arm64/ptrace: Clarify documentation of VL configuration via ptrace

When we configure SVE, SSVE or ZA via ptrace we allow the user to configure
the vector length and specify any of the flags that are accepted when
configuring via prctl(). This includes the S[VM]E_SET_VL_ONEXEC flag which
defers the configuration of the VL until an exec(). We don't do anything to
limit the provision of register data as part of configuring the _ONEXEC VL
but as a function of the VL enumeration support we do this will be
interpreted using the vector length currently configured for the process.

This is all a bit surprising, and probably we should just not have allowed
register data to be specified with _ONEXEC, but it's our ABI so let's
add some explicit documentation in both the ABI documents and the source
calling out what happens.

The comments are also missing the fact that since SME does not have a
mandatory 128 bit VL it is possible for VL enumeration to result in the
configuration of a higher VL than was requested, cover that too.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241106-arm64-sve-ptrace-vl-set-v1-1-3b164e8b559c@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: broonie

Documentation/arch/arm64/sme.rst

@@ -346,6 +346,10 @@ The regset data starts with struct user_za_header, containing:
 
 * Writes to NT_ARM_ZT will set PSTATE.ZA to 1.
 
+* If any register data is provided along with SME_PT_VL_ONEXEC then the
+  registers data will be interpreted with the current vector length, not
+  the vector length configured for use on exec.
+
 
 8.  ELF coredump extensions
 ---------------------------

Documentation/arch/arm64/sve.rst

@@ -402,6 +402,10 @@ 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.
 
+* 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.
+
 * The effect of writing a partial, incomplete payload is unspecified.
 
 

arch/arm64/kernel/ptrace.c

@@ -898,7 +898,11 @@ static int sve_set_common(struct task_struct *target,
 	if (ret)
 		goto out;
 
-	/* Actual VL set may be less than the user asked for: */
+	/*
+	 * Actual VL set may be different from what the user asked
+	 * for, or we may have configured the _ONEXEC VL not the
+	 * current VL:
+	 */
 	vq = sve_vq_from_vl(task_get_vl(target, type));
 
 	/* Enter/exit streaming mode */
@@ -1125,7 +1129,11 @@ static int za_set(struct task_struct *target,
 	if (ret)
 		goto out;
 
-	/* Actual VL set may be less than the user asked for: */
+	/*
+	 * Actual VL set may be different from what the user asked
+	 * for, or we may have configured the _ONEXEC rather than
+	 * current VL:
+	 */
 	vq = sve_vq_from_vl(task_get_sme_vl(target));
 
 	/* Ensure there is some SVE storage for streaming mode */