When FPMR and SME are both present then entering and exiting streaming mode
clears FPMR in the same manner as it clears the V/Z and P registers.
Since entering and exiting streaming mode via ptrace is expected to have
the same effect as doing so via SMSTART/SMSTOP it should clear FPMR too
but this was missed when FPMR support was added. Add the required reset
of FPMR.

Since changing the vector length resets SVCR a SME vector length change
implemented via a write to ZA can trigger an exit of streaming mode and
we need to check when writing to ZA as well.

Fixes: 4035c22ef7d4 ("arm64/ptrace: Expose FPMR via ptrace")
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
---
 arch/arm64/kernel/ptrace.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index b756578aeaeea1d3250276734520e3eaae8a671d..f242df53de2992bf8a3fd51710d6653fe82f7779 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -876,6 +876,7 @@ static int sve_set_common(struct task_struct *target,
 			  const void *kbuf, const void __user *ubuf,
 			  enum vec_type type)
 {
+	u64 old_svcr = target->thread.svcr;
 	int ret;
 	struct user_sve_header header;
 	unsigned int vq;
@@ -903,8 +904,6 @@ static int sve_set_common(struct task_struct *target,
 
 	/* Enter/exit streaming mode */
 	if (system_supports_sme()) {
-		u64 old_svcr = target->thread.svcr;
-
 		switch (type) {
 		case ARM64_VEC_SVE:
 			target->thread.svcr &= ~SVCR_SM_MASK;
@@ -1003,6 +1002,10 @@ static int sve_set_common(struct task_struct *target,
 				 start, end);
 
 out:
+	/* If we entered or exited streaming mode then reset FPMR */
+	if ((target->thread.svcr & SVCR_SM) != (old_svcr & SVCR_SM))
+		target->thread.uw.fpmr = 0;
+
 	fpsimd_flush_task_state(target);
 	return ret;
 }
@@ -1099,6 +1102,7 @@ static int za_set(struct task_struct *target,
 		  unsigned int pos, unsigned int count,
 		  const void *kbuf, const void __user *ubuf)
 {
+	u64 old_svcr = target->thread.svcr;
 	int ret;
 	struct user_za_header header;
 	unsigned int vq;
@@ -1175,6 +1179,10 @@ static int za_set(struct task_struct *target,
 	target->thread.svcr |= SVCR_ZA_MASK;
 
 out:
+	/* If we entered or exited streaming mode then reset FPMR */
+	if ((target->thread.svcr & SVCR_SM) != (old_svcr & SVCR_SM))
+		target->thread.uw.fpmr = 0;
+
 	fpsimd_flush_task_state(target);
 	return ret;
 }

---
base-commit: 8e929cb546ee42c9a61d24fae60605e9e3192354
change-id: 20241106-arm64-ptrace-fpmr-sm-45390f592574

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