Merge branch 'x86-asm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Assembler Annotations

=====================

Copyright (c) 2017-2019 Jiri Slaby

This document describes the new macros for annotation of data and code in

assembly. In particular, it contains information about ``SYM_FUNC_START``,

``SYM_FUNC_END``, ``SYM_CODE_START``, and similar.

Rationale

---------

Some code like entries, trampolines, or boot code needs to be written in

assembly. The same as in C, such code is grouped into functions and

accompanied with data. Standard assemblers do not force users into precisely

marking these pieces as code, data, or even specifying their length.

Nevertheless, assemblers provide developers with such annotations to aid

debuggers throughout assembly. On top of that, developers also want to mark

some functions as *global* in order to be visible outside of their translation

units.

Over time, the Linux kernel has adopted macros from various projects (like

``binutils``) to facilitate such annotations. So for historic reasons,

developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other

annotations in assembly.  Due to the lack of their documentation, the macros

are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was

intended to denote the beginning of global symbols (be it data or code).

``END`` used to mark the end of data or end of special functions with

*non-standard* calling convention. In contrast, ``ENDPROC`` should annotate

only ends of *standard* functions.

When these macros are used correctly, they help assemblers generate a nice

object with both sizes and types set correctly. For example, the result of

``arch/x86/lib/putuser.S``::

   Num:    Value          Size Type    Bind   Vis      Ndx Name

    25: 0000000000000000    33 FUNC    GLOBAL DEFAULT    1 __put_user_1

    29: 0000000000000030    37 FUNC    GLOBAL DEFAULT    1 __put_user_2

    32: 0000000000000060    36 FUNC    GLOBAL DEFAULT    1 __put_user_4

    35: 0000000000000090    37 FUNC    GLOBAL DEFAULT    1 __put_user_8

This is not only important for debugging purposes. When there are properly

annotated objects like this, tools can be run on them to generate more useful

information. In particular, on properly annotated objects, ``objtool`` can be

run to check and fix the object if needed. Currently, ``objtool`` can report

missing frame pointer setup/destruction in functions. It can also

automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>`

for most code. Both of these are especially important to support reliable

stack traces which are in turn necessary for :doc:`Kernel live patching

<livepatch/livepatch>`.

Caveat and Discussion

---------------------

As one might realize, there were only three macros previously. That is indeed

insufficient to cover all the combinations of cases:

* standard/non-standard function

* code/data

* global/local symbol

There was a discussion_ and instead of extending the current ``ENTRY/END*``

macros, it was decided that brand new macros should be introduced instead::

    So how about using macro names that actually show the purpose, instead

    of importing all the crappy, historic, essentially randomly chosen

    debug symbol macro names from the binutils and older kernels?

.. _discussion: https://lkml.kernel.org/r/20170217104757.28588-1-jslaby@suse.cz

Macros Description

------------------

The new macros are prefixed with the ``SYM_`` prefix and can be divided into

three main groups:

1. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with

   standard C calling conventions, i.e. the stack contains a return address at

   the predefined place and a return from the function can happen in a

   standard way. When frame pointers are enabled, save/restore of frame

   pointer shall happen at the start/end of a function, respectively, too.

   Checking tools like ``objtool`` should ensure such marked functions conform

   to these rules. The tools can also easily annotate these functions with

   debugging information (like *ORC data*) automatically.

2. ``SYM_CODE_*`` -- special functions called with special stack. Be it

   interrupt handlers with special stack content, trampolines, or startup

   functions.

   Checking tools mostly ignore checking of these functions. But some debug

   information still can be generated automatically. For correct debug data,

   this code needs hints like ``UNWIND_HINT_REGS`` provided by developers.

3. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to

   ``.text``. Data do not contain instructions, so they have to be treated

   specially by the tools: they should not treat the bytes as instructions,

   nor assign any debug information to them.

Instruction Macros

~~~~~~~~~~~~~~~~~~

This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above.

* ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the

  most frequent markings**. They are used for functions with standard calling

  conventions -- global and local. Like in C, they both align the functions to

  architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants

  for special cases where developers do not want this implicit alignment.

  ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are

  also offered as an assembler counterpart to the *weak* attribute known from

  C.

  All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks

  the sequence of instructions as a function and computes its size to the

  generated object file. Second, it also eases checking and processing such

  object files as the tools can trivially find exact function boundaries.

  So in most cases, developers should write something like in the following

  example, having some asm instructions in between the macros, of course::

    SYM_FUNC_START(memset)

        ... asm insns ...

    SYM_FUNC_END(memset)

  In fact, this kind of annotation corresponds to the now deprecated ``ENTRY``

  and ``ENDPROC`` macros.

* ``SYM_FUNC_START_ALIAS`` and ``SYM_FUNC_START_LOCAL_ALIAS`` serve for those

  who decided to have two or more names for one function. The typical use is::

    SYM_FUNC_START_ALIAS(__memset)

    SYM_FUNC_START(memset)

        ... asm insns ...

    SYM_FUNC_END(memset)

    SYM_FUNC_END_ALIAS(__memset)

  In this example, one can call ``__memset`` or ``memset`` with the same

  result, except the debug information for the instructions is generated to

  the object file only once -- for the non-``ALIAS`` case.

* ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in

  special cases -- if you know what you are doing. This is used exclusively

  for interrupt handlers and similar where the calling convention is not the C

  one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC``

  category above::

    SYM_CODE_START_LOCAL(bad_put_user)

        ... asm insns ...

    SYM_CODE_END(bad_put_user)

  Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``.

  To some extent, this category corresponds to deprecated ``ENTRY`` and

  ``END``. Except ``END`` had several other meanings too.

* ``SYM_INNER_LABEL*`` is used to denote a label inside some

  ``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``.  They are very similar

  to C labels, except they can be made global. An example of use::

    SYM_CODE_START(ftrace_caller)

        /* save_mcount_regs fills in first two parameters */

        ...

    SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL)

        /* Load the ftrace_ops into the 3rd parameter */

        ...

    SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL)

        call ftrace_stub

        ...

        retq

    SYM_CODE_END(ftrace_caller)

Data Macros

~~~~~~~~~~~

Similar to instructions, there is a couple of macros to describe data in the

assembly.

* ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data

  and shall be used in conjunction with either ``SYM_DATA_END``, or

  ``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that

  people can use ``lstack`` and (local) ``lstack_end`` in the following

  example::

    SYM_DATA_START_LOCAL(lstack)

        .skip 4096

    SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end)

* ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line

  data::

    SYM_DATA(HEAP,     .long rm_heap)

    SYM_DATA(heap_end, .long rm_stack)

  In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END``

  internally.

Support Macros

~~~~~~~~~~~~~~

All the above reduce themselves to some invocation of ``SYM_START``,

``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using

these.

Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also

``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a

symbol marked by them. They are used either in ``_LABEL`` variants of the

earlier macros, or in ``SYM_START``.

Overriding Macros

~~~~~~~~~~~~~~~~~

Architecture can also override any of the macros in their own

``asm/linkage.h``, including macros specifying the type of a symbol

(``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``).  As every macro

described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough

to define the macros differently in the aforementioned architecture-dependent

header.

   mic/index

   scheduler/index

Architecture-agnostic documentation

-----------------------------------

.. toctree::

   :maxdepth: 2

   asm-annotations

Architecture-specific documentation

-----------------------------------

/* SPDX-License-Identifier: GPL-2.0 */

#define EMITS_PT_NOTE

#define RO_EXCEPTION_TABLE_ALIGN	16

#include <asm-generic/vmlinux.lds.h>

#include <asm/thread_info.h>

#include <asm/cache.h>

OUTPUT_FORMAT("elf64-alpha")

OUTPUT_ARCH(alpha)

ENTRY(__start)

PHDRS { kernel PT_LOAD; note PT_NOTE; }

PHDRS { text PT_LOAD; note PT_NOTE; }

jiffies = jiffies_64;

SECTIONS

{

		LOCK_TEXT

		*(.fixup)

		*(.gnu.warning)

	} :kernel

	} :text

	swapper_pg_dir = SWAPPER_PGD;

	_etext = .;	/* End of text section */

	NOTES :kernel :note

	.dummy : {

		*(.dummy)

	} :kernel

	RODATA

	EXCEPTION_TABLE(16)

	RO_DATA(4096)

	/* Will be freed after init */

	__init_begin = ALIGN(PAGE_SIZE);

	_sdata = .;	/* Start of rw data section */

	_data = .;

	RW_DATA_SECTION(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	.got : {

		*(.got)

	_etext = .;

	_sdata = .;

	RO_DATA_SECTION(PAGE_SIZE)

	RO_DATA(PAGE_SIZE)

	/*

	 * 1. this is .data essentially

	 * 2. THREAD_SIZE for init.task, must be kernel-stk sz aligned

	 */

	RW_DATA_SECTION(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	_edata = .;

	/DISCARD/ : {	*(.eh_frame) }

#endif

	NOTES

	. = ALIGN(PAGE_SIZE);

	_end = . ;

	ARM_UNWIND_SECTIONS

#endif

	NOTES

	_etext = .;			/* End of text and rodata section */

	ARM_VECTORS

	. = ALIGN(THREAD_SIZE);

	_sdata = .;

	RW_DATA_SECTION(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)

	.data.ro_after_init : AT(ADDR(.data.ro_after_init) - LOAD_OFFSET) {

		*(.data..ro_after_init)

	}