[PATCH v4 08/14] arm64: exec: Adjust affinity for compat tasks with mismatched 32-bit EL0

[Will Deacon <will@xxxxxxxxxx>]

When exec'ing a 32-bit task on a system with mismatched support for
32-bit EL0, try to ensure that it starts life on a CPU that can actually
run it.

Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 arch/arm64/kernel/process.c | 42 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 1540ab0fbf23..72116b0c7c73 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -31,6 +31,7 @@
 #include <linux/interrupt.h>
 #include <linux/init.h>
 #include <linux/cpu.h>
+#include <linux/cpuset.h>
 #include <linux/elfcore.h>
 #include <linux/pm.h>
 #include <linux/tick.h>
@@ -625,6 +626,45 @@ unsigned long arch_align_stack(unsigned long sp)
 	return sp & ~0xf;
 }
 
+static void adjust_compat_task_affinity(struct task_struct *p)
+{
+	cpumask_var_t cpuset_mask;
+	const struct cpumask *possible_mask = system_32bit_el0_cpumask();
+	const struct cpumask *newmask = possible_mask;
+
+	/*
+	 * Restrict the CPU affinity mask for a 32-bit task so that it contains
+	 * only the 32-bit-capable subset of its original CPU mask. If this is
+	 * empty, then try again with the cpuset allowed mask. If that fails,
+	 * forcefully override it with the set of all 32-bit-capable CPUs that
+	 * we know about.
+	 *
+	 * From the perspective of the task, this looks similar to what would
+	 * happen if the 64-bit-only CPUs were hot-unplugged at the point of
+	 * execve().
+	 */
+	if (!restrict_cpus_allowed_ptr(p, possible_mask))
+		goto out;
+
+	if (alloc_cpumask_var(&cpuset_mask, GFP_KERNEL)) {
+		cpuset_cpus_allowed(p, cpuset_mask);
+		if (cpumask_and(cpuset_mask, cpuset_mask, possible_mask)) {
+			newmask = cpuset_mask;
+			goto out_set_mask;
+		}
+	}
+
+	if (printk_ratelimit()) {
+		printk_deferred("Overriding affinity for 32-bit process %d (%s) to CPUs %*pbl\n",
+				task_pid_nr(p), p->comm, cpumask_pr_args(newmask));
+	}
+out_set_mask:
+	set_cpus_allowed_ptr(p, newmask);
+	free_cpumask_var(cpuset_mask);
+out:
+	set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+}
+
 /*
  * Called from setup_new_exec() after (COMPAT_)SET_PERSONALITY.
  */
@@ -635,7 +675,7 @@ void arch_setup_new_exec(void)
 	if (is_compat_task()) {
 		mmflags = MMCF_AARCH32;
 		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
-			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+			adjust_compat_task_affinity(current);
 	}
 
 	current->mm->context.flags = mmflags;
-- 
2.29.2.454.gaff20da3a2-goog