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Full Discussion: Problem about Kernel oops
Top Forums Programming Problem about Kernel oops Post 302588222 by Corona688 on Saturday 7th of January 2012 04:39:04 PM
Old 01-07-2012
Quote:
Originally Posted by homeboy
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;
}

It's writing 20 bytes to an array 4 bytes long, rampaging 16 more bytes across memory it shouldn't, trashing things like local variable values, function parameters, and return locations, all of which are kept in the same general area of stack here.

You're both right, in other words. There's many possible problems that could be caused by this, not just corruption of the return vector. If you're lucky, it crashes immediately. If you're unlucky, it puts bizarre values into your local variables and function parameters, causing behavior that's very difficult to rationally explain.
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mkstr(1)						      General Commands Manual							  mkstr(1)

NAME
mkstr - Creates an error message file SYNOPSIS
mkstr [-] message_file prefix file... The mkstr command is used to create files of error messages that can be removed from a single C source file, or from multiple source files. OPTIONS
Causes messages to be appended to the specified message file, instead of creating a new file. DESCRIPTION
The use of mkstr can reduce the size of programs that contain many error diagnostics and reduce system overhead in running such programs. The mkstr command processes each of the specified files, placing an altered version of the input file in a file whose name consists of the specified prefix and the original name. 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 and is followed by a null character and a newline character. The null character terminates the message so it can be easily used when retrieved; the newline character makes it possible to catalog the error message file neatly to see its contents. The altered copy of the input file then contains a lseek() pointer into the file that can be used to retrieve the message to its appropri- ate source file, as shown in the following example of a program that mkstr produces. char efilname[] = "/usr/lib/pi_strings"; int efil = -1; error(int a1, int a2, int a3, int 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)) == (long)-1 ) || read(efil, buf, 256) <= 0) goto oops; printf(buf, a2, a3, a4); } EXAMPLES
To put the error messages from the current directory C source files into a file called pi_strings, and to put processed copies of the source for these files into filenames prefixed by xx, enter: mkstr pi_strings xx *.c To append the error messages from an additional source file to pi_strings, enter: mkstr - pi_strings xx newfile.c SEE ALSO
Commands: xstr(1) Functions: lseek(2) mkstr(1)
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