[PATCH 0.5/3] livepatch: Convert stack entries array to percpu

[Josh Poimboeuf]

On Fri, Mar 03, 2023 at 03:00:13PM +0100, Petr Mladek wrote:
> > MAX_STACK_ENTRIES is 100, which seems excessive.  If we halved that, the
> > array would be "only" 400 bytes, which is *almost* reasonable to
> > allocate on the stack?
> 
> It is just for the stack in the process context. Right?
> 
> I think that I have never seen a stack with over 50 entries. And in
> the worst case, a bigger amount of entries would "just" result in
> a non-reliable stack which might be acceptable.
> 
> It looks acceptable to me.
> 
> > Alternatively we could have a percpu entries array... :-/
> 
> That said, percpu entries would be fine as well. It sounds like
> a good price for the livepatching feature. I think that livepatching
> is used on big systems anyway.
> 
> I slightly prefer the per-cpu solution.

Booting a kernel with PREEMPT+LOCKDEP gave me a high-water mark of 60+
stack entries, seen when probing a device.  I decided not to mess with
MAX_STACK_ENTRIES, and instead just convert the entries to percpu.  This
patch could be inserted at the beginning of the set.

---8<---

Subject: [PATCH 0.5/3] livepatch: Convert stack entries array to percpu

The entries array in klp_check_stack() is static local because it's too
big to be reasonably allocated on the stack.  Serialized access is
enforced by the klp_mutex.

In preparation for calling klp_check_stack() without the mutex (from
cond_resched), convert it to a percpu variable.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 kernel/livepatch/transition.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c
index f1b25ec581e0..135fc73e2e5d 100644
--- a/kernel/livepatch/transition.c
+++ b/kernel/livepatch/transition.c
@@ -14,6 +14,8 @@
 #include "transition.h"
 
 #define MAX_STACK_ENTRIES  100
+DEFINE_PER_CPU(unsigned long[MAX_STACK_ENTRIES], klp_stack_entries);
+
 #define STACK_ERR_BUF_SIZE 128
 
 #define SIGNALS_TIMEOUT 15
@@ -240,12 +242,15 @@ static int klp_check_stack_func(struct klp_func *func, unsigned long *entries,
  */
 static int klp_check_stack(struct task_struct *task, const char **oldname)
 {
-	static unsigned long entries[MAX_STACK_ENTRIES];
+	unsigned long *entries = this_cpu_ptr(klp_stack_entries);
 	struct klp_object *obj;
 	struct klp_func *func;
 	int ret, nr_entries;
 
-	ret = stack_trace_save_tsk_reliable(task, entries, ARRAY_SIZE(entries));
+	/* Protect 'klp_stack_entries' */
+	lockdep_assert_preemption_disabled();
+
+	ret = stack_trace_save_tsk_reliable(task, entries, MAX_STACK_ENTRIES);
 	if (ret < 0)
 		return -EINVAL;
 	nr_entries = ret;
-- 
2.39.2