tracepoint: Mark __tracepoint_string's __used

__tracepoint_string's have their string data stored in .rodata, and an
address to that data stored in the "__tracepoint_str" section. Functions
that refer to those strings refer to the symbol of the address. Compiler
optimization can replace those address references with references
directly to the string data. If the address doesn't appear to have other
uses, then it appears dead to the compiler and is removed. This can
break the /tracing/printk_formats sysfs node which iterates the
addresses stored in the "__tracepoint_str" section.

Like other strings stored in custom sections in this header, mark these
__used to inform the compiler that there are other non-obvious users of
the address, so they should still be emitted.

Link: https://lkml.kernel.org/r/[email protected]

Cc: Ingo Molnar <[email protected]>
Cc: Miguel Ojeda <[email protected]>
Cc: [email protected]
Fixes: 102c932 ("tracing: Add __tracepoint_string() to export string pointers")
Reported-by: Tim Murray <[email protected]>
Reported-by: Simon MacMullen <[email protected]>
Suggested-by: Greg Hackmann <[email protected]>
Signed-off-by: Nick Desaulniers <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: nickdesaulniers

include/linux/tracepoint.h

@@ -361,7 +361,7 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		static const char *___tp_str __tracepoint_string = str; \
 		___tp_str;						\
 	})
-#define __tracepoint_string	__attribute__((section("__tracepoint_str")))
+#define __tracepoint_string	__attribute__((section("__tracepoint_str"), used))
 #else
 /*
  * tracepoint_string() is used to save the string address for userspace