[PATCH v5 12/15] 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.

Reviewed-by: Quentin Perret <qperret at google.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/kernel/process.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 9a2532d848f0..da313b738c7c 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -633,8 +633,20 @@ void arch_setup_new_exec(void)
 
 	if (is_compat_task()) {
 		mmflags = MMCF_AARCH32;
-		if (static_branch_unlikely(&arm64_mismatched_32bit_el0))
+
+		/*
+		 * Restrict the CPU affinity mask for a 32-bit task so that
+		 * it contains only 32-bit-capable CPUs.
+		 *
+		 * 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(), although we try a bit harder to
+		 * honour the cpuset hierarchy.
+		 */
+		if (static_branch_unlikely(&arm64_mismatched_32bit_el0)) {
+			force_compatible_cpus_allowed_ptr(current);
 			set_tsk_thread_flag(current, TIF_NOTIFY_RESUME);
+		}
 	}
 
 	current->mm->context.flags = mmflags;
-- 
2.29.2.576.ga3fc446d84-goog