tracing: Add __tracepoint_string() to export string pointers

There are several tracepoints (mostly in RCU), that reference a string
pointer and uses the print format of "%s" to display the string that
exists in the kernel, instead of copying the actual string to the
ring buffer (saves time and ring buffer space).

But this has an issue with userspace tools that read the binary buffers
that has the address of the string but has no access to what the string
itself is. The end result is just output that looks like:

 rcu_dyntick:          ffffffff818adeaa 1 0
 rcu_dyntick:          ffffffff818adeb5 0 140000000000000
 rcu_dyntick:          ffffffff818adeb5 0 140000000000000
 rcu_utilization:      ffffffff8184333b
 rcu_utilization:      ffffffff8184333b

The above is pretty useless when read by the userspace tools. Ideally
we would want something that looks like this:

 rcu_dyntick:          Start 1 0
 rcu_dyntick:          End 0 140000000000000
 rcu_dyntick:          Start 140000000000000 0
 rcu_callback:         rcu_preempt rhp=0xffff880037aff710 func=put_cred_rcu 0/4
 rcu_callback:         rcu_preempt rhp=0xffff880078961980 func=file_free_rcu 0/5
 rcu_dyntick:          End 0 1

The trace_printk() which also only stores the address of the string
format instead of recording the string into the buffer itself, exports
the mapping of kernel addresses to format strings via the printk_format
file in the debugfs tracing directory.

The tracepoint strings can use this same method and output the format
to the same file and the userspace tools will be able to decipher
the address without any modification.

The tracepoint strings need its own section to save the strings because
the trace_printk section will cause the trace_printk() buffers to be
allocated if anything exists within the section. trace_printk() is only
used for debugging and should never exist in the kernel, we can not use
the trace_printk sections.

Add a new tracepoint_str section that will also be examined by the output
of the printk_format file.

Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 19 insertions, 0 deletions

diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
index a9077c1b4ad3..2900817ba65c 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -244,12 +244,31 @@ static const char **find_next(void *v, loff_t *pos)
 {
         const char **fmt = v;
         int start_index;
+        int last_index;
 
         start_index = __stop___trace_bprintk_fmt - __start___trace_bprintk_fmt;
 
         if (*pos < start_index)
                 return __start___trace_bprintk_fmt + *pos;
 
+        /*
+         * The __tracepoint_str section is treated the same as the
+         * __trace_printk_fmt section. The difference is that the
+         * __trace_printk_fmt section should only be used by trace_printk()
+         * in a debugging environment, as if anything exists in that section
+         * the trace_prink() helper buffers are allocated, which would just
+         * waste space in a production environment.
+         *
+         * The __tracepoint_str sections on the other hand are used by
+         * tracepoints which need to map pointers to their strings to
+         * the ASCII text for userspace.
+         */
+        last_index = start_index;
+        start_index = __stop___tracepoint_str - __start___tracepoint_str;
+
+        if (*pos < last_index + start_index)
+                return __start___tracepoint_str + (*pos - last_index);
+
         return find_next_mod_format(start_index, v, fmt, pos);
 }