Re: [PATCH v1 2/8] tracing/ftrace: guard syscall probe with preempt_notrace

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-10-04 15:26, Steven Rostedt wrote:
> On Thu, 3 Oct 2024 21:33:16 -0400
> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> 
>> On 2024-10-04 03:04, Steven Rostedt wrote:
>>> On Thu, 3 Oct 2024 20:26:29 -0400
>>> Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
>>>
>>>    
>>>> static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
>>>> {
>>>>            struct trace_array *tr = data;
>>>>            struct trace_event_file *trace_file;
>>>>            struct syscall_trace_enter *entry;
>>>>            struct syscall_metadata *sys_data;
>>>>            struct trace_event_buffer fbuffer;
>>>>            unsigned long args[6];
>>>>            int syscall_nr;
>>>>            int size;
>>>>
>>>>            syscall_nr = trace_get_syscall_nr(current, regs);
>>>>            if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
>>>>                    return;
>>>>
>>>>            /* Here we're inside tp handler's rcu_read_lock_sched (__DO_TRACE) */
>>>>            trace_file = rcu_dereference_sched(tr->enter_syscall_files[syscall_nr]);
>>>>
>>>> ^^^^ this function explicitly states that preempt needs to be disabled by
>>>> tracepoints.
>>>
>>> Ah, I should have known it was the syscall portion. I don't care for this
>>> hidden dependency. I rather add a preempt disable here and not expect it to
>>> be disabled when called.
>>
>> Which is exactly what this patch is doing.
> 
> I was thinking of putting the protection in the function and not the macro.

I'm confused by your comment. The protection is added to the function here:

diff --git a/kernel/trace/trace_syscalls.c b/kernel/trace/trace_syscalls.c
index 67ac5366f724..ab4db8c23f36 100644
--- a/kernel/trace/trace_syscalls.c
+++ b/kernel/trace/trace_syscalls.c
@@ -299,6 +299,12 @@ static void ftrace_syscall_enter(void *data, struct pt_regs *regs, long id)
         int syscall_nr;
         int size;
  
+       /*
+        * Syscall probe called with preemption enabled, but the ring
+        * buffer and per-cpu data require preemption to be disabled.
+        */
+       guard(preempt_notrace)();
+
         syscall_nr = trace_get_syscall_nr(current, regs);
         if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
                 return;
@@ -338,6 +344,12 @@ static void ftrace_syscall_exit(void *data, struct pt_regs *regs, long ret)
         struct trace_event_buffer fbuffer;
         int syscall_nr;
  
+       /*
+        * Syscall probe called with preemption enabled, but the ring
+        * buffer and per-cpu data require preemption to be disabled.
+        */
+       guard(preempt_notrace)();
+
         syscall_nr = trace_get_syscall_nr(current, regs);
         if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
                 return;

(I'll answer to the rest of your message in a separate email)

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com