linux-kernel - Re: [PATCH v2 2/8] powerpc/module: Only try to generate the ftrace

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Date:	Tue, 1 Mar 2016 15:37:39 +0530
From:	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	linuxppc-dev@...abs.org, bsingharora@...il.com, duwe@....de,
	linux-kernel@...r.kernel.org, rostedt@...dmis.org,
	pmladek@...e.com, jeyu@...hat.com, jkosina@...e.cz,
	live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH v2 2/8] powerpc/module: Only try to generate the
 ftrace_caller() stub once

* Michael Ellerman <mpe@...erman.id.au> [2016-02-29 20:26:23]:

> Currently we generate the module stub for ftrace_caller() at the bottom
> of apply_relocate_add(). However apply_relocate_add() is potentially
> called more than once per module, which means we will try to generate
> the ftrace_caller() stub multiple times.
> 
> Although the current code deals with that correctly, ie. it only
> generates a stub the first time, it would be clearer to only try to
> generate the stub once.
> 
> Note also on first reading it may appear that we generate a different
> stub for each section that requires relocation, but that is not the
> case. The code in stub_for_addr() that searches for an existing stub
> uses sechdrs[me->arch.stubs_section], ie. the single stub section for
> this module.
> 
> A cleaner approach is to only generate the ftrace_caller() stub once,
> from module_finalize(). Although the original code didn't check to see
> if the stub was actually generated correctly, it seems prudent to add a
> check, so do that. And an additional benefit is we can clean the ifdefs
> up a little.
> 
> Finally we must propagate the const'ness of some of the pointers passed
> to module_finalize(), but that is also an improvement.
> 
> Reviewed-by: Balbir Singh <bsingharora@...il.com>
> Reviewed-by: Torsten Duwe <duwe@...e.de>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> ---


with !CONFIG_DYNAMIC_FTRACE, build breaks

arch/powerpc/kernel/module_64.c:491:13: error: âsquash_toc_save_instâ used but never defined [-Werror]
 static void squash_toc_save_inst(const char *name, unsigned long addr);
             ^
cc1: all warnings being treated as errors

moving squash_toc_save_inst() definition to #else part
of #ifdef CONFIG_DYNAMIC_FTRACE helps.

 arch/powerpc/kernel/module_64.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index c9c75ceb7ed3..def0b9a013c9 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -774,8 +774,6 @@ static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module
 	return (unsigned long)entry;
 }
 #else
-static void squash_toc_save_inst(const char *name, unsigned long addr) { }
-
 static unsigned long create_ftrace_stub(const Elf64_Shdr *sechdrs, struct module *me)
 {
 	return stub_for_addr(sechdrs, (unsigned long)ftrace_caller, me);
@@ -792,4 +790,8 @@ int module_finalize_ftrace(struct module *mod, const Elf_Shdr *sechdrs)
 
 	return 0;
 }
+
+#else
+static void squash_toc_save_inst(const char *name, unsigned long addr) { }
+
 #endif
-- 
2.6.0.rc1