Kernel module - How to test if file doesn't exist Post: 302501308

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Top Forums Programming Kernel module - How to test if file doesn't exist Post 302501308 by disaster on Thursday 3rd of March 2011 09:20:51 AM

03-03-2011

Registered User

Kernel module - How to test if file doesn't exist

Hey,
I'm currently getting into some kernel module progamming. As a little exercise I want to read the headers out of an ELF file. My code is very simple, here is the important part:

Code:

struct file *fp;
/* ... */
fp = filp_open("some/file/on/my/pc", O_RDONLY, 0);
if(fp == NULL) {
     ALERT("NULLPOINTER"); // predefinied makro for output
     return;
}
if(fp->f_dentry == NULL) {
     ALERT("NULLPOINTER"); 
     return;
}
printk(KERN_INFO "Filname: %\n", fp->f_dentry->d_name.name);

(I know that might not make much sense, it's just to play around and see how the functions and data structures work).

As long as I choose a valid path and filename in the filp_open everything works fine, just as it should.
But if I enter a non-existent filename it always gets killed. The log says:
BUG: Unable to handle kernel NULL pointer dereference at 0000000a
Interestingly, the module always seems to get killed at the 2nd if-clause. That means that even if the file doesn't exist fd itself is not NULL, but then, once I simply check if f_dentry is a NULLpointer, the module crashes.
Can anyone tell me why and how can I write this short code in such a way that it can handle non-existent files (without creating them, but rather exiting with a simple error msg)

disaster

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genders_load_data

GENDERS_LOAD_DATA(3)						    LIBGENDERS						      GENDERS_LOAD_DATA(3)

NAME

       genders_load_data -  read and parse a genders file

SYNOPSIS

       #include <genders.h>

       int genders_load_data(genders_t handle, const char *filename);

DESCRIPTION

       genders_load_data()  read and parses the genders file indicated by filename.  If filename is NULL, genders_load_data() will attempt to read
       and parse the default genders file defined by DEFAULT_GENDERS_FILE. handle is a genders handle created by genders_handle_create(3).

       After genders_load_data() is called, information about the genders file is stored in the genders handle passed  in.   This  information	is
       used  by  other	genders C API functions.  All future use of handle with other genders C API functions will be directly associated with the
       genders file indicated by filename (or the default genders file if filename is NULL).

RETURN VALUES

       On success, 0 is returned.  On error, -1 is returned, and an error code is returned in handle.  The error code can be  retrieved  via  gen-
       ders_errnum(3) , and a description of the error code can be retrieved via genders_strerror(3).  Error codes are defined in genders.h.

ERRORS

       GENDERS_ERR_NULLHANDLE
	      The handle parameter is NULL.  The genders handle must be created with genders_handle_create(3).

       GENDERS_ERR_OPEN
	      The genders file indicated by filename cannot be opened for reading.

       GENDERS_ERR_READ
	      Error reading the genders file indicated by filename.

       GENDERS_ERR_PARSE
	      The genders file indicated by filename is incorrectly formatted.

       GENDERS_ERR_ISLOADED
	      genders_load_data(3) has already been called with handle.

       GENDERS_ERR_OVERFLOW
	      A line in the genders database exceeds the maximum allowed length.

       GENDERS_ERR_OUTMEM
	      malloc(3) has failed internally, system is out of memory.

       GENDERS_ERR_MAGIC
	      handle  has  an  incorrect  magic  number.   handle  does not point to a genders handle or handle has been destroyed by genders_han-
	      dle_destroy(3).

       GENDERS_ERR_INTERNAL
	      An internal system error has occurred.

FILES

       /usr/include/genders.h

SEE ALSO

       libgenders(3), genders_handle_create(3), genders_handle_destroy(3), genders_errnum(3), genders_strerror(3)

LLNL
								    August 2003 					      GENDERS_LOAD_DATA(3)

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