FROMGIT: arm64/fpsimd: ptrace: Gracefully handle errors

Within sve_set_common() we do not handle error conditions correctly:

* When writing to NT_ARM_SSVE, if sme_alloc() fails, the task will be
  left with task->thread.sme_state==NULL, but TIF_SME will be set and
  task->thread.fp_type==FP_STATE_SVE. This will result in a subsequent
  null pointer dereference when the task's state is loaded or otherwise
  manipulated.

* When writing to NT_ARM_SSVE, if sve_alloc() fails, the task will be
  left with task->thread.sve_state==NULL, but TIF_SME will be set,
  PSTATE.SM will be set, and task->thread.fp_type==FP_STATE_FPSIMD.
  This is not a legitimate state, and can result in various problems,
  including a subsequent null pointer dereference and/or the task
  inheriting stale streaming mode register state the next time its state
  is loaded into hardware.

* When writing to NT_ARM_SSVE, if the VL is changed but the resulting VL
  differs from that in the header, the task will be left with TIF_SME
  set, PSTATE.SM set, but task->thread.fp_type==FP_STATE_FPSIMD. This is
  not a legitimate state, and can result in various problems as
  described above.

Avoid these problems by allocating memory earlier, and by changing the
task's saved fp_type to FP_STATE_SVE before skipping register writes due
to a change of VL.

To make early returns simpler, I've moved the call to
fpsimd_flush_task_state() earlier. As the tracee's state has already
been saved, and the tracee is known to be blocked for the duration of
sve_set_common(), it doesn't matter whether this is called at the start
or the end.

For consistency I've moved the setting of TIF_SVE earlier. This will be
cleared when loading FPSIMD-only state, and so moving this has no
resulting functional change.

Note that we only allocate the memory for SVE state when SVE register
contents are provided, avoiding unnecessary memory allocations for tasks
which only use FPSIMD.

Fixes: e12310a0 ("arm64/sme: Implement ptrace support for streaming mode SVE registers")
Fixes: baa85152 ("arm64/fpsimd: Track the saved FPSIMD state type separately to TIF_SVE")
Fixes: 5d0a8d2f ("arm64/ptrace: Ensure that SME is set up for target when writing SSVE state")
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Spickett <david.spickett@arm.com>
Cc: Luis Machado <luis.machado@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20250508132644.1395904-20-mark.rutland@arm.com


Signed-off-by: Will Deacon <will@kernel.org>
(cherry picked from commit 9f8bf718
 https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git


 for-next/sme-fixes)
Bug: 393087661
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I8b194019e77daa5016cae2775c904dde32179c30