How __ATTR macro is used in Linux kernel? - linux-kernel

Question

I would like to create a sysfs entry, which is done using an "attribute" structure. The way recommended to do this is to use the __ATTR macro, as in this:
static ssize_t functionCalledOnRead(struct kobject *, struct attribute *, char *);
static ssize_t functionCalledOnRead(struct kobject *, struct attribute *, char *);
static struct attribute genericSysfsAttribute = __ATTR(attr_name, S_IRUGO | S_IRUSR, functionCalledOnRead, functionCalledOnWrite);
However, this does not work.
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: warning: braces around scalar initializer
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: warning: (near initialization for 'genericSysfsAttribute.name')
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: error: field name not in record or union initializer
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: error: (near initialization for 'genericSysfsAttribute.name')
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: error: field name not in record or union initializer
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:15: error: (near initialization for 'genericSysfsAttribute.name')
In file included from include/linux/thread_info.h:11:0,
from include/asm-generic/preempt.h:4,
from arch/arm/include/generated/asm/preempt.h:1,
from include/linux/preempt.h:59,
from include/linux/spinlock.h:50,
from include/linux/seqlock.h:35,
from include/linux/time.h:5,
from include/linux/stat.h:18,
from include/linux/module.h:10,
from /home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:2:
include/linux/bug.h:34:45: warning: excess elements in scalar initializer
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: warning: (near initialization for 'genericSysfsAttribute.name')
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: error: unknown field 'show' specified in initializer
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: warning: initialization makes integer from pointer without a cast
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: warning: (near initialization for 'genericSysfsAttribute.mode')
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: error: initializer element is not computable at load time
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: error: (near initialization for 'genericSysfsAttribute.mode')
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: error: unknown field 'store' specified in initializer
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: warning: excess elements in struct initializer
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
include/linux/bug.h:34:45: warning: (near initialization for 'genericSysfsAttribute')
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
include/linux/kernel.h:842:3: note: in expansion of macro 'BUILD_BUG_ON_ZERO'
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
include/linux/sysfs.h:102:12: note: in expansion of macro 'VERIFY_OCTAL_PERMISSIONS'
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:49: note: in expansion of macro '__ATTR'
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.c:32:25: warning: 'genericSysfsAttribute' defined but not used [-Wunused-variable]
static struct attribute genericSysfsAttribute = __ATTR(attr_name,S_IRUGO | S_IRUSR,functionCalledOnRead,functionCalledOnWrite);
^
scripts/Makefile.build:297: recipe for target '/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.o' failed
make[2]: *** [/home/osboxes/Documents/kernel_modules/sysfsmodule/sysfsmodule.o] Error 1
Makefile:1425: recipe for target '_module_/home/osboxes/Documents/kernel_modules/sysfsmodule' failed
make[1]: *** [_module_/home/osboxes/Documents/kernel_modules/sysfsmodule] Error 2
make[1]: Leaving directory '/home/osboxes/Documents/linux-mykonos'
Makefile:7: recipe for target 'cross' failed
make: *** [cross] Error 2
I don't really understand what most of these errors are saying since they're talking about included code and not what I wrote. However, since the first argument is just a name and the second is just a number, I don't expect there is anything wrong with those, so it seems like its something to do with how I am giving the names of those two functions isn't working. How does one pass a function like this?

Answer 1

Macro __ATTR is not for direct use, but for use by subsystem developers who defines type for kobject and creates attributes structure "derived" from struct attribute.
Also, plain struct attribute has a little sence:
A bare attribute contains no means to read or write the value of the
attribute. Subsystems are encouraged to define their own attribute
structure and wrapper functions for adding and removing attributes for
a specific object type.

Answer 2

The error arises due to the value of the mode you are passing to the macro, i.e. S_IRUGO|S_IRUSR. In the <linux/kernel.h> module, permission check raises an error if the permissions violate any of the following:
USER_READABLE >= GROUP_READABLE >= OTHER_READABLE
<777 and >0
USER_WRITABLE >= GROUP_WRITABLE
OTHER_WRITABLE = 0 (not a good idea to allow writes here)