Hi,
On Wed, Jan 17 2024, Alexander Graf wrote:
> We want to be able to transfer ftrace state from one kernel to the next.
> To start off with, let's establish all the boiler plate to get a write
> hook when KHO wants to serialize and fill out basic data.
>
> Follow-up patches will fill in serialization of ring buffers and events.
>
> Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>
>
> ---
>
> v1 -> v2:
>
> - Remove ifdefs
> ---
> kernel/trace/trace.c | 47 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 47 insertions(+)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index a0defe156b57..9a0d96975c9c 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -32,6 +32,7 @@
> #include <linux/percpu.h>
> #include <linux/splice.h>
> #include <linux/kdebug.h>
> +#include <linux/kexec.h>
> #include <linux/string.h>
> #include <linux/mount.h>
> #include <linux/rwsem.h>
> @@ -866,6 +867,8 @@ static struct tracer *trace_types __read_mostly;
> */
> DEFINE_MUTEX(trace_types_lock);
>
> +static bool trace_in_kho;
> +
> /*
> * serialize the access of the ring buffer
> *
> @@ -10574,12 +10577,56 @@ void __init early_trace_init(void)
> init_events();
> }
>
> +static int trace_kho_notifier(struct notifier_block *self,
> + unsigned long cmd,
> + void *v)
> +{
> + const char compatible[] = "ftrace-v1";
> + void *fdt = v;
> + int err = 0;
> +
> + switch (cmd) {
> + case KEXEC_KHO_ABORT:
> + if (trace_in_kho)
> + mutex_unlock(&trace_types_lock);
> + trace_in_kho = false;
> + return NOTIFY_DONE;
> + case KEXEC_KHO_DUMP:
> + /* Handled below */
> + break;
> + default:
> + return NOTIFY_BAD;
> + }
> +
> + if (unlikely(tracing_disabled))
> + return NOTIFY_DONE;
> +
> + err |= fdt_begin_node(fdt, "ftrace");
> + err |= fdt_property(fdt, "compatible", compatible, sizeof(compatible));
> + err |= fdt_end_node(fdt);
> +
> + if (!err) {
> + /* Hold all future allocations */
> + mutex_lock(&trace_types_lock);
Say I do "echo 1 | tee /sys/kernel/kho/active". Then the lock is held by
tee, which exits. Then I later I do "echo 0 | tee
/sys/kernel/kho/active". This time another tee task unlocks the lock. So
it is not being unlocked by the same task that locked it. The comment
above mutex_lock() definition says:
The mutex must later on be released by the same task that acquired
it. Recursive locking is not allowed. The task may not exit without
first unlocking the mutex.
I tested your code and it happens to work because the unlock always
happened to take the fast path which does not sanity-check the owner.
Still, this is not the correct thing to do.
> + trace_in_kho = true;
> + }
> +
> + return err ? NOTIFY_BAD : NOTIFY_DONE;
> +}
> +
> +static struct notifier_block trace_kho_nb = {
> + .notifier_call = trace_kho_notifier,
> +};
> +
> void __init trace_init(void)
> {
> trace_event_init();
>
> if (boot_instance_index)
> enable_instances();
> +
> + if (IS_ENABLED(CONFIG_FTRACE_KHO))
> + register_kho_notifier(&trace_kho_nb);
> }
>
> __init static void clear_boot_tracer(void)
--
Regards,
Pratyush Yadav
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879
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