__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@vger.kernel.org Reported-by: Tim Murray timmurray@google.com Reported-by: Simon MacMullen simonmacm@google.com Suggested-by: Greg Hackmann ghackmann@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com --- 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