Commit 67ccddf8 authored by Ilya Leoshkevich's avatar Ilya Leoshkevich Committed by Heiko Carstens
Browse files

ftrace: Introduce ftrace_need_init_nop() 



Implementing live patching on s390 requires each function's prologue to
contain a very special kind of nop, which gcc and clang don't generate.
However, the current code assumes that if CC_USING_NOP_MCOUNT is
defined, then whatever the compiler generates is good enough.

Move the CC_USING_NOP_MCOUNT check into the new ftrace_need_init_nop()
macro, that the architectures can override.

An alternative solution is to disable using -mnop-mcount in the
Makefile, however, this makes the build logic (even) more complicated
and forces the arch-specific code to deal with the useless __fentry__
symbol.

Signed-off-by: default avatarIlya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20210728212546.128248-2-iii@linux.ibm.com


Signed-off-by: default avatarHeiko Carstens <hca@linux.ibm.com>
parent d80d3ea6

include/linux/ftrace.h

+16 −0
Original line number Diff line number Diff line
@@ -643,6 +643,22 @@ static inline int ftrace_disable_ftrace_graph_caller(void) { return 0; } 
extern int ftrace_make_nop(struct module *mod,

			   struct dyn_ftrace *rec, unsigned long addr);



/**

 * ftrace_need_init_nop - return whether nop call sites should be initialized

 *

 * Normally the compiler's -mnop-mcount generates suitable nops, so we don't

 * need to call ftrace_init_nop() if the code is built with that flag.

 * Architectures where this is not always the case may define their own

 * condition.

 *

 * Return must be:

 *  0	    if ftrace_init_nop() should be called

 *  Nonzero if ftrace_init_nop() should not be called

 */



#ifndef ftrace_need_init_nop

#define ftrace_need_init_nop() (!__is_defined(CC_USING_NOP_MCOUNT))

#endif



/**

 * ftrace_init_nop - initialize a nop call site

kernel/trace/ftrace.c

+2 −2
Original line number Diff line number Diff line
@@ -3100,6 +3100,7 @@ ops_references_rec(struct ftrace_ops *ops, struct dyn_ftrace *rec) 


static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs)

{

	bool init_nop = ftrace_need_init_nop();

	struct ftrace_page *pg;

	struct dyn_ftrace *p;

	u64 start, stop;
@@ -3138,8 +3139,7 @@ static int ftrace_update_code(struct module *mod, struct ftrace_page *new_pgs) 
			 * Do the initial record conversion from mcount jump

			 * to the NOP instructions.

			 */

			if (!__is_defined(CC_USING_NOP_MCOUNT) &&

			    !ftrace_nop_initialize(mod, p))

			if (init_nop && !ftrace_nop_initialize(mod, p))

				break;



			update_cnt++;