More info about kobj_attribute store and show functions

More info about kobj_attribute store and show functions

[Lucas Tanure]

Hi,

I'm looking for some information about :

struct kobj_attribute {
        struct attribute attr;
        ssize_t (*show)(struct kobject *kobj, struct kobj_attribute
*attr, char *buf);
        ssize_t (*store)(struct kobject *kobj, struct kobj_attribute
*attr, const char *buf, size_t count);
};


My function to handle the operation:

static ssize_t my_store(struct kobject *kobj, struct kobj_attribute *attr,
 const char *buf, size_t count)
{
char *buffer;

buffer = (char*) kcalloc(count, sizeof(char), GFP_KERNEL);

if (count == 13) {
 if (buffer) {
        copy_from_user(buffer, buf, count);
printk("A %s",buffer);
 if (0) {
if (strcmp(buffer, eudyptula_id) == 0)
 return count;
        printk("erro na strcmp\n");
}
 else
printk("erro na copia\n");
 }
}
return -EINVAL;
}

So, this *const char *buf, *where ti come from ? Kernel space


--
Lucas Tanure
+55 (19) 988176559
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More info about kobj_attribute store and show functions

[Lucas Tanure]

Sorry, didn't finish to write the email and Gmail sent.
I will re-send.

Sorry.

--
Lucas Tanure
+55 (19) 988176559
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More info about kobj_attribute store and show functions

[Lucas Tanure]

(Forget the last e-mail, please, stupid Gmail)

Hi,

I'm looking for some information about :

struct kobj_attribute {
        struct attribute attr;
        ssize_t (*show)(struct kobject *kobj, struct kobj_attribute
*attr, char *buf);
        ssize_t (*store)(struct kobject *kobj, struct kobj_attribute
*attr, const char *buf, size_t count);
};


My function to handle the operation:

static ssize_t my_store(struct kobject *kobj, struct kobj_attribute *attr,
const char *buf, size_t count) {
 char *buffer;

        buffer = (char*) kcalloc(count, sizeof(char), GFP_KERNEL);

        copy_from_user(buffer, buf, count);
        .....
        .....
        etc....
}

So, this *const char *buf, *where ti come from ? Kernel space?
My copy_from_user it's not working. Should return zero.

There any Macro or function to test if the pointer *buf *is a kernel
pointer ?

Thanks

Lucas Tanure
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More info about kobj_attribute store and show functions

[Greg KH]

On Wed, Sep 03, 2014 at 09:10:41PM -0300, Lucas Tanure wrote:
> (Forget the last e-mail, please, stupid Gmail)
> 
> Hi,
> 
> I'm looking for some information about :
> 
> struct kobj_attribute {
>         struct attribute attr;
>         ssize_t (*show)(struct kobject *kobj, struct kobj_attribute
> *attr, char *buf);
>         ssize_t (*store)(struct kobject *kobj, struct kobj_attribute
> *attr, const char *buf, size_t count);
> };
> 
> 
> My function to handle the operation:
> 
> static ssize_t my_store(struct kobject *kobj, struct kobj_attribute *attr,
> const char *buf, size_t count) {
>  char *buffer;
> 
>         buffer = (char*) kcalloc(count, sizeof(char), GFP_KERNEL);
> 
>         copy_from_user(buffer, buf, count);
>         .....
>         .....
>         etc....
> }
> 
> So, this *const char *buf, *where ti come from ? Kernel space?

I'm guessing you didn't read the kobject documentation that is in the
kernel tree?

More info about kobj_attribute store and show functions

[Lucas Tanure]

Greg,

I'm still trying to understand the sysfs.
I indeed read the documentation, I was using your "Sample kobject
implementation" as base of my code. But as 4h of work there some points
that I didn't understand yet.

Thanks

--
Lucas Tanure
+55 (19) 988176559


On Wed, Sep 3, 2014 at 9:23 PM, Greg KH <greg@kroah.com> wrote:

> On Wed, Sep 03, 2014 at 09:10:41PM -0300, Lucas Tanure wrote:
> > (Forget the last e-mail, please, stupid Gmail)
> >
> > Hi,
> >
> > I'm looking for some information about :
> >
> > struct kobj_attribute {
> >         struct attribute attr;
> >         ssize_t (*show)(struct kobject *kobj, struct kobj_attribute
> > *attr, char *buf);
> >         ssize_t (*store)(struct kobject *kobj, struct kobj_attribute
> > *attr, const char *buf, size_t count);
> > };
> >
> >
> > My function to handle the operation:
> >
> > static ssize_t my_store(struct kobject *kobj, struct kobj_attribute
> *attr,
> > const char *buf, size_t count) {
> >  char *buffer;
> >
> >         buffer = (char*) kcalloc(count, sizeof(char), GFP_KERNEL);
> >
> >         copy_from_user(buffer, buf, count);
> >         .....
> >         .....
> >         etc....
> > }
> >
> > So, this *const char *buf, *where ti come from ? Kernel space?
>
> I'm guessing you didn't read the kobject documentation that is in the
> kernel tree?
>
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More info about kobj_attribute store and show functions

[Greg KH]

A: No.
Q: Should I include quotations after my reply?

http://daringfireball.net/2007/07/on_top

On Wed, Sep 03, 2014 at 09:39:50PM -0300, Lucas Tanure wrote:
> Greg,?
> 
> I'm still trying to understand the sysfs.?

Why?  What are you using "raw kobjects" for?  You should almost never
need them.

> I indeed read the documentation, I was using your "Sample kobject
> implementation" as base of my code.

Great, then it should "just work", right?  :)

Care to post your non-working code?

thanks,

greg k-h

More info about kobj_attribute store and show functions

[Jeff Haran]

From: kernelnewbies-bounces+jharan=bytemobile.com@kernelnewbies.org [mailto:kernelnewbies-bounces+jharan=bytemobile.com at kernelnewbies.org] On Behalf Of Lucas Tanure
Sent: Wednesday, September 03, 2014 5:11 PM
To: kernelnewbies
Subject: Re: More info about kobj_attribute store and show functions

(Forget the last e-mail, please, stupid Gmail)

Hi,

I'm looking for some information about :

struct kobj_attribute {
        struct attribute attr;
        ssize_t (*show)(struct kobject *kobj, struct kobj_attribute *attr, char *buf);
        ssize_t (*store)(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count);
};


My function to handle the operation:

static ssize_t my_store(struct kobject *kobj, struct kobj_attribute *attr, const char *buf, size_t count) {
char *buffer;

        buffer = (char*) kcalloc(count, sizeof(char), GFP_KERNEL);

        copy_from_user(buffer, buf, count);
        .....
        .....
        etc....
}

So, this const char *buf, where ti come from ? Kernel space?
My copy_from_user it's not working. Should return zero.

There any Macro or function to test if the pointer buf is a kernel pointer ?

Thanks

Lucas Tanure

I haven?t written too many of these things, but the last time I wrote a store function I didn?t need to do the copy_to_user().
buf seemed to point to directly accessible kernel memory. I would think a simple memcpy above would do the trick.

Jeff

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More info about kobj_attribute store and show functions

[Lucas Tanure]

Greg,

I was looking again to documentation, but didn't find a explanation about
the show and store functions. I'm missing something ?

Yes, everything work if I simple use the buffer from store function, but I
was looking for where it came from.
Like, It was from user space? So, I need to take care like
simple_write_to_buffer
does with user space memory ?

I said that because of file file_operations read and write, that I know
that comes from user space. But kobj_attribute  I didn't understand yet.

What I meant posting one piece of my code was just show what I doing in
high level.
But, for some reason gmail sent in the middle of my writing.



static ssize_t id_store(struct kobject *kobj, struct kobj_attribute *attr,
 const char *buf, size_t count)
{
char *buffer;
buffer = kcalloc(cnt, sizeof(char), GFP_KERNEL);
if (buffer) {
 if (copy_from_user(buffer, buf, cnt) == cnt) {
if (strcmp(buffer, "Tanure") == 0)
return cnt;
 }
}
}
return -EINVAL;
}

The copy_from_user returns 6 when I write my name in the file. So, 6 means
that copy_from_user fails to copy 6 bytes from buf.
So there is my question, buf is a user pointer ? Or Can I just copy without
problems ?

All this e-mails got me in trouble here. I got kicked from eudyptula
challenge, what I still begging to be able to finish.

Thanks Greg for all the help.



--
Lucas Tanure
+55 (19) 988176559


On Wed, Sep 3, 2014 at 9:56 PM, Greg KH <greg@kroah.com> wrote:

>
> A: No.
> Q: Should I include quotations after my reply?
>
> http://daringfireball.net/2007/07/on_top
>
> On Wed, Sep 03, 2014 at 09:39:50PM -0300, Lucas Tanure wrote:
> > Greg,
> >
> > I'm still trying to understand the sysfs.
>
> Why?  What are you using "raw kobjects" for?  You should almost never
> need them.
>
> > I indeed read the documentation, I was using your "Sample kobject
> > implementation" as base of my code.
>
> Great, then it should "just work", right?  :)
>
> Care to post your non-working code?
>
> thanks,
>
> greg k-h
>
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More info about kobj_attribute store and show functions

[Greg KH]

On Wed, Sep 03, 2014 at 10:39:06PM -0300, Lucas Tanure wrote:
> Greg,?
> 
> I was looking again to?documentation, but didn't find a explanation about the
> show and store functions. I'm missing something ??
> 
> Yes, everything work if I simple use the buffer from store function, but I was
> looking for where it came from.?
> Like, It was from user space? So, I need to take care like?
> simple_write_to_buffer does with user space memory ??

If it came from userspace, it would have been marked with a __user
marking, so it's a kernel pointer.

> static ssize_t id_store(struct kobject *kobj, struct kobj_attribute *attr,
> const char *buf, size_t count)
> {
> char *buffer;
> buffer = kcalloc(cnt, sizeof(char), GFP_KERNEL);
> if (buffer) {
> if (copy_from_user(buffer, buf, cnt) == cnt) {
> if (strcmp(buffer, "Tanure") == 0)
> return cnt;
> }
> }
> }
> return -EINVAL;
> }

Just work with the buffer directly, sysfs is supposed to be "easy" to
use.

greg k-h