From: Nick Desaulniers ndesaulniers@google.com
__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/20200730224555.2142154-2-ndesaulniers@google.com
Cc: Ingo Molnar mingo@redhat.com Cc: Miguel Ojeda miguel.ojeda.sandonis@gmail.com Cc: stable@vger.kernel.org Fixes: 102c9323c35a8 ("tracing: Add __tracepoint_string() to export string pointers") 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 Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- 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