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
