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ftrace: Introduce ftrace_need_init_nop()

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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: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: https://lore.kernel.org/r/20210728212546.128248-2-iii@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
This commit is contained in:
Ilya Leoshkevich 2021-07-28 23:25:45 +02:00 committed by Heiko Carstens
parent d80d3ea64e
commit 67ccddf866
2 changed files with 18 additions and 2 deletions
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16
include/linux/ftrace.h
View File

@ -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

4
kernel/trace/ftrace.c
View File

@ -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++;