arm64: Advertise CPUs capable of running 32-bit applications in sysfs

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210730112443.23245-14-will@kernel.org

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 160b10c..69edbd9 100644 (file)
--- 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 0af5845..0a9bc7e 100644 (file)
--- 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>
@@ -1321,6 +1322,24 @@ 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))