| 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(function_hook) |
| ... asm insns ... |
| SYM_FUNC_END(function_hook) |
| |
| 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. |