se, but we do end up using an uninitialized
variable if reading the instruction that triggered the alignment fault
from user space faults, even if the fault ensures that the uninitialized
value doesn't propagate any further.

Given that we just give up and return 1 if any fault occurs when reading
the instruction, let's get rid of the 'success handling' pattern that
captures the fault in a variable and aborts later, and instead, just
return 1 immediately if any of the get_user() calls result in an
exception.

Fixes: 3fc24ef32d3b ("arm64: compat: Implement misalignment fixups for multiword loads")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/202304021214.gekJ8yRc-lkp@intel.com/
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20230404103625.2386382-1-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arm64: compat: Implement misalignment fixups for multiword loads

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 <vagrant@debian.org>
Cc: Riku Voipio <riku.voipio@iki.fi>
Cc: Steve McIntyre <steve@einval.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20220701135322.3025321-1-ardb@kernel.org
[catalin.marinas@arm.com: change the option to 'default n']
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>