[RFC][PATCH 00/16] objtool: vmlinux.o and noinstr validation

[Peter Zijlstra <peterz@infradead.org>]

Hi all,

These patches extend objtool to be able to run on vmlinux.o and validate
Thomas's proposed noinstr annotation:

  https://lkml.kernel.org/r/20200310170951.87c29e9c1cfbddd93ccd92b3@kernel.org

 "That's why we want the sections and the annotation. If something calls
  out of a noinstr section into a regular text section and the call is not
  annotated at the call site, then objtool can complain and tell you. What
  Peter and I came up with looks like this:

  noinstr foo()
	do_protected(); <- Safe because in the noinstr section
	instr_begin();  <- Marks the begin of a safe region, ignored
			   by objtool
	do_stuff();     <- All good
	instr_end();    <- End of the safe region. objtool starts
			   looking again
	do_other_stuff();  <- Unsafe because do_other_stuff() is
			      not protected

  and:

  noinstr do_protected()
	bar();          <- objtool will complain here
  "

It should be accompanied by something like the below; which you'll find in a
series by Thomas.

---  

--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -53,6 +53,9 @@ extern char __ctors_start[], __ctors_end
 /* Start and end of .opd section - used for function descriptors. */
 extern char __start_opd[], __end_opd[];
 
+/* Start and end of instrumentation protected text section */
+extern char __noinstr_text_start[], __noinstr_text_end[];
+
 extern __visible const void __nosave_begin, __nosave_end;
 
 /* Function descriptor handling (if any).  Override in asm/sections.h */
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -550,6 +550,10 @@
 #define TEXT_TEXT							\
 		ALIGN_FUNCTION();					\
 		*(.text.hot TEXT_MAIN .text.fixup .text.unlikely)	\
+		ALIGN_FUNCTION();					\
+		__noinstr_text_start = .;				\
+		*(.noinstr.text)					\
+		__noinstr_text_end = .;					\
 		*(.text..refcount)					\
 		*(.ref.text)						\
 	MEM_KEEP(init.text*)						\
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -120,12 +120,37 @@ void ftrace_likely_update(struct ftrace_
 /* Annotate a C jump table to allow objtool to follow the code flow */
 #define __annotate_jump_table __section(.rodata..c_jump_table)
 
+/* Begin/end of an instrumentation safe region */
+#define instr_begin() ({						\
+	asm volatile("%c0:\n\t"						\
+		     ".pushsection .discard.instr_begin\n\t"		\
+		     ".long %c0b - .\n\t"				\
+		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+})
+
+#define instr_end() ({							\
+	asm volatile("%c0:\n\t"						\
+		     ".pushsection .discard.instr_end\n\t"		\
+		     ".long %c0b - .\n\t"				\
+		     ".popsection\n\t" : : "i" (__COUNTER__));		\
+})
+
 #else
 #define annotate_reachable()
 #define annotate_unreachable()
 #define __annotate_jump_table
+#define instr_begin()		do { } while(0)
+#define instr_end()		do { } while(0)
 #endif
 
+#define INSTR(expr) ({			\
+	typeof(({ expr; })) __ret;	\
+	instr_begin();			\
+	__ret = ({ expr; });		\
+	instr_end();			\
+	__ret;				\
+})
+
 #ifndef ASM_UNREACHABLE
 # define ASM_UNREACHABLE
 #endif
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -118,6 +118,11 @@ struct ftrace_likely_data {
 #define notrace			__attribute__((__no_instrument_function__))
 #endif
 
+/* Section for code which can't be instrumented at all */
+#define noinstr								\
+	notrace __attribute((__section__(".noinstr.text")))
+
+
 /*
  * it doesn't make sense on ARM (currently the only user of __naked)
  * to trace naked functions because then mcount is called without
--- a/scripts/mod/modpost.c
+++ b/scripts/mod/modpost.c
@@ -953,7 +953,7 @@ static void check_section(const char *mo
 
 #define DATA_SECTIONS ".data", ".data.rel"
 #define TEXT_SECTIONS ".text", ".text.unlikely", ".sched.text", \
-		".kprobes.text", ".cpuidle.text"
+		".kprobes.text", ".cpuidle.text", ".noinstr.text"
 #define OTHER_TEXT_SECTIONS ".ref.text", ".head.text", ".spinlock.text", \
 		".fixup", ".entry.text", ".exception.text", ".text.*", \
 		".coldtext"