arm64/fpsimd: ptrace: Do not present register data for inactive mode

The SME ptrace ABI is written around the incorrect assumption that
SVE_PT_REGS_FPSIMD and SVE_PT_REGS_SVE are independent bit flags, where
it is possible for both to be clear. In reality they are different
values for bit 0 of the header flags, where SVE_PT_REGS_FPSIMD is 0 and
SVE_PT_REGS_SVE is 1. In cases where code was written expecting that
neither bit flag would be set, the value is equivalent to
SVE_PT_REGS_FPSIMD.

One consequence of this is that reads of the NT_ARM_SVE or NT_ARM_SSVE
will erroneously present data from the other mode:

* When PSTATE.SM==1, reads of NT_ARM_SVE will present a header with
  SVE_PT_REGS_FPSIMD, and FPSIMD-formatted data from streaming mode.

* When PSTATE.SM==0, reads of NT_ARM_SSVE will present a header with
  SVE_PT_REGS_FPSIMD, and FPSIMD-formatted data from non-streaming mode.

The original intent was that no register data would be provided in these
cases, as described in commit:

  e12310a0d30f ("arm64/sme: Implement ptrace support for streaming mode SVE registers")

Luckily, debuggers do not consume the bogus register data. Both GDB and
LLDB read the NT_ARM_SSVE regset before the NT_ARM_SVE regset, and
assume that when the NT_ARM_SSVE header presents SVE_PT_REGS_FPSIMD, it
is necessary to read register contents from the NT_ARM_SVE regset,
regardless of whether the NT_ARM_SSVE regset provided bogus register
data.

Fix the code to stop presenting register data from the inactive mode.
At the same time, make the manipulation of the flag clearer, and remove
the bogus comment from sve_set_common(). I've given this a quick spin
with GDB and LLDB, and both seem happy.

Fixes: e12310a0d30f ("arm64/sme: Implement ptrace support for streaming mode SVE registers")
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/20250508132644.1395904-18-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 18 insertions, 14 deletions

diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 67f3843de51f..a2075e1df27c 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -775,6 +775,11 @@ static void sve_init_header_from_task(struct user_sve_header *header,
 		task_type = ARM64_VEC_SVE;
 	active = (task_type == type);
 
+	if (active && target->thread.fp_type == FP_STATE_SVE)
+		header->flags = SVE_PT_REGS_SVE;
+	else
+		header->flags = SVE_PT_REGS_FPSIMD;
+
 	switch (type) {
 	case ARM64_VEC_SVE:
 		if (test_tsk_thread_flag(target, TIF_SVE_VL_INHERIT))
@@ -789,19 +794,14 @@ static void sve_init_header_from_task(struct user_sve_header *header,
 		return;
 	}
 
-	if (active) {
-		if (target->thread.fp_type == FP_STATE_FPSIMD) {
-			header->flags |= SVE_PT_REGS_FPSIMD;
-		} else {
-			header->flags |= SVE_PT_REGS_SVE;
-		}
-	}
-
 	header->vl = task_get_vl(target, type);
 	vq = sve_vq_from_vl(header->vl);
 
 	header->max_vl = vec_max_vl(type);
-	header->size = SVE_PT_SIZE(vq, header->flags);
+	if (active)
+		header->size = SVE_PT_SIZE(vq, header->flags);
+	else
+		header->size = sizeof(header);
 	header->max_size = SVE_PT_SIZE(sve_vq_from_vl(header->max_vl),
 				      SVE_PT_REGS_SVE);
 }
@@ -832,6 +832,13 @@ static int sve_get_common(struct task_struct *target,
 	BUILD_BUG_ON(SVE_PT_FPSIMD_OFFSET != sizeof(header));
 	BUILD_BUG_ON(SVE_PT_SVE_OFFSET != sizeof(header));
 
+	/*
+	 * When the requested vector type is not active, do not present data
+	 * from the other mode to userspace.
+	 */
+	if (header.size == sizeof(header))
+		return 0;
+
 	switch ((header.flags & SVE_PT_REGS_MASK)) {
 	case SVE_PT_REGS_FPSIMD:
 		return __fpr_get(target, regset, to);
@@ -859,7 +866,7 @@ static int sve_get_common(struct task_struct *target,
 		return membuf_zero(&to, end - start);
 
 	default:
-		return 0;
+		BUILD_BUG();
 	}
 }
 
@@ -946,10 +953,7 @@ static int sve_set_common(struct task_struct *target,
 		goto out;
 	}
 
-	/*
-	 * Otherwise: no registers or full SVE case.  For backwards
-	 * compatibility reasons we treat empty flags as SVE registers.
-	 */
+	/* Otherwise: no registers or full SVE case. */
 
 	/*
 	 * If setting a different VL from the requested VL and there is