On Fri, 22 Jul 2022 at 12:11, Will Deacon [off-list ref] wrote:

quoted

quoted

quoted

[adding Will]

On Mon, Jul 04, 2022 at 05:05:01PM +0200, Marco Elver wrote:

quoted

Add KUnit test for hw_breakpoint constraints accounting, with various
interesting mixes of breakpoint targets (some care was taken to catch
interesting corner cases via bug-injection).

The test cannot be built as a module because it requires access to
hw_breakpoint_slots(), which is not inlinable or exported on all
architectures.

Signed-off-by: Marco Elver <elver@google.com>

As mentioned on IRC, I'm seeing these tests fail on arm64 when applied atop
v5.19-rc7:

| TAP version 14
| 1..1
|     # Subtest: hw_breakpoint
|     1..9
|     ok 1 - test_one_cpu
|     ok 2 - test_many_cpus
|     # test_one_task_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 3 - test_one_task_on_all_cpus
|     # test_two_tasks_on_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 4 - test_two_tasks_on_all_cpus
|     # test_one_task_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 5 - test_one_task_on_one_cpu
|     # test_one_task_mixed: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 6 - test_one_task_mixed
|     # test_two_tasks_on_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 7 - test_two_tasks_on_one_cpu
|     # test_two_tasks_on_one_all_cpus: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 8 - test_two_tasks_on_one_all_cpus
|     # test_task_on_all_and_one_cpu: ASSERTION FAILED at kernel/events/hw_breakpoint_test.c:70
|     Expected IS_ERR(bp) to be false, but is true
|     not ok 9 - test_task_on_all_and_one_cpu
| # hw_breakpoint: pass:2 fail:7 skip:0 total:9
| # Totals: pass:2 fail:7 skip:0 total:9

... which seems to be becasue arm64 currently forbids per-task
breakpoints/watchpoints in hw_breakpoint_arch_parse(), where we have:

        /*
         * Disallow per-task kernel breakpoints since these would
         * complicate the stepping code.
         */
        if (hw->ctrl.privilege == AARCH64_BREAKPOINT_EL1 && bp->hw.target)
                return -EINVAL;

... which has been the case since day one in commit:

  478fcb2cdb2351dc ("arm64: Debugging support")

I'm not immediately sure what would be necessary to support per-task kernel
breakpoints, but given a lot of that state is currently per-cpu, I imagine it's
invasive.

I would actually like to remove HW_BREAKPOINT completely for arm64 as it
doesn't really work and causes problems for other interfaces such as ptrace
and kgdb.

Will it be a localized removal of code that will be easy to revert in
future? Or will it touch lots of code here and there?
Let's say we come up with a very important use case for HW_BREAKPOINT
and will need to make it work on arm64 as well in future.

My (rough) plan is to implement a lower-level abstraction for handling the
underlying hardware resources, so we can layer consumers on top of that
instead of funneling through hw_breakpoint. So if we figure out how to make
bits of hw_breakpoint work on arm64, then it should just go on top.

The main pain point for hw_breakpoint is kernel-side {break,watch}points
and I think there are open design questions about how they should work
on arm64, particularly when considering the interaction with user
watchpoints triggering on uaccess routines and the possibility of hitting
a kernel watchpoint in irq context.

I see. Our main interest would be break/watchpoints on user addresses
firing from both user-space and kernel (uaccess), so at least on irqs.