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221 lines
9.4 KiB
221 lines
9.4 KiB
Assembler Annotations |
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===================== |
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Copyright (c) 2017-2019 Jiri Slaby |
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This document describes the new macros for annotation of data and code in |
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assembly. In particular, it contains information about ``SYM_FUNC_START``, |
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``SYM_FUNC_END``, ``SYM_CODE_START``, and similar. |
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Rationale |
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--------- |
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Some code like entries, trampolines, or boot code needs to be written in |
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assembly. The same as in C, such code is grouped into functions and |
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accompanied with data. Standard assemblers do not force users into precisely |
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marking these pieces as code, data, or even specifying their length. |
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Nevertheless, assemblers provide developers with such annotations to aid |
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debuggers throughout assembly. On top of that, developers also want to mark |
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some functions as *global* in order to be visible outside of their translation |
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units. |
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Over time, the Linux kernel has adopted macros from various projects (like |
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``binutils``) to facilitate such annotations. So for historic reasons, |
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developers have been using ``ENTRY``, ``END``, ``ENDPROC``, and other |
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annotations in assembly. Due to the lack of their documentation, the macros |
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are used in rather wrong contexts at some locations. Clearly, ``ENTRY`` was |
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intended to denote the beginning of global symbols (be it data or code). |
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``END`` used to mark the end of data or end of special functions with |
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*non-standard* calling convention. In contrast, ``ENDPROC`` should annotate |
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only ends of *standard* functions. |
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When these macros are used correctly, they help assemblers generate a nice |
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object with both sizes and types set correctly. For example, the result of |
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``arch/x86/lib/putuser.S``:: |
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Num: Value Size Type Bind Vis Ndx Name |
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25: 0000000000000000 33 FUNC GLOBAL DEFAULT 1 __put_user_1 |
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29: 0000000000000030 37 FUNC GLOBAL DEFAULT 1 __put_user_2 |
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32: 0000000000000060 36 FUNC GLOBAL DEFAULT 1 __put_user_4 |
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35: 0000000000000090 37 FUNC GLOBAL DEFAULT 1 __put_user_8 |
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This is not only important for debugging purposes. When there are properly |
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annotated objects like this, tools can be run on them to generate more useful |
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information. In particular, on properly annotated objects, ``objtool`` can be |
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run to check and fix the object if needed. Currently, ``objtool`` can report |
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missing frame pointer setup/destruction in functions. It can also |
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automatically generate annotations for :doc:`ORC unwinder <x86/orc-unwinder>` |
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for most code. Both of these are especially important to support reliable |
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stack traces which are in turn necessary for :doc:`Kernel live patching |
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<livepatch/livepatch>`. |
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Caveat and Discussion |
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--------------------- |
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As one might realize, there were only three macros previously. That is indeed |
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insufficient to cover all the combinations of cases: |
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* standard/non-standard function |
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* code/data |
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* global/local symbol |
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There was a discussion_ and instead of extending the current ``ENTRY/END*`` |
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macros, it was decided that brand new macros should be introduced instead:: |
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So how about using macro names that actually show the purpose, instead |
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of importing all the crappy, historic, essentially randomly chosen |
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debug symbol macro names from the binutils and older kernels? |
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.. _discussion: https://lore.kernel.org/r/[email protected] |
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Macros Description |
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------------------ |
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The new macros are prefixed with the ``SYM_`` prefix and can be divided into |
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three main groups: |
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1. ``SYM_FUNC_*`` -- to annotate C-like functions. This means functions with |
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standard C calling conventions. For example, on x86, this means that the |
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stack contains a return address at the predefined place and a return from |
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the function can happen in a standard way. When frame pointers are enabled, |
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save/restore of frame pointer shall happen at the start/end of a function, |
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respectively, too. |
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Checking tools like ``objtool`` should ensure such marked functions conform |
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to these rules. The tools can also easily annotate these functions with |
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debugging information (like *ORC data*) automatically. |
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2. ``SYM_CODE_*`` -- special functions called with special stack. Be it |
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interrupt handlers with special stack content, trampolines, or startup |
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functions. |
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Checking tools mostly ignore checking of these functions. But some debug |
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information still can be generated automatically. For correct debug data, |
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this code needs hints like ``UNWIND_HINT_REGS`` provided by developers. |
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3. ``SYM_DATA*`` -- obviously data belonging to ``.data`` sections and not to |
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``.text``. Data do not contain instructions, so they have to be treated |
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specially by the tools: they should not treat the bytes as instructions, |
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nor assign any debug information to them. |
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Instruction Macros |
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~~~~~~~~~~~~~~~~~~ |
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This section covers ``SYM_FUNC_*`` and ``SYM_CODE_*`` enumerated above. |
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``objtool`` requires that all code must be contained in an ELF symbol. Symbol |
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names that have a ``.L`` prefix do not emit symbol table entries. ``.L`` |
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prefixed symbols can be used within a code region, but should be avoided for |
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denoting a range of code via ``SYM_*_START/END`` annotations. |
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* ``SYM_FUNC_START`` and ``SYM_FUNC_START_LOCAL`` are supposed to be **the |
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most frequent markings**. They are used for functions with standard calling |
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conventions -- global and local. Like in C, they both align the functions to |
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architecture specific ``__ALIGN`` bytes. There are also ``_NOALIGN`` variants |
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for special cases where developers do not want this implicit alignment. |
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``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` markings are |
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also offered as an assembler counterpart to the *weak* attribute known from |
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C. |
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All of these **shall** be coupled with ``SYM_FUNC_END``. First, it marks |
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the sequence of instructions as a function and computes its size to the |
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generated object file. Second, it also eases checking and processing such |
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object files as the tools can trivially find exact function boundaries. |
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So in most cases, developers should write something like in the following |
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example, having some asm instructions in between the macros, of course:: |
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SYM_FUNC_START(memset) |
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... asm insns ... |
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SYM_FUNC_END(memset) |
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In fact, this kind of annotation corresponds to the now deprecated ``ENTRY`` |
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and ``ENDPROC`` macros. |
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* ``SYM_FUNC_ALIAS``, ``SYM_FUNC_ALIAS_LOCAL``, and ``SYM_FUNC_ALIAS_WEAK`` can |
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be used to define multiple names for a function. The typical use is:: |
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SYM_FUNC_START(__memset) |
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... asm insns ... |
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SYN_FUNC_END(__memset) |
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SYM_FUNC_ALIAS(memset, __memset) |
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In this example, one can call ``__memset`` or ``memset`` with the same |
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result, except the debug information for the instructions is generated to |
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the object file only once -- for the non-``ALIAS`` case. |
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* ``SYM_CODE_START`` and ``SYM_CODE_START_LOCAL`` should be used only in |
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special cases -- if you know what you are doing. This is used exclusively |
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for interrupt handlers and similar where the calling convention is not the C |
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one. ``_NOALIGN`` variants exist too. The use is the same as for the ``FUNC`` |
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category above:: |
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SYM_CODE_START_LOCAL(bad_put_user) |
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... asm insns ... |
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SYM_CODE_END(bad_put_user) |
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Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``. |
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To some extent, this category corresponds to deprecated ``ENTRY`` and |
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``END``. Except ``END`` had several other meanings too. |
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* ``SYM_INNER_LABEL*`` is used to denote a label inside some |
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``SYM_{CODE,FUNC}_START`` and ``SYM_{CODE,FUNC}_END``. They are very similar |
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to C labels, except they can be made global. An example of use:: |
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SYM_CODE_START(ftrace_caller) |
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/* save_mcount_regs fills in first two parameters */ |
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... |
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SYM_INNER_LABEL(ftrace_caller_op_ptr, SYM_L_GLOBAL) |
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/* Load the ftrace_ops into the 3rd parameter */ |
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... |
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SYM_INNER_LABEL(ftrace_call, SYM_L_GLOBAL) |
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call ftrace_stub |
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... |
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retq |
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SYM_CODE_END(ftrace_caller) |
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Data Macros |
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~~~~~~~~~~~ |
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Similar to instructions, there is a couple of macros to describe data in the |
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assembly. |
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* ``SYM_DATA_START`` and ``SYM_DATA_START_LOCAL`` mark the start of some data |
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and shall be used in conjunction with either ``SYM_DATA_END``, or |
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``SYM_DATA_END_LABEL``. The latter adds also a label to the end, so that |
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people can use ``lstack`` and (local) ``lstack_end`` in the following |
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example:: |
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SYM_DATA_START_LOCAL(lstack) |
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.skip 4096 |
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SYM_DATA_END_LABEL(lstack, SYM_L_LOCAL, lstack_end) |
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* ``SYM_DATA`` and ``SYM_DATA_LOCAL`` are variants for simple, mostly one-line |
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data:: |
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SYM_DATA(HEAP, .long rm_heap) |
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SYM_DATA(heap_end, .long rm_stack) |
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In the end, they expand to ``SYM_DATA_START`` with ``SYM_DATA_END`` |
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internally. |
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Support Macros |
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~~~~~~~~~~~~~~ |
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All the above reduce themselves to some invocation of ``SYM_START``, |
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``SYM_END``, or ``SYM_ENTRY`` at last. Normally, developers should avoid using |
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these. |
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Further, in the above examples, one could see ``SYM_L_LOCAL``. There are also |
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``SYM_L_GLOBAL`` and ``SYM_L_WEAK``. All are intended to denote linkage of a |
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symbol marked by them. They are used either in ``_LABEL`` variants of the |
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earlier macros, or in ``SYM_START``. |
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Overriding Macros |
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~~~~~~~~~~~~~~~~~ |
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Architecture can also override any of the macros in their own |
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``asm/linkage.h``, including macros specifying the type of a symbol |
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(``SYM_T_FUNC``, ``SYM_T_OBJECT``, and ``SYM_T_NONE``). As every macro |
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described in this file is surrounded by ``#ifdef`` + ``#endif``, it is enough |
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to define the macros differently in the aforementioned architecture-dependent |
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header.
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