kernel-hacking.tmpl

讲述linux的初始化过程

   <function>test_and_change_bit()</function> do the same thing,
   except return true if the bit was previously set; these are
   particularly useful for very simple locking.
  </para>
  
  <para>
   It is possible to call these operations with bit indices greater
   than BITS_PER_LONG.  The resulting behavior is strange on big-endian
   platforms though so it is a good idea not to do this.
  </para>

  <para>
   Note that the order of bits depends on the architecture, and in
   particular, the bitfield passed to these operations must be at
   least as large as a <type>long</type>.
  </para>
 </chapter>

 <chapter id="symbols">
  <title>Symbols</title>

  <para>
   Within the kernel proper, the normal linking rules apply
   (ie. unless a symbol is declared to be file scope with the
   <type>static</type> keyword, it can be used anywhere in the
   kernel).  However, for modules, a special exported symbol table is
   kept which limits the entry points to the kernel proper.  Modules
   can also export symbols.
  </para>

  <sect1 id="sym-exportsymbols">
   <title><function>EXPORT_SYMBOL()</function>
    <filename class=headerfile>include/linux/module.h</filename></title>

   <para>
    This is the classic method of exporting a symbol, and it works
    for both modules and non-modules.  In the kernel all these
    declarations are often bundled into a single file to help
    genksyms (which searches source files for these declarations).
    See the comment on genksyms and Makefiles below.
   </para>
  </sect1>

  <sect1 id="sym-exportsymtab">
   <title><function>EXPORT_SYMTAB</function></title>

   <para>
    For convenience, a module usually exports all non-file-scope
    symbols (ie. all those not declared <type>static</type>).  If this
    is defined before

    <filename class=headerfile>include/linux/module.h</filename> is
    included, then only symbols explicit exported with
    <function>EXPORT_SYMBOL()</function> will be exported.
   </para>
  </sect1>
 </chapter>

 <chapter id="conventions">
  <title>Routines and Conventions</title>

  <sect1 id="conventions-doublelinkedlist">
   <title>Double-linked lists
    <filename class=headerfile>include/linux/list.h</filename></title>

   <para>
    There are three sets of linked-list routines in the kernel
    headers, but this one seems to be winning out (and Linus has
    used it).  If you don't have some particular pressing need for
    a single list, it's a good choice.  In fact, I don't care
    whether it's a good choice or not, just use it so we can get
    rid of the others.
   </para>
  </sect1>

  <sect1 id="convention-returns">
   <title>Return Conventions</title>

   <para>
    For code called in user context, it's very common to defy C
    convention, and return <returnvalue>0</returnvalue> for success,
    and a negative error number
    (eg. <returnvalue>-EFAULT</returnvalue>) for failure.  This can be
    unintuitive at first, but it's fairly widespread in the networking
    code, for example.
   </para>

   <para>
    The filesystem code uses <function>ERR_PTR()</function>

    <filename class=headerfile>include/linux/fs.h</filename>; to
    encode a negative error number into a pointer, and
    <function>IS_ERR()</function> and <function>PTR_ERR()</function>
    to get it back out again: avoids a separate pointer parameter for
    the error number.  Icky, but in a good way.
   </para>
  </sect1>

  <sect1 id="conventions-borkedcompile">
   <title>Breaking Compilation</title>

   <para>
    Linus and the other developers sometimes change function or
    structure names in development kernels; this is not done just to
    keep everyone on their toes: it reflects a fundamental change
    (eg. can no longer be called with interrupts on, or does extra
    checks, or doesn't do checks which were caught before).  Usually
    this is accompanied by a fairly complete note to the linux-kernel
    mailing list; search the archive.  Simply doing a global replace
    on the file usually makes things <emphasis>worse</emphasis>.
   </para>
  </sect1>

  <sect1 id="conventions-initialising">
   <title>Initializing structure members</title>

   <para>
    The preferred method of initializing structures is to use the
    gcc Labeled Elements extension, eg:
   </para>
   <programlisting>
static struct block_device_operations opt_fops = {
        open:                   opt_open,
        release:                opt_release,
        ioctl:                  opt_ioctl,
        check_media_change:     opt_media_change,
};
   </programlisting>

   <para>
    This makes it easy to grep for, and makes it clear which
    structure fields are set.  You should do this because it looks
    cool.
   </para>
  </sect1>

  <sect1 id="conventions-gnu-extns">
   <title>GNU Extensions</title>

   <para>
    GNU Extensions are explicitly allowed in the Linux kernel.
    Note that some of the more complex ones are not very well
    supported, due to lack of general use, but the following are
    considered standard (see the GCC info page section "C
    Extensions" for more details - Yes, really the info page, the
    man page is only a short summary of the stuff in info):
   </para>
   <itemizedlist>
    <listitem>
     <para>
      Inline functions
     </para>
    </listitem>
    <listitem>
     <para>
      Statement expressions (ie. the ({ and }) constructs).
     </para>
    </listitem>
    <listitem>
     <para>
      Declaring attributes of a function / variable / type
      (__attribute__)
     </para>
    </listitem>
    <listitem>
     <para>
      Labeled elements
     </para>
    </listitem>
    <listitem>
     <para>
      typeof
     </para>
    </listitem>
    <listitem>
     <para>
      Zero length arrays
     </para>
    </listitem>
    <listitem>
     <para>
      Macro varargs
     </para>
    </listitem>
    <listitem>
     <para>
      Arithmetic on void pointers
     </para>
    </listitem>
    <listitem>
     <para>
      Non-Constant initializers
     </para>
    </listitem>
    <listitem>
     <para>
      Assembler Instructions (not outside arch/ and include/asm/)
     </para>
    </listitem>
    <listitem>
     <para>
      Function names as strings (__FUNCTION__)
     </para>
    </listitem>
    <listitem>
     <para>
      __builtin_constant_p()
     </para>
    </listitem>
   </itemizedlist>

   <para>
    Be wary when using long long in the kernel, the code gcc generates for
    it is horrible and worse: division and multiplication does not work
    on i386 because the GCC runtime functions for it are missing from
    the kernel environment.
   </para>

    <!-- FIXME: add a note about ANSI aliasing cleanness -->
  </sect1>

  <sect1 id="conventions-cplusplus">
   <title>C++</title>
   
   <para>
    Using C++ in the kernel is usually a bad idea, because the
    kernel does not provide the necessary runtime environment
    and the include files are not tested for it.  It is still
    possible, but not recommended.  If you really want to do
    this, forget about exceptions at least.
   </para>
  </sect1>

  <sect1 id="conventions-ifdef">
   <title>&num;if</title>
   
   <para>
    It is generally considered cleaner to use macros in header files
    (or at the top of .c files) to abstract away functions rather than
    using `#if' pre-processor statements throughout the source code.
   </para>
  </sect1>
 </chapter>

 <chapter id="submitting">
  <title>Putting Your Stuff in the Kernel</title>

  <para>
   In order to get your stuff into shape for official inclusion, or
   even to make a neat patch, there's administrative work to be
   done:
  </para>
  <itemizedlist>
   <listitem>
    <para>
     Figure out whose pond you've been pissing in.  Look at the top of
     the source files, inside the <filename>MAINTAINERS</filename>
     file, and last of all in the <filename>CREDITS</filename> file.
     You should coordinate with this person to make sure you're not
     duplicating effort, or trying something that's already been
     rejected.
    </para>

    <para>
     Make sure you put your name and EMail address at the top of
     any files you create or mangle significantly.  This is the
     first place people will look when they find a bug, or when
     <emphasis>they</emphasis> want to make a change.
    </para>
   </listitem>

   <listitem>
    <para>
     Usually you want a configuration option for your kernel hack.
     Edit <filename>Config.in</filename> in the appropriate directory
     (but under <filename>arch/</filename> it's called
     <filename>config.in</filename>).  The Config Language used is not
     bash, even though it looks like bash; the safe way is to use only
     the constructs that you already see in
     <filename>Config.in</filename> files (see
     <filename>Documentation/kbuild/config-language.txt</filename>).
     It's good to run "make xconfig" at least once to test (because
     it's the only one with a static parser).
    </para>

    <para>
     Variables which can be Y or N use <type>bool</type> followed by a
     tagline and the config define name (which must start with
     CONFIG_).  The <type>tristate</type> function is the same, but
     allows the answer M (which defines
     <symbol>CONFIG_foo_MODULE</symbol> in your source, instead of
     <symbol>CONFIG_FOO</symbol>) if <symbol>CONFIG_MODULES</symbol>
     is enabled.
    </para>

    <para>
     You may well want to make your CONFIG option only visible if
     <symbol>CONFIG_EXPERIMENTAL</symbol> is enabled: this serves as a
     warning to users.  There many other fancy things you can do: see
     the the various <filename>Config.in</filename> files for ideas.
    </para>
   </listitem>

   <listitem>
    <para>
     Edit the <filename>Makefile</filename>: the CONFIG variables are
     exported here so you can conditionalize compilation with `ifeq'.
     If your file exports symbols then add the names to
     <varname>MX_OBJS</varname> or <varname>OX_OBJS</varname> instead
     of <varname>M_OBJS</varname> or <varname>O_OBJS</varname>, so
     that genksyms will find them.
    </para>
   </listitem>

   <listitem>
    <para>
     Document your option in Documentation/Configure.help.  Mention
     incompatibilities and issues here.  <emphasis> Definitely
     </emphasis> end your description with <quote> if in doubt, say N
     </quote> (or, occasionally, `Y'); this is for people who have no
     idea what you are talking about.
    </para>
   </listitem>

   <listitem>
    <para>
     Put yourself in <filename>CREDITS</filename> if you've done
     something noteworthy, usually beyond a single file (your name
     should be at the top of the source files anyway).
     <filename>MAINTAINERS</filename> means you want to be consulted
     when changes are made to a subsystem, and hear about bugs; it
     implies a more-than-passing commitment to some part of the code.
    </para>
   </listitem>
  </itemizedlist>
 </chapter>

 <chapter id="cantrips">
  <title>Kernel Cantrips</title>

  <para>
   Some favorites from browsing the source.  Feel free to add to this
   list.
  </para>

  <para>
   <filename>include/linux/brlock.h:</filename>
  </para>
  <programlisting>
extern inline void br_read_lock (enum brlock_indices idx)
{
        /*
         * This causes a link-time bug message if an
         * invalid index is used:
         */
        if (idx >= __BR_END)
                __br_lock_usage_bug();

        read_lock(&amp;__brlock_array[smp_processor_id()][idx]);
}
  </programlisting>

  <para>
   <filename>include/linux/fs.h</filename>:
  </para>
  <programlisting>
/*
 * Kernel pointers have redundant information, so we can use a
 * scheme where we can return either an error code or a dentry
 * pointer with the same return value.
 *
 * This should be a per-architecture thing, to allow different
 * error and pointer decisions.
 */
 #define ERR_PTR(err)    ((void *)((long)(err)))
 #define PTR_ERR(ptr)    ((long)(ptr))
 #define IS_ERR(ptr)     ((unsigned long)(ptr) > (unsigned long)(-1000))
</programlisting>

  <para>
   <filename>include/asm-i386/uaccess.h:</filename>
  </para>

  <programlisting>
#define copy_to_user(to,from,n)                         \
        (__builtin_constant_p(n) ?                      \
         __constant_copy_to_user((to),(from),(n)) :     \
         __generic_copy_to_user((to),(from),(n)))
  </programlisting>

  <para>
   <filename>arch/sparc/kernel/head.S:</filename>
  </para>

  <programlisting>
/*
 * Sun people can't spell worth damn. "compatability" indeed.
 * At least we *know* we can't spell, and use a spell-checker.
 */

/* Uh, actually Linus it is I who cannot spell. Too much murky
 * Sparc assembly will do this to ya.
 */
C_LABEL(cputypvar):
        .asciz "compatability"

/* Tested on SS-5, SS-10. Probably someone at Sun applied a spell-checker. */
        .align 4
C_LABEL(cputypvar_sun4m):
        .asciz "compatible"
  </programlisting>

  <para>
   <filename>arch/sparc/lib/checksum.S:</filename>
  </para>

  <programlisting>
        /* Sun, you just can't beat me, you just can't.  Stop trying,
         * give up.  I'm serious, I am going to kick the living shit
         * out of you, game over, lights out.
         */
  </programlisting>
 </chapter>

 <chapter id="credits">
  <title>Thanks</title>

  <para>
   Thanks to Andi Kleen for the idea, answering my questions, fixing
   my mistakes, filling content, etc.  Philipp Rumpf for more spelling
   and clarity fixes, and some excellent non-obvious points.  Werner
   Almesberger for giving me a great summary of
   <function>disable_irq()</function>, and Jes Sorensen and Andrea
   Arcangeli added caveats. Michael Elizabeth Chastain for checking
   and adding to the Configure section. <!-- Rusty insisted on this
   bit; I didn't do it! --> Telsa Gwynne for teaching me DocBook. 
  </para>
 </chapter>
</book>