[PATCH 0/5] arm64/sme: Support disabling streaming mode via ptrace on SME only systems

[PATCH 0/5] arm64/sme: Support disabling streaming mode via ptrace on SME only systems

[Mark Brown]

Currently it is not possible to disable streaming mode via ptrace on SME
only systems, the interface for doing this is to write via NT_ARM_SVE but
such writes will be rejected on a system without SVE support. Enable this
functionality by allowing userspace to write SVE_PT_REGS_FPSIMD format data
via NT_ARM_SVE with the vector length set to 0 on SME only systems. Such
writes currently error since we require that a vector length is specified
which should minimise the risk that existing software is relying on current
behaviour.

Reads are not supported since I am not aware of any use case for this and
there is some risk that an existing userspace application may be confused if
it reads NT_ARM_SVE on a system without SVE. Existing kernels will return
FPSIMD formatted register state from NT_ARM_SVE if full SVE state is not
stored, for example if the task has not used SVE. Returning a vector length
of 0 would create a risk that software could try to do things like allocate
space for register state with zero sizes, while returning a vector length of
128 bits would look like SVE is supported. It seems safer to just not make
the changes to add read support.

It remains possible for userspace to detect a SME only system via the ptrace
interface only since reads of NT_ARM_SSVE and NT_ARM_ZA will suceed while
reads of NT_ARM_SVE will fail. Read/write access to the FPSIMD registers in
non-streaming mode is available via REGSET_FPR.

The aim is is to make a minimally invasive change, no operation that would
previously have succeeded will be affected, and we use a previously defined
interface in new circumstances rather than define completely new ABI.

The series starts with some enhancements to sve-ptrace to cover some
further corners of existing behaviours in order to reduce the risk of
inadvertent changes, implements the proposed new ABI, then extends both
sve-ptrace and fp-ptrace to exercise it.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
Mark Brown (5):
      kselftest/arm64: Verify that we reject out of bounds VLs in sve-ptrace
      kselftest/arm64: Check that unsupported regsets fail in sve-ptrace
      arm64/sme: Support disabling streaming mode via ptrace on SME only systems
      kselftst/arm64: Test NT_ARM_SVE FPSIMD format writes on non-SVE systems
      kselftest/arm64: Cover disabling streaming mode without SVE in fp-ptrace

 Documentation/arch/arm64/sve.rst              |   5 +
 arch/arm64/kernel/ptrace.c                    |  40 ++++++--
 tools/testing/selftests/arm64/fp/fp-ptrace.c  |   5 +-
 tools/testing/selftests/arm64/fp/sve-ptrace.c | 139 +++++++++++++++++++++++++-
 4 files changed, 177 insertions(+), 12 deletions(-)
---
base-commit: 768361ab16ce943ef3577cea204dc81aa4a47517
change-id: 20250717-arm64-sme-ptrace-sme-only-1fb850600ea0
prerequisite-change-id: 20250808-arm64-fp-trace-macro-02ede083da51

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

[PATCH 1/5] kselftest/arm64: Verify that we reject out of bounds VLs in sve-ptrace

[Mark Brown]

We do not currently have a test that asserts that we reject attempts to set
a vector length smaller than SVE_VL_MIN or larger than SVE_VL_MAX, add one
since that is our current behaviour.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/arm64/fp/sve-ptrace.c | 32 ++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/arm64/fp/sve-ptrace.c b/tools/testing/selftests/arm64/fp/sve-ptrace.c
index b22303778fb0..a9b2377c46b8 100644
--- a/tools/testing/selftests/arm64/fp/sve-ptrace.c
+++ b/tools/testing/selftests/arm64/fp/sve-ptrace.c
@@ -66,7 +66,7 @@ static const struct vec_type vec_types[] = {
 };
 
 #define VL_TESTS (((TEST_VQ_MAX - SVE_VQ_MIN) + 1) * 4)
-#define FLAG_TESTS 2
+#define FLAG_TESTS 4
 #define FPSIMD_TESTS 2
 
 #define EXPECTED_TESTS ((VL_TESTS + FLAG_TESTS + FPSIMD_TESTS) * ARRAY_SIZE(vec_types))
@@ -270,6 +270,25 @@ static void check_u32(unsigned int vl, const char *reg,
 	}
 }
 
+/* Set out of range VLs */
+static void ptrace_set_vl_ranges(pid_t child, const struct vec_type *type)
+{
+	struct user_sve_header sve;
+	int ret;
+
+	memset(&sve, 0, sizeof(sve));
+	sve.flags = SVE_PT_REGS_SVE;
+	sve.size = sizeof(sve);
+
+	ret = set_sve(child, type, &sve);
+	ksft_test_result(ret != 0, "%s Set invalid VL 0\n", type->name);
+
+	sve.vl = SVE_VL_MAX + SVE_VQ_BYTES;
+	ret = set_sve(child, type, &sve);
+	ksft_test_result(ret != 0, "%s Set invalid VL %d\n", type->name,
+			 SVE_VL_MAX + SVE_VQ_BYTES);
+}
+
 /* Access the FPSIMD registers via the SVE regset */
 static void ptrace_sve_fpsimd(pid_t child, const struct vec_type *type)
 {
@@ -703,6 +722,17 @@ static int do_parent(pid_t child)
 					      vec_types[i].name);
 		}
 
+		/* Setting out of bounds VLs should fail */
+		if (getauxval(vec_types[i].hwcap_type) & vec_types[i].hwcap) {
+			ptrace_set_vl_ranges(child, &vec_types[i]);
+		} else {
+			ksft_test_result_skip("%s Set invalid VL 0\n",
+					      vec_types[i].name);
+			ksft_test_result_skip("%s Set invalid VL %d\n",
+					      vec_types[i].name,
+					      SVE_VL_MAX + SVE_VQ_BYTES);
+		}
+
 		/* Step through every possible VQ */
 		for (vq = SVE_VQ_MIN; vq <= TEST_VQ_MAX; vq++) {
 			vl = sve_vl_from_vq(vq);

-- 
2.39.5

[PATCH 2/5] kselftest/arm64: Check that unsupported regsets fail in sve-ptrace

[Mark Brown]

Add a test which verifies that NT_ARM_SVE and NT_ARM_SSVE reads and writes
are rejected as expected when the relevant architecture feature is not
supported.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/arm64/fp/sve-ptrace.c | 46 +++++++++++++++++++++++++++
 1 file changed, 46 insertions(+)

diff --git a/tools/testing/selftests/arm64/fp/sve-ptrace.c b/tools/testing/selftests/arm64/fp/sve-ptrace.c
index a9b2377c46b8..5f39ca0b4194 100644
--- a/tools/testing/selftests/arm64/fp/sve-ptrace.c
+++ b/tools/testing/selftests/arm64/fp/sve-ptrace.c
@@ -158,6 +158,38 @@ static int set_sve(pid_t pid, const struct vec_type *type,
 	return ptrace(PTRACE_SETREGSET, pid, type->regset, &iov);
 }
 
+/* A read operation fails */
+static void read_fails(pid_t child, const struct vec_type *type)
+{
+	struct user_sve_header *new_sve = NULL;
+	size_t new_sve_size = 0;
+	void *ret;
+
+	ret = get_sve(child, type, (void **)&new_sve, &new_sve_size);
+
+	ksft_test_result(ret == NULL, "%s unsupported read fails\n",
+			 type->name);
+
+	free(new_sve);
+}
+
+/* A write operation fails */
+static void write_fails(pid_t child, const struct vec_type *type)
+{
+	struct user_sve_header sve;
+	int ret;
+
+	/* Just the header, no data */
+	memset(&sve, 0, sizeof(sve));
+	sve.size = sizeof(sve);
+	sve.flags = SVE_PT_REGS_SVE;
+	sve.vl = SVE_VL_MIN;
+	ret = set_sve(child, type, &sve);
+
+	ksft_test_result(ret != 0, "%s unsupported write fails\n",
+			 type->name);
+}
+
 /* Validate setting and getting the inherit flag */
 static void ptrace_set_get_inherit(pid_t child, const struct vec_type *type)
 {
@@ -702,6 +734,20 @@ static int do_parent(pid_t child)
 	}
 
 	for (i = 0; i < ARRAY_SIZE(vec_types); i++) {
+		/*
+		 * If the vector type isn't supported reads and writes
+		 * should fail.
+		 */
+		if (!(getauxval(vec_types[i].hwcap_type) & vec_types[i].hwcap)) {
+			read_fails(child, &vec_types[i]);
+			write_fails(child, &vec_types[i]);
+		} else {
+			ksft_test_result_skip("%s unsupported read fails\n",
+					      vec_types[i].name);
+			ksft_test_result_skip("%s unsupported write fails\n",
+					      vec_types[i].name);
+		}
+
 		/* FPSIMD via SVE regset */
 		if (getauxval(vec_types[i].hwcap_type) & vec_types[i].hwcap) {
 			ptrace_sve_fpsimd(child, &vec_types[i]);

-- 
2.39.5

[PATCH 3/5] arm64/sme: Support disabling streaming mode via ptrace on SME only systems

[Mark Brown]

Currently it is not possible to disable streaming mode via ptrace on SME
only systems, the interface for doing this is to write via NT_ARM_SVE but
such writes will be rejected on a system without SVE support. Enable this
functionality by allowing userspace to write SVE_PT_REGS_FPSIMD format data
via NT_ARM_SVE with the vector length set to 0 on SME only systems. Such
writes currently error since we require that a vector length is specified
which should minimise the risk that existing software is relying on current
behaviour.

Reads are not supported since I am not aware of any use case for this and
there is some risk that an existing userspace application may be confused if
it reads NT_ARM_SVE on a system without SVE. Existing kernels will return
FPSIMD formatted register state from NT_ARM_SVE if full SVE state is not
stored, for example if the task has not used SVE. Returning a vector length
of 0 would create a risk that software would try to do things like allocate
space for register state with zero sizes, while returning a vector length of
128 bits would look like SVE is supported. It seems safer to just not make
the changes to add read support.

It remains possible for userspace to detect a SME only system via the ptrace
interface only since reads of NT_ARM_SSVE and NT_ARM_ZA will succeed while
reads of NT_ARM_SVE will fail. Read/write access to the FPSIMD registers in
non-streaming mode is available via REGSET_FPR.

sve_set_common() already avoids allocating SVE storage when doing a FPSIMD
formatted write and allocating SME storage when doing a NT_ARM_SVE write so
we change the function to validate the new case and skip setting a vector
length for it.

The aim is to make a minimally invasive change, no operation that would
previously have succeeded will be affected, and we use a previously
defined interface in new circumstances rather than define completely new
ABI.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 Documentation/arch/arm64/sve.rst |  5 +++++
 arch/arm64/kernel/ptrace.c       | 40 +++++++++++++++++++++++++++++++++-------
 2 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/Documentation/arch/arm64/sve.rst b/Documentation/arch/arm64/sve.rst
index 28152492c29c..a61c9d0efe4d 100644
--- a/Documentation/arch/arm64/sve.rst
+++ b/Documentation/arch/arm64/sve.rst
@@ -402,6 +402,11 @@ The regset data starts with struct user_sve_header, containing:
   streaming mode and any SETREGSET of NT_ARM_SSVE will enter streaming mode
   if the target was not in streaming mode.
 
+* On systems that do not support SVE it is permitted to use SETREGSET to
+  write SVE_PT_REGS_FPSIMD formatted data via NT_ARM_SVE, in this case the
+  vector length should be specified as 0. This allows streaming mode to be
+  disabled on systems with SME but not SVE.
+
 * If any register data is provided along with SVE_PT_VL_ONEXEC then the
   registers data will be interpreted with the current vector length, not
   the vector length configured for use on exec.
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 4b001121c72d..b9bdd83fbbca 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -912,13 +912,39 @@ static int sve_set_common(struct task_struct *target,
 		return -EINVAL;
 
 	/*
-	 * Apart from SVE_PT_REGS_MASK, all SVE_PT_* flags are consumed by
-	 * vec_set_vector_length(), which will also validate them for us:
+	 * On systems without SVE we accept FPSIMD format writes with
+	 * a VL of 0 to allow exiting streaming mode, otherwise a VL
+	 * is required.
 	 */
-	ret = vec_set_vector_length(target, type, header.vl,
-		((unsigned long)header.flags & ~SVE_PT_REGS_MASK) << 16);
-	if (ret)
-		return ret;
+	if (header.vl) {
+		/*
+		 * If the system does not support SVE we can't
+		 * configure a SVE VL.
+		 */
+		if (!system_supports_sve() && type == ARM64_VEC_SVE)
+			return -EINVAL;
+
+		/*
+		 * Apart from SVE_PT_REGS_MASK, all SVE_PT_* flags are
+		 * consumed by vec_set_vector_length(), which will
+		 * also validate them for us:
+		 */
+		ret = vec_set_vector_length(target, type, header.vl,
+					    ((unsigned long)header.flags & ~SVE_PT_REGS_MASK) << 16);
+		if (ret)
+			return ret;
+	} else {
+		/* If the system supports SVE we require a VL. */
+		if (system_supports_sve())
+			return -EINVAL;
+
+		/*
+		 * Only FPSIMD formatted data with no flags set is
+		 * supported.
+		 */
+		if (header.flags != SVE_PT_REGS_FPSIMD)
+			return -EINVAL;
+	}
 
 	/* Allocate SME storage if necessary, preserving any existing ZA/ZT state */
 	if (type == ARM64_VEC_SME) {
@@ -1016,7 +1042,7 @@ static int sve_set(struct task_struct *target,
 		   unsigned int pos, unsigned int count,
 		   const void *kbuf, const void __user *ubuf)
 {
-	if (!system_supports_sve())
+	if (!system_supports_sve() && !system_supports_sme())
 		return -EINVAL;
 
 	return sve_set_common(target, regset, pos, count, kbuf, ubuf,

-- 
2.39.5

[PATCH 4/5] kselftst/arm64: Test NT_ARM_SVE FPSIMD format writes on non-SVE systems

[Mark Brown]

In order to allow exiting streaming mode on systems with SME but not SVE
we allow writes of FPSIMD format data via NT_ARM_SVE even when SVE is not
supported, add a test case that covers this to sve-ptrace.

We do not support reads.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/arm64/fp/sve-ptrace.c | 61 +++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/tools/testing/selftests/arm64/fp/sve-ptrace.c b/tools/testing/selftests/arm64/fp/sve-ptrace.c
index 5f39ca0b4194..fe43fb07ee6c 100644
--- a/tools/testing/selftests/arm64/fp/sve-ptrace.c
+++ b/tools/testing/selftests/arm64/fp/sve-ptrace.c
@@ -378,6 +378,58 @@ static void ptrace_sve_fpsimd(pid_t child, const struct vec_type *type)
 	free(svebuf);
 }
 
+/* Write the FPSIMD registers via the SVE regset when SVE is not supported */
+static void ptrace_sve_fpsimd_no_sve(pid_t child)
+{
+	void *svebuf;
+	struct user_sve_header *sve;
+	struct user_fpsimd_state *fpsimd, new_fpsimd;
+	unsigned int i, j;
+	unsigned char *p;
+	int ret;
+
+	svebuf = malloc(SVE_PT_SIZE(0, SVE_PT_REGS_FPSIMD));
+	if (!svebuf) {
+		ksft_test_result_fail("Failed to allocate FPSIMD buffer\n");
+		return;
+	}
+
+	/* On a system without SVE the VL should be set to 0 */
+	memset(svebuf, 0, SVE_PT_SIZE(0, SVE_PT_REGS_FPSIMD));
+	sve = svebuf;
+	sve->flags = SVE_PT_REGS_FPSIMD;
+	sve->size = SVE_PT_SIZE(0, SVE_PT_REGS_FPSIMD);
+	sve->vl = 0;
+
+	/* Try to set a known FPSIMD state via PT_REGS_SVE */
+	fpsimd = (struct user_fpsimd_state *)((char *)sve +
+					      SVE_PT_FPSIMD_OFFSET);
+	for (i = 0; i < 32; ++i) {
+		p = (unsigned char *)&fpsimd->vregs[i];
+
+		for (j = 0; j < sizeof(fpsimd->vregs[i]); ++j)
+			p[j] = j;
+	}
+
+	ret = set_sve(child, &vec_types[0], sve);
+	ksft_test_result(ret == 0, "FPSIMD write via SVE\n");
+	if (ret) {
+		ksft_test_result_skip("Verify FPSIMD write via SVE\n");
+		goto out;
+	}
+
+	/* Verify via the FPSIMD regset */
+	if (get_fpsimd(child, &new_fpsimd)) {
+		ksft_test_result_skip("Verify FPSIMD write via SVE\n");
+		goto out;
+	}
+	ksft_test_result(memcmp(fpsimd, &new_fpsimd, sizeof(*fpsimd)) == 0,
+			 "Verify FPSIMD write via SVE\n");
+
+out:
+	free(svebuf);
+}
+
 /* Validate attempting to set SVE data and read SVE data */
 static void ptrace_set_sve_get_sve_data(pid_t child,
 					const struct vec_type *type,
@@ -810,6 +862,15 @@ static int do_parent(pid_t child)
 		}
 	}
 
+	/* We support SVE writes of FPSMID format on SME only systems */
+	if (!(getauxval(AT_HWCAP) & HWCAP_SVE) &&
+	    (getauxval(AT_HWCAP2) & HWCAP2_SME)) {
+		ptrace_sve_fpsimd_no_sve(child);
+	} else {
+		ksft_test_result_skip("FPSIMD write via SVE\n");
+		ksft_test_result_skip("Verify FPSIMD write via SVE\n");
+	}
+
 	ret = EXIT_SUCCESS;
 
 error:

-- 
2.39.5

[PATCH 5/5] kselftest/arm64: Cover disabling streaming mode without SVE in fp-ptrace

[Mark Brown]

On a system which support SME but not SVE we can now disable streaming mode
via ptrace by writing FPSIMD formatted data through NT_ARM_SVE with a VL of
0. Extend fp-ptrace to cover rather than skip these cases, relax the check
for SVE writes of FPSIMD format data to not skip if SME is supported and
accept 0 as the VL when performing the ptrace write.

Signed-off-by: Mark Brown <broonie@kernel.org>
---
 tools/testing/selftests/arm64/fp/fp-ptrace.c | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/tools/testing/selftests/arm64/fp/fp-ptrace.c b/tools/testing/selftests/arm64/fp/fp-ptrace.c
index cdd7a45c045d..3bb4bbff120f 100644
--- a/tools/testing/selftests/arm64/fp/fp-ptrace.c
+++ b/tools/testing/selftests/arm64/fp/fp-ptrace.c
@@ -1071,7 +1071,7 @@ static bool sve_write_supported(struct test_config *config)
 
 static bool sve_write_fpsimd_supported(struct test_config *config)
 {
-	if (!sve_supported())
+	if (!sve_supported() && !sme_supported())
 		return false;
 
 	if ((config->svcr_in & SVCR_ZA) != (config->svcr_expected & SVCR_ZA))
@@ -1231,9 +1231,6 @@ static void sve_write_fpsimd(pid_t child, struct test_config *config)
 	vl = vl_expected(config);
 	vq = __sve_vq_from_vl(vl);
 
-	if (!vl)
-		return;
-
 	iov.iov_len = SVE_PT_SIZE(vq, SVE_PT_REGS_FPSIMD);
 	iov.iov_base = malloc(iov.iov_len);
 	if (!iov.iov_base) {

-- 
2.39.5

Re: [PATCH 3/5] arm64/sme: Support disabling streaming mode via ptrace on SME only systems

[Randy Dunlap]

On 8/20/25 11:29 AM, Mark Brown wrote:
> diff --git a/Documentation/arch/arm64/sve.rst b/Documentation/arch/arm64/sve.rst
> index 28152492c29c..a61c9d0efe4d 100644
> --- a/Documentation/arch/arm64/sve.rst
> +++ b/Documentation/arch/arm64/sve.rst
> @@ -402,6 +402,11 @@ The regset data starts with struct user_sve_header, containing:
>    streaming mode and any SETREGSET of NT_ARM_SSVE will enter streaming mode
>    if the target was not in streaming mode.
>  
> +* On systems that do not support SVE it is permitted to use SETREGSET to
> +  write SVE_PT_REGS_FPSIMD formatted data via NT_ARM_SVE, in this case the

s/,/;/
or
                                                 NT_ARM_SVE. In this case

> +  vector length should be specified as 0. This allows streaming mode to be
> +  disabled on systems with SME but not SVE.
> +
>  * If any register data is provided along with SVE_PT_VL_ONEXEC then the
>    registers data will be interpreted with the current vector length, not
>    the vector length configured for use on exec.

-- 
~Randy