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SubjectRe: [PATCH v2] tracing, printk: Force no hashing when trace_printk() is used
On Wed, 4 Apr 2018 11:34:55 +0900
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> wrote:

> On (04/03/18 18:03), Steven Rostedt wrote:
> >
> > > he'd want you to change all the trace_printk()s to %px with
> > > justifications, though.
> >
> > What trace_printk()s do you want to change? They are throw away
> > functions. trace_printk() is not something that stays in the kernel.
> > It's added during debugging and then removed before submitting what you
> > are working on upstream.
>
> Seems that your patch also can fix up bpf_trace_printk() - it doesn't
> even support %px, only hashed %p, which it passes to __trace_printk().
>

Crap, that should not work, but it does. I need to update the code to
prevent that from happening. I don't want any user space code to be
able to enable this.

Something like this will even prevent modules from disabling the printk
hash...

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 2a1d01666dd2..c4af083742b0 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -2834,6 +2834,14 @@ static int alloc_percpu_trace_buffer(void)

static int buffers_allocated;

+/* Will become read only after boot */
+static const bool disable_printk_hash;
+
+static void set_printk_hash(bool val)
+{
+ *(bool *)&disable_printk_hash = val;
+}
+
void trace_printk_init_buffers(void)
{
if (buffers_allocated)
@@ -2864,9 +2872,11 @@ void trace_printk_init_buffers(void)

buffers_allocated = 1;

- /* This is a debug kernel, allow pointers to be shown */
- static_branch_enable(&trace_debug);
- kptr_restrict = 0;
+ if (disable_printk_hash) {
+ /* This is a debug kernel, allow pointers to be shown */
+ static_branch_enable(&trace_debug);
+ kptr_restrict = 0;
+ }

/*
* trace_printk_init_buffers() can be called by modules.
@@ -8456,10 +8466,13 @@ __init static int tracer_alloc_buffers(void)
if (!alloc_cpumask_var(&global_trace.tracing_cpumask, GFP_KERNEL))
goto out_free_buffer_mask;

+ /* Only trace_printk() in the core kernel can disable hashing */
+ set_printk_hash(true);
/* Only allocate trace_printk buffers if a trace_printk exists */
if (__stop___trace_bprintk_fmt != __start___trace_bprintk_fmt)
/* Must be called before global_trace.buffer is allocated */
trace_printk_init_buffers();
+ set_printk_hash(false);

/* To save memory, keep the ring buffer size to its minimum */
if (ring_buffer_expanded)


-- Steve

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