[PATCH 1/2] tracepoint: mark __tracepoint_string's __used

[Nick Desaulniers]

__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.

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Tim Murray <timmurray@xxxxxxxxxx>
Reported-by: Simon MacMullen <simonmacm@xxxxxxxxxx>
Suggested-by: Greg Hackmann <ghackmann@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
We observe this in Clang; it seems that GCC doesn't do the "cleanup" of
the dead address.

Specifically, the Clang passes "Interprocedural Sparse Conditional
Constant Propagation" (IPSCCP) and GlobalOpt both try to removed the
address if no other uses exist after inlining the reference directly to
the string data.

We don't want to change the linkage of these variables, but we kind of
want optimization behavior to treat these function static strings as if
they had `extern` linkage, at least by not removing the address of the
string data from the custom section.

 include/linux/tracepoint.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index a1fecf311621..3a5b717d92e8 100644
--- a/include/linux/tracepoint.h
+++ b/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
-- 
2.28.0.rc0.105.gf9edc3c819-goog