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Diffstat (limited to 'Documentation/kernel-hacking')
| -rw-r--r-- | Documentation/kernel-hacking/hacking.rst | 179 |
1 files changed, 98 insertions, 81 deletions
diff --git a/Documentation/kernel-hacking/hacking.rst b/Documentation/kernel-hacking/hacking.rst index 75597e627791..1a456b60a7cf 100644 --- a/Documentation/kernel-hacking/hacking.rst +++ b/Documentation/kernel-hacking/hacking.rst @@ -56,7 +56,7 @@ interrupts. You can sleep, by calling :c:func:`schedule()`. In user context, the ``current`` pointer (indicating the task we are currently executing) is valid, and :c:func:`in_interrupt()` -(``include/linux/interrupt.h``) is false. +(``include/linux/preempt.h``) is false. .. warning:: @@ -114,12 +114,12 @@ time, although different tasklets can run simultaneously. Kuznetsov had at the time. You can tell you are in a softirq (or tasklet) using the -:c:func:`in_softirq()` macro (``include/linux/interrupt.h``). +:c:func:`in_softirq()` macro (``include/linux/preempt.h``). .. warning:: - Beware that this will return a false positive if a bh lock (see - below) is held. + Beware that this will return a false positive if a + :ref:`botton half lock <local_bh_disable>` is held. Some Basic Rules ================ @@ -154,9 +154,7 @@ The Linux kernel is portable ioctls: Not writing a new system call ===================================== -A system call generally looks like this - -:: +A system call generally looks like this:: asmlinkage long sys_mycall(int arg) { @@ -175,7 +173,9 @@ If all your routine does is read or write some parameter, consider implementing a :c:func:`sysfs()` interface instead. Inside the ioctl you're in user context to a process. When a error -occurs you return a negated errno (see ``include/linux/errno.h``), +occurs you return a negated errno (see +``include/uapi/asm-generic/errno-base.h``, +``include/uapi/asm-generic/errno.h`` and ``include/linux/errno.h``), otherwise you return 0. After you slept you should check if a signal occurred: the Unix/Linux @@ -195,9 +195,7 @@ some data structure. If you're doing longer computations: first think userspace. If you **really** want to do it in kernel you should regularly check if you need to give up the CPU (remember there is cooperative multitasking per CPU). -Idiom: - -:: +Idiom:: cond_resched(); /* Will sleep */ @@ -231,26 +229,24 @@ Really. Common Routines =============== -:c:func:`printk()` ``include/linux/kernel.h`` ---------------------------------------------- +:c:func:`printk()` +------------------ + +Defined in ``include/linux/printk.h`` :c:func:`printk()` feeds kernel messages to the console, dmesg, and the syslog daemon. It is useful for debugging and reporting errors, and can be used inside interrupt context, but use with caution: a machine which has its console flooded with printk messages is unusable. It uses a format string mostly compatible with ANSI C printf, and C string -concatenation to give it a first "priority" argument: - -:: +concatenation to give it a first "priority" argument:: printk(KERN_INFO "i = %u\n", i); -See ``include/linux/kernel.h``; for other ``KERN_`` values; these are +See ``include/linux/kern_levels.h``; for other ``KERN_`` values; these are interpreted by syslog as the level. Special case: for printing an IP -address use - -:: +address use:: __be32 ipaddress; printk(KERN_INFO "my ip: %pI4\n", &ipaddress); @@ -270,8 +266,10 @@ overruns. Make sure that will be enough. on top of its printf function: "Printf should not be used for chit-chat". You should follow that advice. -:c:func:`copy_[to/from]_user()` / :c:func:`get_user()` / :c:func:`put_user()` ``include/linux/uaccess.h`` ---------------------------------------------------------------------------------------------------------- +:c:func:`copy_to_user()` / :c:func:`copy_from_user()` / :c:func:`get_user()` / :c:func:`put_user()` +--------------------------------------------------------------------------------------------------- + +Defined in ``include/linux/uaccess.h`` / ``asm/uaccess.h`` **[SLEEPS]** @@ -297,8 +295,10 @@ The functions may sleep implicitly. This should never be called outside user context (it makes no sense), with interrupts disabled, or a spinlock held. -:c:func:`kmalloc()`/:c:func:`kfree()` ``include/linux/slab.h`` --------------------------------------------------------------- +:c:func:`kmalloc()`/:c:func:`kfree()` +------------------------------------- + +Defined in ``include/linux/slab.h`` **[MAY SLEEP: SEE BELOW]** @@ -324,9 +324,9 @@ message, then maybe you called a sleeping allocation function from interrupt context without ``GFP_ATOMIC``. You should really fix that. Run, don't walk. -If you are allocating at least ``PAGE_SIZE`` (``include/asm/page.h``) -bytes, consider using :c:func:`__get_free_pages()` -(``include/linux/mm.h``). It takes an order argument (0 for page sized, +If you are allocating at least ``PAGE_SIZE`` (``asm/page.h`` or +``asm/page_types.h``) bytes, consider using :c:func:`__get_free_pages()` +(``include/linux/gfp.h``). It takes an order argument (0 for page sized, 1 for double page, 2 for four pages etc.) and the same memory priority flag word as above. @@ -344,24 +344,30 @@ routine. Before inventing your own cache of often-used objects consider using a slab cache in ``include/linux/slab.h`` -:c:func:`current()` ``include/asm/current.h`` ---------------------------------------------- +:c:func:`current()` +------------------- + +Defined in ``include/asm/current.h`` This global variable (really a macro) contains a pointer to the current task structure, so is only valid in user context. For example, when a process makes a system call, this will point to the task structure of the calling process. It is **not NULL** in interrupt context. -:c:func:`mdelay()`/:c:func:`udelay()` ``include/asm/delay.h`` ``include/linux/delay.h`` ---------------------------------------------------------------------------------------- +:c:func:`mdelay()`/:c:func:`udelay()` +------------------------------------- + +Defined in ``include/asm/delay.h`` / ``include/linux/delay.h`` The :c:func:`udelay()` and :c:func:`ndelay()` functions can be used for small pauses. Do not use large values with them as you risk overflow - the helper function :c:func:`mdelay()` is useful here, or consider :c:func:`msleep()`. -:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` ``include/asm/byteorder.h`` ---------------------------------------------------------------------------------------------------------------------------- +:c:func:`cpu_to_be32()`/:c:func:`be32_to_cpu()`/:c:func:`cpu_to_le32()`/:c:func:`le32_to_cpu()` +----------------------------------------------------------------------------------------------- + +Defined in ``include/asm/byteorder.h`` The :c:func:`cpu_to_be32()` family (where the "32" can be replaced by 64 or 16, and the "be" can be replaced by "le") are the general way @@ -375,8 +381,10 @@ to the given type, and return the converted value. The other variation is the "in-situ" family, such as :c:func:`cpu_to_be32s()`, which convert value referred to by the pointer, and return void. -:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` ``include/linux/irqflags.h`` -------------------------------------------------------------------------------------- +:c:func:`local_irq_save()`/:c:func:`local_irq_restore()` +-------------------------------------------------------- + +Defined in ``include/linux/irqflags.h`` These routines disable hard interrupts on the local CPU, and restore them. They are reentrant; saving the previous state in their one @@ -384,16 +392,23 @@ them. They are reentrant; saving the previous state in their one enabled, you can simply use :c:func:`local_irq_disable()` and :c:func:`local_irq_enable()`. -:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` ``include/linux/interrupt.h`` --------------------------------------------------------------------------------------- +.. _local_bh_disable: + +:c:func:`local_bh_disable()`/:c:func:`local_bh_enable()` +-------------------------------------------------------- + +Defined in ``include/linux/bottom_half.h`` + These routines disable soft interrupts on the local CPU, and restore them. They are reentrant; if soft interrupts were disabled before, they will still be disabled after this pair of functions has been called. They prevent softirqs and tasklets from running on the current CPU. -:c:func:`smp_processor_id()`() ``include/asm/smp.h`` ----------------------------------------------------- +:c:func:`smp_processor_id()` +---------------------------- + +Defined in ``include/linux/smp.h`` :c:func:`get_cpu()` disables preemption (so you won't suddenly get moved to another CPU) and returns the current processor number, between @@ -405,8 +420,10 @@ If you know you cannot be preempted by another task (ie. you are in interrupt context, or have preemption disabled) you can use smp_processor_id(). -``__init``/``__exit``/``__initdata`` ``include/linux/init.h`` -------------------------------------------------------------- +``__init``/``__exit``/``__initdata`` +------------------------------------ + +Defined in ``include/linux/init.h`` After boot, the kernel frees up a special section; functions marked with ``__init`` and data structures marked with ``__initdata`` are dropped @@ -415,10 +432,13 @@ initialization. ``__exit`` is used to declare a function which is only required on exit: the function will be dropped if this file is not compiled as a module. See the header file for use. Note that it makes no sense for a function marked with ``__init`` to be exported to modules -with :c:func:`EXPORT_SYMBOL()` - this will break. +with :c:func:`EXPORT_SYMBOL()` or :c:func:`EXPORT_SYMBOL_GPL()`- this +will break. + +:c:func:`__initcall()`/:c:func:`module_init()` +---------------------------------------------- -:c:func:`__initcall()`/:c:func:`module_init()` ``include/linux/init.h`` ------------------------------------------------------------------------ +Defined in ``include/linux/init.h`` / ``include/linux/module.h`` Many parts of the kernel are well served as a module (dynamically-loadable parts of the kernel). Using the @@ -438,8 +458,11 @@ to fail (unfortunately, this has no effect if the module is compiled into the kernel). This function is called in user context with interrupts enabled, so it can sleep. -:c:func:`module_exit()` ``include/linux/init.h`` ------------------------------------------------- +:c:func:`module_exit()` +----------------------- + + +Defined in ``include/linux/module.h`` This macro defines the function to be called at module removal time (or never, in the case of the file compiled into the kernel). It will only @@ -450,8 +473,10 @@ it returns. Note that this macro is optional: if it is not present, your module will not be removable (except for 'rmmod -f'). -:c:func:`try_module_get()`/:c:func:`module_put()` ``include/linux/module.h`` ----------------------------------------------------------------------------- +:c:func:`try_module_get()`/:c:func:`module_put()` +------------------------------------------------- + +Defined in ``include/linux/module.h`` These manipulate the module usage count, to protect against removal (a module also can't be removed if another module uses one of its exported @@ -472,8 +497,8 @@ Wait Queues ``include/linux/wait.h`` A wait queue is used to wait for someone to wake you up when a certain condition is true. They must be used carefully to ensure there is no -race condition. You declare a ``wait_queue_head_t``, and then processes -which want to wait for that condition declare a ``wait_queue_t`` +race condition. You declare a :c:type:`wait_queue_head_t`, and then processes +which want to wait for that condition declare a :c:type:`wait_queue_t` referring to themselves, and place that in the queue. Declaring @@ -490,15 +515,15 @@ Queuing Placing yourself in the waitqueue is fairly complex, because you must put yourself in the queue before checking the condition. There is a macro to do this: :c:func:`wait_event_interruptible()` -``include/linux/wait.h`` The first argument is the wait queue head, and +(``include/linux/wait.h``) The first argument is the wait queue head, and the second is an expression which is evaluated; the macro returns 0 when -this expression is true, or -ERESTARTSYS if a signal is received. The +this expression is true, or ``-ERESTARTSYS`` if a signal is received. The :c:func:`wait_event()` version ignores signals. Waking Up Queued Tasks ---------------------- -Call :c:func:`wake_up()` ``include/linux/wait.h``;, which will wake +Call :c:func:`wake_up()` (``include/linux/wait.h``);, which will wake up every process in the queue. The exception is if one has ``TASK_EXCLUSIVE`` set, in which case the remainder of the queue will not be woken. There are other variants of this basic function available @@ -508,9 +533,9 @@ Atomic Operations ================= Certain operations are guaranteed atomic on all platforms. The first -class of operations work on ``atomic_t`` ``include/asm/atomic.h``; this -contains a signed integer (at least 32 bits long), and you must use -these functions to manipulate or read atomic_t variables. +class of operations work on :c:type:`atomic_t` (``include/asm/atomic.h``); +this contains a signed integer (at least 32 bits long), and you must use +these functions to manipulate or read :c:type:`atomic_t` variables. :c:func:`atomic_read()` and :c:func:`atomic_set()` get and set the counter, :c:func:`atomic_add()`, :c:func:`atomic_sub()`, :c:func:`atomic_inc()`, :c:func:`atomic_dec()`, and @@ -534,7 +559,7 @@ true if the bit was previously set; these are particularly useful for atomically setting flags. It is possible to call these operations with bit indices greater than -BITS_PER_LONG. The resulting behavior is strange on big-endian +``BITS_PER_LONG``. The resulting behavior is strange on big-endian platforms though so it is a good idea not to do this. Symbols @@ -546,14 +571,18 @@ 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. -:c:func:`EXPORT_SYMBOL()` ``include/linux/export.h`` ----------------------------------------------------- +:c:func:`EXPORT_SYMBOL()` +------------------------- + +Defined in ``include/linux/export.h`` This is the classic method of exporting a symbol: dynamically loaded modules will be able to use the symbol as normal. -:c:func:`EXPORT_SYMBOL_GPL()` ``include/linux/export.h`` --------------------------------------------------------- +:c:func:`EXPORT_SYMBOL_GPL()` +----------------------------- + +Defined in ``include/linux/export.h`` Similar to :c:func:`EXPORT_SYMBOL()` except that the symbols exported by :c:func:`EXPORT_SYMBOL_GPL()` can only be seen by @@ -579,11 +608,11 @@ Return Conventions ------------------ For code called in user context, it's very common to defy C convention, -and return 0 for success, and a negative error number (eg. -EFAULT) for +and return 0 for success, and a negative error number (eg. ``-EFAULT``) for failure. This can be unintuitive at first, but it's fairly widespread in the kernel. -Using :c:func:`ERR_PTR()` ``include/linux/err.h``; to encode a +Using :c:func:`ERR_PTR()` (``include/linux/err.h``) to encode a negative error number into a pointer, and :c:func:`IS_ERR()` and :c:func:`PTR_ERR()` to get it back out again: avoids a separate pointer parameter for the error number. Icky, but in a good way. @@ -603,9 +632,7 @@ Initializing structure members ------------------------------ The preferred method of initializing structures is to use designated -initialisers, as defined by ISO C99, eg: - -:: +initialisers, as defined by ISO C99, eg:: static struct block_device_operations opt_fops = { .open = opt_open, @@ -716,18 +743,14 @@ Kernel Cantrips Some favorites from browsing the source. Feel free to add to this list. -``arch/x86/include/asm/delay.h:`` - -:: +``arch/x86/include/asm/delay.h``:: #define ndelay(n) (__builtin_constant_p(n) ? \ ((n) > 20000 ? __bad_ndelay() : __const_udelay((n) * 5ul)) : \ __ndelay(n)) -``include/linux/fs.h``: - -:: +``include/linux/fs.h``:: /* * Kernel pointers have redundant information, so we can use a @@ -741,9 +764,7 @@ Some favorites from browsing the source. Feel free to add to this list. #define PTR_ERR(ptr) ((long)(ptr)) #define IS_ERR(ptr) ((unsigned long)(ptr) > (unsigned long)(-1000)) -``arch/x86/include/asm/uaccess_32.h:`` - -:: +``arch/x86/include/asm/uaccess_32.h:``:: #define copy_to_user(to,from,n) \ (__builtin_constant_p(n) ? \ @@ -751,9 +772,7 @@ Some favorites from browsing the source. Feel free to add to this list. __generic_copy_to_user((to),(from),(n))) -``arch/sparc/kernel/head.S:`` - -:: +``arch/sparc/kernel/head.S:``:: /* * Sun people can't spell worth damn. "compatability" indeed. @@ -772,9 +791,7 @@ Some favorites from browsing the source. Feel free to add to this list. .asciz "compatible" -``arch/sparc/lib/checksum.S:`` - -:: +``arch/sparc/lib/checksum.S:``:: /* 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 |