Since 32-bit applications will be killed if they are caught trying to
execute on a 64-bit-only CPU in a mismatched system, advertise the set
of 32-bit capable CPUs to userspace in sysfs.

Signed-off-by: Will Deacon <will@kernel.org>
---
 .../ABI/testing/sysfs-devices-system-cpu      |  8 ++++++++
 arch/arm64/kernel/cpufeature.c                | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index b555df825447..19893fb8e870 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -472,6 +472,14 @@ Description:	AArch64 CPU registers
 		'identification' directory exposes the CPU ID registers for
 		 identifying model and revision of the CPU.
 
+What:		/sys/devices/system/cpu/aarch32_el0
+Date:		October 2020
+Contact:	Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
+Description:	Identifies the subset of CPUs in the system that can execute
+		AArch32 (32-bit ARM) applications. If absent, then all or none
+		of the CPUs can execute AArch32 applications and execve() will
+		behave accordingly.

+
 What:		/sys/devices/system/cpu/cpu#/cpu_capacity
 Date:		December 2016
 Contact:	Linux kernel mailing list [off-list ref]

diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 2e2219cbd54c..9f29d4d1ef7e 100644
--- a/arch/arm64/kernel/cpufeature.c
+++ b/arch/arm64/kernel/cpufeature.c

@@ -67,6 +67,7 @@
 #include <linux/crash_dump.h>
 #include <linux/sort.h>
 #include <linux/stop_machine.h>
+#include <linux/sysfs.h>
 #include <linux/types.h>
 #include <linux/mm.h>
 #include <linux/cpu.h>

@@ -1236,6 +1237,24 @@ bool system_has_mismatched_32bit_el0(void)
 	return fld == ID_AA64PFR0_EL0_64BIT_ONLY;
 }
 
+static ssize_t aarch32_el0_show(struct kobject *kobj,
+				struct kobj_attribute *attr, char *buf)
+{
+	const struct cpumask *mask = system_32bit_el0_cpumask();
+	return sprintf(buf, "%*pbl\n", cpumask_pr_args(mask));