sys_read buffer too large

[Rajat Sharma]

On Sat, Jul 20, 2013 at 8:49 PM, anish singh <anish198519851985 at gmail.com>wrote:

>
>
>
> On Sat, Jul 20, 2013 at 2:04 PM, Don Raikes <don.raikes at oracle.com> wrote:
>
>> Hello all,****
>>
>> ** **
>>
>> I am very new to kernel programming. In fact, I have been working on it
>> for a week now.****
>>
>> ** **
>>
>> I am taking a computer science class, and one of our assignments is to
>> hook some of the system calls in a 2.6.28 kernel.****
>>
>> ** **
>>
>> I have the basic module created, and I am hooking into the sys_read
>> function.  The assignment is to print to the log what is being read using
>> sys_read.****
>>
>> ** **
>>
>> I have some checks in my function to limit my printing to only a certain
>> file, so I don’t get the contents of every file being read on the system.
>> ****
>>
>> ** **
>>
>> OnceI know I have the right file, I decided to print the size of the
>> buffer.****
>>
>> ** **
>>
>> The function signature is:****
>>
>> ** **
>>
>> Asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);
>> ****
>>
>> ** **
>>
>> So I do:****
>>
>> ** **
>>
>> Printk(KERN_INFO “Read buffer is [%d] bytes.\n”,count);****
>>
>> ** **
>>
>> The particular file I am reading has 2 lines the first is 14 bytes long
>> and the second is 30 bytes long.****
>>
>> **
>>
>
What utility are you using to read files? It is better to write your own
program using glibc 'read' routine. Also you can use 'strace' utility to
trace read system call and its arguments from user space.


> **
>>
>> The output from my printk statements are 8192 and 4096 respectively.****
>>
>> Why is the count so large?
>>
> I think it is the size of one PAGE.I think reading from sys_read
> is PAGE aligned.Just print the size of the page and i think it will
> be 4K.
>

I doubt it is true, sys_read will merely pass down arguments which it
received from user space. My hunch is it is user mode utility which is
doing buffered read.


> ****
>>
>> ** **
>>
>> Also I do a strlen(buf) and print the value as well, and it is 29 and 16
>> respectively.****
>>
>> ** **
>>
>> Next I did  copy_from_user(tbuf,buf,strlen(buf))****
>>
>> And tried to print the contents of the buffer but it came out garbage.***
>> *
>>
>> ** **
>>
>> Does anyone have any ideas how I can get this to work?****
>>
>> ** **
>>
>> BTW: the assignment is due on Sunday so any help would be aappreciated.**
>> **
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> --
>> Best Regards, Donald****
>>
>> [image: Oracle] <http://www.oracle.com/>
>> Donald raikes | Accessibility Specialist/ QA Engineer
>> Phone: +15202717608 | Mobile: +15202717608
>> Oracle Quality Assurance
>> | Tucson, Arizona ****
>>
>> [image: Green Oracle] <http://www.oracle.com/commitment>****
>>
>> Oracle is committed to developing practices and products that help
>> protect the environment****
>>
>> ** **
>>
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>>
>
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