Re: [PATCH] fix vmstat per cpu usage

[Jan Blunck]

On Wed, Aug 02, Andrew Morton wrote:

> > +#define per_cpu(var, cpu) (*({				\
> > +	int user_error_##var __attribute__ ((unused));	\
> > +	RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))
> 
> What's it do?  Forces a syntax error if `var' isn't a simple identifier?
> 
> Seems sane, although I'd check that the compiler doesn't accidentally
> waste a stack slot for that local.  Perhaps it's be safer to make
> it a non-existing function:
> 
> 	extern int user_error#var(void);
> 

Yes, that is even better. See attached patch with the modifications for my
major archs (s390, x86-64, generic). I did a run of 'make allmodconfig' with
the attached patch and it produced some errors. I'll send the patches in a
seperate mail.
From: Jan Blunck <jblunck@xxxxxxx>
Subject: trigger a syntax error if percpu macros are incorrectly used

get_cpu_var()/per_cpu()/__get_cpu_var() arguments must be simple identifiers.
Otherwise the arch dependant implementations might break. This patch enforces
the correct usage of the macros by producing a syntax error if the variable is
not a simple identifier.

Signed-off-by: Jan Blunck <jblunck@xxxxxxx>
---
 include/asm-generic/percpu.h |    4 +++-
 include/asm-s390/percpu.h    |   20 +++++++++++---------
 include/asm-x86_64/percpu.h  |   12 +++++++++---
 include/linux/percpu.h       |   10 ++++++++--
 4 files changed, 31 insertions(+), 15 deletions(-)

Index: linux-2.6/include/asm-generic/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-generic/percpu.h
+++ linux-2.6/include/asm-generic/percpu.h
@@ -14,7 +14,9 @@ extern unsigned long __per_cpu_offset[NR
     __attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name
 
 /* var is in discarded region: offset to particular copy we want */
-#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]))
+#define per_cpu(var, cpu) (*({				\
+	extern int simple_indentifier_##var(void);	\
+	RELOC_HIDE(&per_cpu__##var, __per_cpu_offset[cpu]); }))
 #define __get_cpu_var(var) per_cpu(var, smp_processor_id())
 #define __raw_get_cpu_var(var) per_cpu(var, raw_smp_processor_id())
 
Index: linux-2.6/include/asm-s390/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-s390/percpu.h
+++ linux-2.6/include/asm-s390/percpu.h
@@ -15,18 +15,20 @@
  */
 #if defined(__s390x__) && defined(MODULE)
 
-#define __reloc_hide(var,offset) \
-  (*({ unsigned long *__ptr; \
-       asm ( "larl %0,per_cpu__"#var"@GOTENT" \
-             : "=a" (__ptr) : "X" (per_cpu__##var) ); \
-       (typeof(&per_cpu__##var))((*__ptr) + (offset)); }))
+#define __reloc_hide(var,offset) (*({			\
+	extern int simple_indentifier_##var(void);	\
+	unsigned long *__ptr;				\
+	asm ( "larl %0,per_cpu__"#var"@GOTENT"		\
+	    : "=a" (__ptr) : "X" (per_cpu__##var) );	\
+	(typeof(&per_cpu__##var))((*__ptr) + (offset));	}))
 
 #else
 
-#define __reloc_hide(var, offset) \
-  (*({ unsigned long __ptr; \
-       asm ( "" : "=a" (__ptr) : "0" (&per_cpu__##var) ); \
-       (typeof(&per_cpu__##var)) (__ptr + (offset)); }))
+#define __reloc_hide(var, offset) (*({				\
+	extern int simple_indentifier_##var(void);		\
+	unsigned long __ptr;					\
+	asm ( "" : "=a" (__ptr) : "0" (&per_cpu__##var) );	\
+	(typeof(&per_cpu__##var)) (__ptr + (offset)); }))
 
 #endif
 
Index: linux-2.6/include/asm-x86_64/percpu.h
===================================================================
--- linux-2.6.orig/include/asm-x86_64/percpu.h
+++ linux-2.6/include/asm-x86_64/percpu.h
@@ -21,9 +21,15 @@
     __attribute__((__section__(".data.percpu"))) __typeof__(type) per_cpu__##name
 
 /* var is in discarded region: offset to particular copy we want */
-#define per_cpu(var, cpu) (*RELOC_HIDE(&per_cpu__##var, __per_cpu_offset(cpu)))
-#define __get_cpu_var(var) (*RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()))
-#define __raw_get_cpu_var(var) (*RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()))
+#define per_cpu(var, cpu) (*({				\
+	extern int simple_indentifier_##var(void);	\
+	RELOC_HIDE(&per_cpu__##var, __per_cpu_offset(cpu)); }))
+#define __get_cpu_var(var) (*({				\
+	extern int simple_indentifier_##var(void);	\
+	RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()); }))
+#define __raw_get_cpu_var(var) (*({			\
+	extern int simple_indentifier_##var(void);	\
+	RELOC_HIDE(&per_cpu__##var, __my_cpu_offset()); }))
 
 /* A macro to avoid #include hell... */
 #define percpu_modcopy(pcpudst, src, size)			\
Index: linux-2.6/include/linux/percpu.h
===================================================================
--- linux-2.6.orig/include/linux/percpu.h
+++ linux-2.6/include/linux/percpu.h
@@ -11,8 +11,14 @@
 #define PERCPU_ENOUGH_ROOM 32768
 #endif
 
-/* Must be an lvalue. */
-#define get_cpu_var(var) (*({ preempt_disable(); &__get_cpu_var(var); }))
+/*
+ * Must be an lvalue. Since @var must be a simple identifier,
+ * we force a syntax error here if it isn't.
+ */
+#define get_cpu_var(var) (*({				\
+	extern int simple_indentifier_##var(void);	\
+	preempt_disable();				\
+	&__get_cpu_var(var); }))
 #define put_cpu_var(var) preempt_enable()
 
 #ifdef CONFIG_SMP