linux-kernel - [PATCH v5] mm: Uninline copy

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Message-Id: <b4b538f8301922d696a49e3e98d83a08a8895d3b.1644780738.git.christophe.leroy@csgroup.eu>
Date:   Sun, 13 Feb 2022 20:32:23 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: [PATCH v5] mm: Uninline copy_overflow()

While building a small config with CONFIG_CC_OPTIMISE_FOR_SIZE,
I ended up with more than 50 times the following function in vmlinux
because GCC doesn't honor the 'inline' keyword:

	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

With -Winline, GCC tells:

	/include/linux/thread_info.h:212:20: warning: inlining failed in call to 'copy_overflow': call is unlikely and code size would grow [-Winline]

copy_overflow() is a non conditional warning called by
check_copy_size() on an error path.

check_copy_size() have to remain inlined in order to benefit
from constant folding, but copy_overflow() is not worth inlining.

Uninline it when CONFIG_BUG is selected.

When CONFIG_BUG is not selected, WARN() does nothing so skip it.

This reduces the size of vmlinux by almost 4kbytes.

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
v5: Change to EXPORT_SYMBOL() instead of EXPORT_SYMBOL_GPL()

v4: Make copy_overflow() a no-op when CONFIG_BUG is not selected

v3: Added missing ; after EXPORT_SYMBOL()

v2: Added missing EXPORT_SYMBOL() and enhanced commit message
---
 include/linux/thread_info.h | 5 ++++-
 mm/maccess.c                | 6 ++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
index 73a6f34b3847..9f392ec76f2b 100644
--- a/include/linux/thread_info.h
+++ b/include/linux/thread_info.h
@@ -209,9 +209,12 @@ __bad_copy_from(void);
 extern void __compiletime_error("copy destination size is too small")
 __bad_copy_to(void);
 
+void __copy_overflow(int size, unsigned long count);
+
 static inline void copy_overflow(int size, unsigned long count)
 {
-	WARN(1, "Buffer overflow detected (%d < %lu)!\n", size, count);
+	if (IS_ENABLED(CONFIG_BUG))
+		__copy_overflow(size, count);
 }
 
 static __always_inline __must_check bool
diff --git a/mm/maccess.c b/mm/maccess.c
index d3f1a1f0b1c1..a1b2a3a4559e 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -335,3 +335,9 @@ 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);
+}
+EXPORT_SYMBOL(copy_overflow);
-- 
2.34.1