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

[Will Deacon]

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 <w...@kernel.org>
---
 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