[PATCH v2] arm64/ptrace: Ensure that the task sees ZT writes on first use

[Mark Brown <broonie@kernel.org>]

When the value of ZT is set via ptrace we don't disable traps for SME.
This means that when a the task has never used SME before then the value
set via ptrace will never be seen by the target task since it will
trigger a SME access trap which will flush the register state.

Disable SME traps when setting ZT, this means we also need to allocate
storage for SVE if it is not already allocated, for the benefit of
streaming SVE.

Fixes: f90b529bcbe5 ("arm64/sme: Implement ZT0 ptrace support")
Signed-off-by: Mark Brown <broonie@kernel.org>
---
Changes in v2:
- Only set TIF_SME if we copied in data successfully to match ZA.
- Link to v1: https://lore.kernel.org/r/20230814-arm64-zt-ptrace-first-use-v1-1-fc8e8022140f@kernel.org
---
 arch/arm64/kernel/ptrace.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 5b9b4305248b..8206127d6e19 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -1170,6 +1170,11 @@ static int zt_set(struct task_struct *target,
 	if (!system_supports_sme2())
 		return -EINVAL;
 
+	/* Ensure SVE storage in case this is first use of SME */
+	sve_alloc(target, false);
+	if (!target->thread.sve_state)
+		return -ENOMEM;
+
 	if (!thread_za_enabled(&target->thread)) {
 		sme_alloc(target);
 		if (!target->thread.sme_state)
@@ -1179,8 +1184,10 @@ static int zt_set(struct task_struct *target,
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf,
 				 thread_zt_state(&target->thread),
 				 0, ZT_SIG_REG_BYTES);
-	if (ret == 0)
+	if (ret == 0) {
 		target->thread.svcr |= SVCR_ZA_MASK;
+		set_tsk_thread_flag(target, TIF_SME);
+	}
 
 	fpsimd_flush_task_state(target);
 

---
base-commit: 2ccdd1b13c591d306f0401d98dedc4bdcd02b421
change-id: 20230814-arm64-zt-ptrace-first-use-e6595b2162fc

Best regards,
-- 
Mark Brown <broonie@kernel.org>