[PATCH] bug.h: Work around GCC PR82365 in BUG()

From: Arnd Bergmann
Date: Tue Dec 19 2017 - 06:42:33 EST

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Looking at functions with large stack frames across all architectures
led me discovering that BUG() suffers from the same problem as
fortify_panic(), which I've added a workaround for already. In short,
variables that go out of scope by calling a noreturn function or
__builtin_unreachable() keep using stack space in functions afterwards.

A workaround that was identified is to insert an empty assembler statement
just before calling the function that doesn't return. I'm adding a macro
"barrier_before_unreachable()" to document this, and insert calls to
that in all instances of BUG() that currently suffer from this problem.

The files that saw the largest change from this had these frame sizes
before, and much less with my patch:

fs/ext4/inode.c:82:1: warning: the frame size of 1672 bytes is larger than 800 bytes [-Wframe-larger-than=]
fs/ext4/namei.c:434:1: warning: the frame size of 904 bytes is larger than 800 bytes [-Wframe-larger-than=]
fs/ext4/super.c:2279:1: warning: the frame size of 1160 bytes is larger than 800 bytes [-Wframe-larger-than=]
fs/ext4/xattr.c:146:1: warning: the frame size of 1168 bytes is larger than 800 bytes [-Wframe-larger-than=]
fs/f2fs/inode.c:152:1: warning: the frame size of 1424 bytes is larger than 800 bytes [-Wframe-larger-than=]
net/netfilter/ipvs/ip_vs_core.c:1195:1: warning: the frame size of 1068 bytes is larger than 800 bytes [-Wframe-larger-than=]
net/netfilter/ipvs/ip_vs_core.c:395:1: warning: the frame size of 1084 bytes is larger than 800 bytes [-Wframe-larger-than=]
net/netfilter/ipvs/ip_vs_ftp.c:298:1: warning: the frame size of 928 bytes is larger than 800 bytes [-Wframe-larger-than=]
net/netfilter/ipvs/ip_vs_ftp.c:418:1: warning: the frame size of 908 bytes is larger than 800 bytes [-Wframe-larger-than=]
net/netfilter/ipvs/ip_vs_lblcr.c:718:1: warning: the frame size of 960 bytes is larger than 800 bytes [-Wframe-larger-than=]
drivers/net/xen-netback/netback.c:1500:1: warning: the frame size of 1088 bytes is larger than 800 bytes [-Wframe-larger-than=]

In case of ARC and CRIS, it turns out that the BUG() implementation
actually does return (or at least the compiler thinks it does), resulting
in lots of warnings about uninitialized variable use and leaving noreturn
functions, such as:

block/cfq-iosched.c: In function 'cfq_async_queue_prio':
block/cfq-iosched.c:3804:1: error: control reaches end of non-void function [-Werror=return-type]
include/linux/dmaengine.h: In function 'dma_maxpq':
include/linux/dmaengine.h:1123:1: error: control reaches end of non-void function [-Werror=return-type]

This makes them call __builtin_trap() instead, which should normally
dump the stack and kill the current process, like some of the other
architectures already do.

I tried adding barrier_before_unreachable() to panic() and fortify_panic()
as well, but that had very little effect, so I'm not submitting that
patch.

Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
---
The name barrier_before_unreachable() is a bit suboptimal here,
as it fails to describe the fact that it is needed for both
__builtin_unreachable() and for calling noreturn functions. Any other
suggestions would be welcome here.
---
arch/arc/include/asm/bug.h | 3 ++-
arch/cris/include/arch-v10/arch/bug.h | 11 +++++++++--
arch/ia64/include/asm/bug.h | 6 +++++-
arch/m68k/include/asm/bug.h | 3 +++
arch/sparc/include/asm/bug.h | 6 +++++-
include/asm-generic/bug.h | 1 +
include/linux/compiler-gcc.h | 15 ++++++++++++++-
include/linux/compiler.h | 5 +++++
8 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/arch/arc/include/asm/bug.h b/arch/arc/include/asm/bug.h
index ea022d47896c..21ec82466d62 100644
--- a/arch/arc/include/asm/bug.h
+++ b/arch/arc/include/asm/bug.h
@@ -23,7 +23,8 @@ void die(const char *str, struct pt_regs *regs, unsigned long address);

#define BUG() do { \
pr_warn("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
- dump_stack(); \
+ barrier_before_unreachable(); \
+ __builtin_trap(); \
} while (0)

#define HAVE_ARCH_BUG
diff --git a/arch/cris/include/arch-v10/arch/bug.h b/arch/cris/include/arch-v10/arch/bug.h
index 905afeacfedf..06da9d49152a 100644
--- a/arch/cris/include/arch-v10/arch/bug.h
+++ b/arch/cris/include/arch-v10/arch/bug.h
@@ -44,18 +44,25 @@ struct bug_frame {
* not be used like this with newer versions of gcc.
*/
#define BUG() \
+do { \
__asm__ __volatile__ ("clear.d [" __stringify(BUG_MAGIC) "]\n\t"\
"movu.w " __stringify(__LINE__) ",$r0\n\t"\
"jump 0f\n\t" \
".section .rodata\n" \
"0:\t.string \"" __FILE__ "\"\n\t" \
- ".previous")
+ ".previous"); \
+ unreachable(); \
+} while (0)
#endif

#else

/* This just causes an oops. */
-#define BUG() (*(int *)0 = 0)
+#define BUG() \
+do { \
+ barrier_before_unreachable(); \
+ __builtin_trap(); \
+} while (0)

#endif

diff --git a/arch/ia64/include/asm/bug.h b/arch/ia64/include/asm/bug.h
index bd3eeb8d1cfa..66b37a532765 100644
--- a/arch/ia64/include/asm/bug.h
+++ b/arch/ia64/include/asm/bug.h
@@ -4,7 +4,11 @@

#ifdef CONFIG_BUG
#define ia64_abort() __builtin_trap()
-#define BUG() do { printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); ia64_abort(); } while (0)
+#define BUG() do { \
+ printk("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
+ barrier_before_unreachable(); \
+ ia64_abort(); \
+} while (0)

/* should this BUG be made generic? */
#define HAVE_ARCH_BUG
diff --git a/arch/m68k/include/asm/bug.h b/arch/m68k/include/asm/bug.h
index b7e2bf1ba4a6..275dca1435bf 100644
--- a/arch/m68k/include/asm/bug.h
+++ b/arch/m68k/include/asm/bug.h
@@ -8,16 +8,19 @@
#ifndef CONFIG_SUN3
#define BUG() do { \
pr_crit("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
+ barrier_before_unreachable(); \
__builtin_trap(); \
} while (0)
#else
#define BUG() do { \
pr_crit("kernel BUG at %s:%d!\n", __FILE__, __LINE__); \
+ barrier_before_unreachable(); \
panic("BUG!"); \
} while (0)
#endif
#else
#define BUG() do { \
+ barrier_before_unreachable(); \
__builtin_trap(); \
} while (0)
#endif
diff --git a/arch/sparc/include/asm/bug.h b/arch/sparc/include/asm/bug.h
index 6f17528356b2..ea53e418f6c0 100644
--- a/arch/sparc/include/asm/bug.h
+++ b/arch/sparc/include/asm/bug.h
@@ -9,10 +9,14 @@
void do_BUG(const char *file, int line);
#define BUG() do { \
do_BUG(__FILE__, __LINE__); \
+ barrier_before_unreachable(); \
__builtin_trap(); \
} while (0)
#else
-#define BUG() __builtin_trap()
+#define BUG() do { \
+ barrier_before_unreachable(); \
+ __builtin_trap(); \
+} while (0)
#endif

#define HAVE_ARCH_BUG
diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
index 963b755d19b0..a7613e1b0c87 100644
--- a/include/asm-generic/bug.h
+++ b/include/asm-generic/bug.h
@@ -52,6 +52,7 @@ struct bug_entry {
#ifndef HAVE_ARCH_BUG
#define BUG() do { \
printk("BUG: failure at %s:%d/%s()!\n", __FILE__, __LINE__, __func__); \
+ barrier_before_unreachable(); \
panic("BUG!"); \
} while (0)
#endif
diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
index 5d595cfdb2c4..66cfdad68f7e 100644
--- a/include/linux/compiler-gcc.h
+++ b/include/linux/compiler-gcc.h
@@ -205,6 +205,15 @@
#endif

/*
+ * calling noreturn functions, __builtin_unreachable() and __builtin_trap()
+ * confuse the stack allocation in gcc, leading to overly large stack
+ * frames, see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82365
+ *
+ * Adding an empty inline assembly before it works around the problem
+ */
+#define barrier_before_unreachable() asm volatile("")
+
+/*
* Mark a position in code as unreachable. This can be used to
* suppress control flow warnings after asm blocks that transfer
* control elsewhere.
@@ -214,7 +223,11 @@
* unreleased. Really, we need to have autoconf for the kernel.
*/
#define unreachable() \
- do { annotate_unreachable(); __builtin_unreachable(); } while (0)
+ do { \
+ annotate_unreachable(); \
+ barrier_before_unreachable(); \
+ __builtin_unreachable(); \
+ } while (0)

/* Mark a function definition as prohibited from being cloned. */
#define __noclone __attribute__((__noclone__, __optimize__("no-tracer")))
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 52e611ab9a6c..97847f2f86cf 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -86,6 +86,11 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
# define barrier_data(ptr) barrier()
#endif

+/* workaround for GCC PR82365 if needed */
+#ifndef barrier_before_unreachable
+# define barrier_before_unreachable() do { } while (0)
+#endif
+
/* Unreachable code */
#ifdef CONFIG_STACK_VALIDATION
/*
--
2.9.0

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