[PATCH] mm: Outline copy_overflow()

[PATCH] mm: Outline copy_overflow()

[Christophe Leroy]

While building a small config with CONFIG_CC_OPTIMISE_FOR_SIZE,
I ended up with more than 50 times the following function in vmlinux:

	c00243bc <copy_overflow>:
	c00243bc:	94 21 ff f0 	stwu    r1,-16(r1)
	c00243c0:	7c 85 23 78 	mr      r5,r4
	c00243c4:	7c 64 1b 78 	mr      r4,r3
	c00243c8:	3c 60 c0 62 	lis     r3,-16286
	c00243cc:	7c 08 02 a6 	mflr    r0
	c00243d0:	38 63 5e e5 	addi    r3,r3,24293
	c00243d4:	90 01 00 14 	stw     r0,20(r1)
	c00243d8:	4b ff 82 45 	bl      c001c61c <__warn_printk>
	c00243dc:	0f e0 00 00 	twui    r0,0
	c00243e0:	80 01 00 14 	lwz     r0,20(r1)
	c00243e4:	38 21 00 10 	addi    r1,r1,16
	c00243e8:	7c 08 03 a6 	mtlr    r0
	c00243ec:	4e 80 00 20 	blr

That function being a non conditional warning on an error path,
it is not worth inlining.

Outline it.

This reduces the size of vmlinux by almost 4kbytes.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 include/linux/thread_info.h | 5 +----
 mm/maccess.c                | 5 +++++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 73a6f34b3847..1087d1e2be5c 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -209,10 +209,7 @@ __bad_copy_from(void);
 extern void __compiletime_error("copy destination size is too small")
 __bad_copy_to(void);
 
-static inline void copy_overflow(int size, unsigned long count)
-{
-	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
-}
+void copy_overflow(int size, unsigned long count);
 
 static __always_inline __must_check bool
 check_copy_size(const void *addr, size_t bytes, bool is_source)
diff --git a/mm/maccess.c b/mm/maccess.c
index d3f1a1f0b1c1..d15c7b2324b4 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -335,3 +335,8 @@ long strnlen_user_nofault(const void __user *unsafe_addr, long count)
 
 	return ret;
 }
+
+void copy_overflow(int size, unsigned long count)
+{
+	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
+}
-- 
2.33.1

Re: [PATCH] mm: Outline copy_overflow()

[kernel test robot]

Hi Christophe,

I love your patch! Yet something to improve:

[auto build test ERROR on hnaz-mm/master]

url:    https://github.com/0day-ci/linux/commits/Christophe-Leroy/mm-Outline-copy_overflow/20220207-190441
base:   https://github.com/hnaz/linux-mm master
config: i386-randconfig-a013-20220207 (https://download.01.org/0day-ci/archive/20220207/202202072148.OAzMGrDo-lkp@intel.com/config)
compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
reproduce (this is a W=1 build):
        # https://github.com/0day-ci/linux/commit/2ae7547d08e23e529db61b3de0cb93879630a102
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Christophe-Leroy/mm-Outline-copy_overflow/20220207-190441
        git checkout 2ae7547d08e23e529db61b3de0cb93879630a102
        # save the config file to linux build tree
        mkdir build_dir
        make W=1 O=build_dir ARCH=i386 SHELL=/bin/bash

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>, old ones prefixed by <<):

>> ERROR: modpost: "copy_overflow" [drivers/usb/gadget/function/usb_f_fs.ko] undefined!
>> ERROR: modpost: "copy_overflow" [drivers/scsi/sg.ko] undefined!

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

Re: [PATCH] mm: Outline copy_overflow()

[Andrew Morton]

On Mon,  7 Feb 2022 11:55:18 +0100 Christophe Leroy <christophe.leroy@csgroup.eu> wrote:

> While building a small config with CONFIG_CC_OPTIMISE_FOR_SIZE,
> I ended up with more than 50 times the following function in vmlinux:
> 
> 	c00243bc <copy_overflow>:
> 	c00243bc:	94 21 ff f0 	stwu    r1,-16(r1)
> 	c00243c0:	7c 85 23 78 	mr      r5,r4
> 	c00243c4:	7c 64 1b 78 	mr      r4,r3
> 	c00243c8:	3c 60 c0 62 	lis     r3,-16286
> 	c00243cc:	7c 08 02 a6 	mflr    r0
> 	c00243d0:	38 63 5e e5 	addi    r3,r3,24293
> 	c00243d4:	90 01 00 14 	stw     r0,20(r1)
> 	c00243d8:	4b ff 82 45 	bl      c001c61c <__warn_printk>
> 	c00243dc:	0f e0 00 00 	twui    r0,0
> 	c00243e0:	80 01 00 14 	lwz     r0,20(r1)
> 	c00243e4:	38 21 00 10 	addi    r1,r1,16
> 	c00243e8:	7c 08 03 a6 	mtlr    r0
> 	c00243ec:	4e 80 00 20 	blr
> 
> That function being a non conditional warning on an error path,
> it is not worth inlining.
> 
> Outline it.

"uninline" is the conventional term for this.

> This reduces the size of vmlinux by almost 4kbytes.

Did you consider uninlining check_copy_size() instead?

Re: [PATCH] mm: Outline copy_overflow()

[Christophe Leroy]

Le 07/02/2022 à 21:40, Andrew Morton a écrit :
> On Mon,  7 Feb 2022 11:55:18 +0100 Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
> 
>> While building a small config with CONFIG_CC_OPTIMISE_FOR_SIZE,
>> I ended up with more than 50 times the following function in vmlinux:
>>
>> 	c00243bc <copy_overflow>:
>> 	c00243bc:	94 21 ff f0 	stwu    r1,-16(r1)
>> 	c00243c0:	7c 85 23 78 	mr      r5,r4
>> 	c00243c4:	7c 64 1b 78 	mr      r4,r3
>> 	c00243c8:	3c 60 c0 62 	lis     r3,-16286
>> 	c00243cc:	7c 08 02 a6 	mflr    r0
>> 	c00243d0:	38 63 5e e5 	addi    r3,r3,24293
>> 	c00243d4:	90 01 00 14 	stw     r0,20(r1)
>> 	c00243d8:	4b ff 82 45 	bl      c001c61c <__warn_printk>
>> 	c00243dc:	0f e0 00 00 	twui    r0,0
>> 	c00243e0:	80 01 00 14 	lwz     r0,20(r1)
>> 	c00243e4:	38 21 00 10 	addi    r1,r1,16
>> 	c00243e8:	7c 08 03 a6 	mtlr    r0
>> 	c00243ec:	4e 80 00 20 	blr
>>
>> That function being a non conditional warning on an error path,
>> it is not worth inlining.
>>
>> Outline it.
> 
> "uninline" is the conventional term for this.
> 
>> This reduces the size of vmlinux by almost 4kbytes.
> 
> Did you consider uninlining check_copy_size() instead?
> 

If you uninline check_copy_size() you loose the benefit of constant 
folding and build time overflow detection for constant sizes, and I'm 
not sure __builtin_object_size() works at all if uninlined.

Christophe