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Using file operations in kernel level: llseek

Posted on 2003-11-23
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Last Modified: 2008-02-26
Hello, I try to write a patch for exec() in linux kernel. I try to control if the binay has been patched before it is executed. So I try to patch the fs/exec.c file do_execve function.

I try to set the file position to N bytes from the beginning and read just 3 bytes from the file but while using llseek function I got segmentation fault.

Here is my code:

.....
                        mm_segment_t old_fs;
                        old_fs = get_fs();
                        set_fs(get_ds());
                        offset=bprm.file->f_op->llseek(bprm.file,VERIFYSIZE+1,0)
;
                        retval=offset;
                        if (retval < 0)
                                goto out;
                        retval=bprm.file->f_op->read(bprm.file,bufferAuth,3, &of
fset);
                        if (retval < 0)
                              goto out;
                        if ((retval=strcmp(CMPSTR,bufferAuth)) !=0 ){
                              goto out;
                              goto out;
                        }
                        set_fs(old_fs);
                        break;
                }
        }

VERIFYSIZE is the bytes that the file position must go.

Regards

--feyza

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Question by:feyzataskazan
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3 Comments
 
LVL 2

Expert Comment

by:asbharadwaj
ID: 9808797
May be f_op->llseek is NULL
Its a function pointer and may not have been set

Just check for this
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LVL 24

Accepted Solution

by:
shivsa earned 500 total points
ID: 9808907
try to use lseek64, it is more safe than llseek.
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Author Comment

by:feyzataskazan
ID: 9814443
Shivsa,
After your comment, I used sys_open, sys_lseek(lseek64), sys_read and sys_close functions and I have been succesfull. Thank you..

Before that I've controlled llseek function, it is not null, file position does not go to the correct offset adress..

Thank you for your answer asbharadwaj..
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