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From:   Will Deacon <will@kernel.org>
To:     linux-arm-kernel@lists.infradead.org
Cc:     linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, kernel-team@android.com
Subject: [PATCH v6 05/21] arm64: Advertise CPUs capable of running 32-bit applications in sysfs
Date:   Tue, 18 May 2021 10:47:09 +0100
Message-Id: <20210518094725.7701-6-will@kernel.org>
In-Reply-To: <20210518094725.7701-1-will@kernel.org>
References: <20210518094725.7701-1-will@kernel.org>
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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>
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 4194a47de62d..959442f76ed7 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))
-- 
2.31.1.751.gd2f1c929bd-goog