[PATCH v9 17/20] arm64: Advertise CPUs capable of running 32-bit applications in sysfs

Will Deacon
2021-06-08T18:04:53Z

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.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
.../ABI/testing/sysfs-devices-system-cpu | 9 +++++++++
arch/arm64/kernel/cpufeature.c | 19 +++++++++++++++++++
2 files changed, 28 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index fe13baa53c59..899377b2715a 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -494,6 +494,15 @@ 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: May 2021
+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 present, the same format as
+ /sys/devices/system/cpu/{offline,online,possible,present} is used.
+ 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 <linux-kernel@vger.kernel.org>
diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
index 55a58933e3b2..854ade5b1de1 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/minmax.h>
#include <linux/mm.h>
@@ -1297,6 +1298,24 @@ const struct cpumask *system_32bit_el0_cpumask(void)
return cpu_possible_mask;
}

+static ssize_t aarch32_el0_show(struct device *dev,
+ struct device_attribute *attr, char *buf)
+{
+ const struct cpumask *mask = system_32bit_el0_cpumask();
+
+ return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(mask));
+}
+static const DEVICE_ATTR_RO(aarch32_el0);
+
+static int __init aarch32_el0_sysfs_init(void)
+{
+ if (!allow_mismatched_32bit_el0)
+ return 0;
+
+ return device_create_file(cpu_subsys.dev_root, &dev_attr_aarch32_el0);
+}
+device_initcall(aarch32_el0_sysfs_init);
+
static bool has_32bit_el0(const struct arm64_cpu_capabilities *entry, int scope)
{
if (!has_cpuid_feature(entry, scope))
--
2.32.0.rc1.229.g3e70b5a671-goog