On Thu, Nov 19, 2020 at 11:06:04AM +0000, Will Deacon wrote:

On Thu, Nov 19, 2020 at 09:24:07AM +0000, Quentin Perret wrote:

quoted

On Friday 13 Nov 2020 at 09:37:13 (+0000), Will Deacon wrote:

quoted

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@kernel.org>
---
 arch/arm64/kernel/process.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 1540ab0fbf23..17b94007fed4 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c

@@ -625,6 +625,16 @@ unsigned long arch_align_stack(unsigned long sp)
 	return sp & ~0xf;
 }
 
+static void adjust_compat_task_affinity(struct task_struct *p)
+{
+	const struct cpumask *mask = system_32bit_el0_cpumask();
+
+	if (restrict_cpus_allowed_ptr(p, mask))
+		set_cpus_allowed_ptr(p, mask);

My understanding of this call to set_cpus_allowed_ptr() is that you're
mimicking the hotplug vs affinity case behaviour in some ways. That is,
if a task is pinned to a CPU and userspace hotplugs that CPU, then the
kernel will reset the affinity of the task to the remaining online CPUs.

Correct. It looks to the 32-bit application like all the 64-bit-only CPUs
were hotplugged off at the point of the execve().

This doesn't respect cpusets though :/

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