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write to userspace files in kernel

Posted on 2001-08-16
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Last Modified: 2012-05-04
 I use kernel 2.4.2 ,add following code into
  ipchains_core.c, to record every packet :

  struct file *myfile;
  char log_file[100];
  char buf[100];
  ...
  strcpy(log_file,"/var/log/my.log");

  strcpy(buf,"This is a test!\n");
  ..
  myfile=filp_open(log_file,O_WRONLY|O_APPEND,0600);
  ...
  myfile->f_op->write(myfile,buf,strlen(buf),    \
                      &myfile- >f_pos);
  ...

  filp_close(myfile,NULL);
   
  ...
  when the new kernel go in work, code is called,
  this kernel breaks down at once!
  I don't know why this kernel breaks down,
  I need your help, please.

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Question by:verybigcat
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bryanh earned 200 total points
ID: 6399472
This is a very common error.  The "buf" parameter is an address in user space.   You're passing an address in kernel space, which as a user space address is rather arbitrary.

You have to use get_ds() and set_ds() to make the kernel address space the user address space while the write operation runs.

Because this is a frequent question on EE, I have an example program at <ftp://giraffe-data.com/pub/filewrite.c> that shows this.
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by:jmcg
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