Interrupt Enable

/* TODO */

2011-06-30T13:36:00.000-07:00

This is a list of topics I plan to write some posts about:

* softlockup and hung tasks.
* kernel debugging tools: kmememleak, lockdep, kmemcheck, perf
* how to submit patches to Linux kernel.
* getting started with the Linux kernel.
* anatomy of the TCP/IP Linux stack.
* analyze TCP connection setup using tcpdump.
* kernel/userspace communication.

Introducing pread

2009-05-06T01:50:00.000-07:00

There are many situations when you just want to read some data from a file starting with a given offset. The usual way of doing this is:

 
     lseek(fd, offset, SEEK_SET);
     read(fd, buffer, LEN);

You can do this in one function call using pread(2) . The above code is equivalent with:


    pread(fd, buffer, LEN, offset);

In a similar manner you can notice the existence of pwrite(2) which writes some data into a file at a given position.

call stack without words

2009-03-18T02:59:00.000-07:00

ltrace

2009-03-11T00:29:00.000-07:00

Usage: ltrace [option ...] [command [arg ...]]

ltrace is a tool that simply runs the specified command until it exits. It intercepts and records the dynamic library calls which are called by the executed process.


dan@lemon:~/playground$ ltrace -n 8 -C date
__libc_start_main(0x8049780, 1, 0xbfb1e874, 0x8052c50, 0x8052c40 
       setlocale(6, "")                                                                         = "en_US.UTF-8"
       bindtextdomain("coreutils", "/usr/share/locale")                                         = "/usr/share/locale"
       textdomain("coreutils")                                                                  = "coreutils"
       __cxa_atexit(0x804cd70, 0, 0, 0xb7ee4ff4, 0xbfb1e7d8)                                    = 0
       getopt_long(1, 0xbfb1e874, "d:f:I::r:Rs:u", 0x8053ea0, NULL)                             = -1
       nl_langinfo(131180, 0xbfb1e874, 0x8053e85, 0x8053ea0, 0)                                 = 0xb7ef3915
       clock_gettime(0, 0xbfb1e7b8, 0xb7dae8f5, 0xb7ee4ff4, 0)                                  = 0
       localtime(0xbfb1e6c8)                                                                    = 0xb7ee8380
       strftime(" Wed", 1024, " %a", 0xb7ee8380)                                                = 4
       fwrite("Wed", 3, 1, 0xb7ee54c0)                                                          = 1
       fputc(' ', 0xb7ee54c0)                                                                   = 32
       strftime(" Mar", 1024, " %b", 0xb7ee8380)                                                = 4
       fwrite("Mar", 3, 1, 0xb7ee54c0)                                                          = 1
       fputc(' ', 0xb7ee54c0)                                                                   = 32
       fwrite("11", 2, 1, 0xb7ee54c0)                                                           = 1
       fputc(' ', 0xb7ee54c0)                                                                   = 32
       fputc('0', 0xb7ee54c0)                                                                   = 48
       fwrite("9", 1, 1, 0xb7ee54c0)                                                            = 1
       fputc(':', 0xb7ee54c0)                                                                   = 58
       fwrite("38", 2, 1, 0xb7ee54c0)                                                           = 1
       fputc(':', 0xb7ee54c0)                                                                   = 58
       fwrite("33", 2, 1, 0xb7ee54c0)                                                           = 1
       fputc(' ', 0xb7ee54c0)                                                                   = 32
       strlen("EET")                                                                            = 3
       fwrite("EET", 3, 1, 0xb7ee54c0)                                                          = 1
       fputc(' ', 0xb7ee54c0)                                                                   = 32
       fwrite("2009", 4, 1, 0xb7ee54c0)                                                         = 1
       __overflow(0xb7ee54c0, 10, 0x1e08f70c, 0, 0x1e08f70cWed Mar 11 09:38:33 EET 2009
)                                    = 10
       exit(0 
               __fpending(0xb7ee54c0, 0xb7f20ff4, 0x8048704, 0xb7ee4ff4, 0)                     = 0
               fclose(0xb7ee54c0)                                                               = 0
               __fpending(0xb7ee5560, 0xb7f20ff4, 0x8048704, 0xb7ee4ff4, 0)                     = 0
               fclose(0xb7ee5560)                                                               = 0
+++ exited (status 0) +++

ltrace accepts a lot of options. You can trace also for system calls, count time , follow forks etc. A short list with options can be found below:


  -a, --align=COLUMN  align return values in a secific column.
  -c                  count time and calls, and report a summary on exit.
  -C, --demangle      decode low-level symbol names into user-level names.
  -d, --debug         print debugging info.
  -e expr             modify which events to trace.
  -f                  follow forks.
  -h, --help          display this help and exit.
  -i                  print instruction pointer at time of library call.
  -l, --library=FILE  print library calls from this library only.
  -L                  do NOT display library calls.
  -n, --indent=NR     indent output by NR spaces for each call level nesting.
  -o, --output=FILE   write the trace output to that file.
  -p PID              attach to the process with the process ID pid.
  -r                  print relative timestamps.
  -s STRLEN           specify the maximum string size to print.
  -S                  display system calls.
  -t, -tt, -ttt       print absolute timestamps.
  -T                  show the time spent inside each call.
  -u USERNAME         run command with the userid, groupid of username.
  -V, --version       output version information and exit.
  -x NAME             treat the global NAME like a library subroutine.

dmidecode

2009-03-03T04:10:00.000-08:00

dmidecode is a tool for dumping a computer's DMI table contents in a human readable format. This table contains information regarding system's hardware components as described in BIOS.

More information you can find here or in dmidecode(8) manpage.

Compilation without words

2009-02-22T02:40:00.001-08:00

Click on the image to have a better view.

Lindent

2009-02-13T23:09:00.000-08:00

Lindent is a script that changes the appearance of a C program according to linux kernel coding style. It can be found in linux/scripts/ and it makes use of indent.(see indent(1) manual page).

For a debian based distribution running Lindent is straightforward:

#install indent
apt-get install indent
#run Lindent over a file
linux/scripts/Lindent file.c

The preferred coding style used in linux kernel is described in linux/Documentation/CodingStyle.

Cycle detection in linked lists

2008-12-03T08:58:00.000-08:00

Find if a linked list has a cycle given a pointer to it's first element.

There are several ways of solving this problem.

iterate over the list and insert elements into a hash table. We have a cycle if we insert the same element twice. This solution requires 0(n) time and 0(n) memory.

adapt the list to this particular situation. Insert a field that marks the node as visited or not. This solution has the drawback of modifying the list which is not always possible.

Floyd algorithm which will be presented below its an efficient and easy to understand algorithm and requires O(n) time and O(1) additional memory.

Also known as the tortoise and hare algorithm it uses two pointers a and b , where a moves two times faster than b.
The pseudocode for the algorithm is as follows:

 
      a <- list_head
      b <- list_head
      do
           a <- a.next.next
           b <- b.next
      while a <> b 
//if there is a cycle then a will be equal at some point with b.

An immediate application of Floyd's algorithm is retrieving the element in the middle of a list without computing it's length.

GCC hacks in the Linux kernel

2008-12-01T06:21:00.000-08:00

Discover GCC extensions for the C language here.
This article talks about functionality extensions that bring new functionality from GCC and about optimization extensions that are used for generating more efficient code.

One thing that it's easy to remember is using ranges with case statements.
So instead of:

  switch(level){
         case 1:
         case 2:
         case 3: /* do something */
         ......
         default: /* */
        }

you can use:

  switch(level){
         case 1..3: /* do something */
         break;
       }

isTwoPower

2008-11-30T02:42:00.000-08:00

Very simple and elegant function to test if a number is a power of two.


int isTwoPower(int n){
     if( n < 1 )
         return 0;
     return !(n & (n-1));
}

Hello World!

2008-11-29T06:37:00.000-08:00

.data
hello:
   .ascii "Hello World!\n"
   len = . - hello
.text
   .global _start
_start:
   movl $4,     %eax
   movl $1,     %ebx
   movl $hello, %ecx
   movl $len,   %edx
   int $0x80

   movl $1, %eax
   movl $0, %ebx
   int $0x80