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Full Discussion: Problem about Kernel oops
Top Forums Programming Problem about Kernel oops Post 302588173 by homeboy on Saturday 7th of January 2012 08:39:39 AM
Old 01-07-2012
Problem about Kernel oops

hello,everyone,I'm reading LDD3.Topics about oops,it said the code bellow would cause a fault condition because "this method copies a string to a local variable,unfortunately,the string is longer than the destination array".Well,hard to understand....Is that right?I thought the fault should be caused by the wrong memset,it overwrite the function stack,the value of __user is 0xff now.will anyone tell me what happened here?Thanks;
Code:
size_t fauly_read(struct file *filp, char __user *buf, size_t count, loff_t *pos)
{
      int ret;
      char stack_buf[4];

      memset(stack_buf, 0xff, 20);
      if(count > 4)
           count  = 4;

      ret = copy_to_user(buf, stack_buf, count);
      if(!ret)
          return count;

      return ret;
}

 

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MKSTR(1)						      General Commands Manual							  MKSTR(1)

NAME
mkstr - create an error message file by massaging C source SYNOPSIS
mkstr [ - ] messagefile prefix file ... DESCRIPTION
Mkstr is used to create files of error messages. Its use can make programs with large numbers of error diagnostics much smaller, and reduce system overhead in running the program as the error messages do not have to be constantly swapped in and out. Mkstr will process each of the specified files, placing a massaged version of the input file in a file whose name consists of the specified prefix and the original name. A typical usage of mkstr would be mkstr pistrings xx *.c This command would cause all the error messages from the C source files in the current directory to be placed in the file pistrings and processed copies of the source for these files to be placed in files whose names are prefixed with xx. To process the error messages in the source to the message file mkstr keys on the string `error("' in the input stream. Each time it occurs, the C string starting at the `"' is placed in the message file followed by a null character and a new-line character; the null character terminates the message so it can be easily used when retrieved, the new-line character makes it possible to sensibly cat the error message file to see its contents. The massaged copy of the input file then contains a lseek pointer into the file which can be used to retrieve the message, i.e.: char efilname[] = "/usr/share/pascal/pi_strings"; int efil = -1; error(a1, a2, a3, a4) { char buf[256]; if (efil < 0) { efil = open(efilname, 0); if (efil < 0) { oops: perror(efilname); exit(1); } } if (lseek(efil, (long) a1, 0) || read(efil, buf, 256) <= 0) goto oops; printf(buf, a2, a3, a4); } The optional - causes the error messages to be placed at the end of the specified message file for recompiling part of a large mkstred pro- gram. SEE ALSO
lseek(2), xstr(1) 3rd Berkeley Distribution October 22, 1996 MKSTR(1)
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