Hi, this time a more detailed look. :) > Subject: Re: [PATCH v8 01/28] linkage: new macros for assembler symbols Patch subject needs a verb, like, for example: "linkage: Introduce new macros for assembler symbols" On Thu, Aug 08, 2019 at 12:38:27PM +0200, Jiri Slaby wrote: > Introduce new C macros for annotations of functions and data in > assembly. There is a long-standing mess in macros like ENTRY, END, > ENDPROC and similar. They are used in different manners and sometimes > incorrectly. > > So introduce macros with clear use to annotate assembly as follows: > > a) Support macros for the ones below > SYM_T_FUNC -- type used by assembler to mark functions > SYM_T_OBJECT -- type used by assembler to mark data > SYM_T_NONE -- type used by assembler to mark entries of unknown type > > They are defined as STT_FUNC, STT_OBJECT, and STT_NOTYPE > respectively. According to the gas manual, this is the most portable > way. I am not sure about other assemblers, so we can switch this back > to %function and %object if this turns into a problem. Architectures > can also override them by something like ", @function" if they need. > > SYM_A_ALIGN, SYM_A_NONE -- align the symbol? > SYM_L_GLOBAL, SYM_L_WEAK, SYM_L_LOCAL -- linkage of symbols > > b) Mostly internal annotations, used by the ones below > SYM_ENTRY -- use only if you have to (for non-paired symbols) > SYM_START -- use only if you have to (for paired symbols) > SYM_END -- use only if you have to (for paired symbols) > > c) Annotations for code > SYM_INNER_LABEL_ALIGN -- only for labels in the middle of code > SYM_INNER_LABEL -- only for labels in the middle of code > > SYM_FUNC_START_LOCAL_ALIAS -- use where there are two local names for > one function > SYM_FUNC_START_ALIAS -- use where there are two global names for one > function > SYM_FUNC_END_ALIAS -- the end of LOCAL_ALIASed or ALIASed function > > SYM_FUNC_START -- use for global functions > SYM_FUNC_START_NOALIGN -- use for global functions, w/o alignment > SYM_FUNC_START_LOCAL -- use for local functions > SYM_FUNC_START_LOCAL_NOALIGN -- use for local functions, w/o > alignment > SYM_FUNC_START_WEAK -- use for weak functions > SYM_FUNC_START_WEAK_NOALIGN -- use for weak functions, w/o alignment > SYM_FUNC_END -- the end of SYM_FUNC_START_LOCAL, SYM_FUNC_START, > SYM_FUNC_START_WEAK, ... > > For functions with special (non-C) calling conventions: > SYM_CODE_START -- use for non-C (special) functions > SYM_CODE_START_NOALIGN -- use for non-C (special) functions, w/o > alignment > SYM_CODE_START_LOCAL -- use for local non-C (special) functions > SYM_CODE_START_LOCAL_NOALIGN -- use for local non-C (special) > functions, w/o alignment > SYM_CODE_END -- the end of SYM_CODE_START_LOCAL or SYM_CODE_START > > d) For data > SYM_DATA_START -- global data symbol > SYM_DATA_START_LOCAL -- local data symbol > SYM_DATA_END -- the end of the SYM_DATA_START symbol > SYM_DATA_END_LABEL -- the labeled end of SYM_DATA_START symbol > SYM_DATA -- start+end wrapper around simple global data > SYM_DATA_LOCAL -- start+end wrapper around simple local data > > ========== > > The macros allow to pair starts and ends of functions and mark functions > correctly in the output ELF objects. > > All users of the old macros in x86 are converted to use these in further > patches. > > [v2] > * use SYM_ prefix and sane names > * add SYM_START and SYM_END and parametrize all the macros > > [v3] > * add SYM_DATA, SYM_DATA_LOCAL, and SYM_DATA_END_LABEL > > [v4] > * add _NOALIGN versions of some macros > * add _CODE_ derivates of _FUNC_ macros > > [v5] > * drop "SIMPLE" from data annotations > * switch NOALIGN and ALIGN variants of inner labels > * s/visibility/linkage/; s@SYM_V_@SYM_L_@ > * add Documentation > > [v6] > * fixed typos found by Randy Dunlap > * remove doubled INNER_LABEL macros, one pair was unused > > [v8] > * use lkml.kernel.org for links > * link the docs from index.rst (by bpetkov) > * fixed typos on the docs Patch version history which doesn't belong in the commit message goes... > Signed-off-by: Jiri Slaby <jslaby@xxxxxxx> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> > Cc: hpa@xxxxxxxxx > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: jpoimboe@xxxxxxxxxx > Cc: Juergen Gross <jgross@xxxxxxxx> > Cc: Len Brown <len.brown@xxxxxxxxx> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Cc: linux-kernel@xxxxxxxxxxxxxxx > Cc: linux-pm@xxxxxxxxxxxxxxx > Cc: mingo@xxxxxxxxxx > Cc: Pavel Machek <pavel@xxxxxx> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx > Cc: x86@xxxxxxxxxx > --- ... here, under the "---" so that git can ignore it when applying. > Documentation/asm-annotations.rst | 217 ++++++++++++++++++++++++++ > Documentation/index.rst | 8 + > arch/x86/include/asm/linkage.h | 10 +- > include/linux/linkage.h | 245 +++++++++++++++++++++++++++++- > 4 files changed, 469 insertions(+), 11 deletions(-) > create mode 100644 Documentation/asm-annotations.rst > > diff --git a/Documentation/asm-annotations.rst b/Documentation/asm-annotations.rst > new file mode 100644 > index 000000000000..a967648e4325 > --- /dev/null > +++ b/Documentation/asm-annotations.rst > @@ -0,0 +1,217 @@ > +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, we group such code into functions and accompany "we" is? Make this passive: "... such code is grouped into... " > +them 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 marks to aid debuggers throughout s/marks/annotations/ > +assembly. On the top of that, developers also want to stamp some functions as On top of that, ... s/stamp/mark/ > +*global* to be visible outside of their translation units. s/to be visible/in order to be visible/ > + > +Over time, the Linux kernel took over macros from various projects (like s/took over/has adopted/ > +``binutils``) to ease these markings. s/ease these markings/facilitate such annotations/ > So for historic reasons, we have been s/we have been/people 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 for starts of > +global symbols (be it data or code). s/for starts of global symbols/to denote the beginning of global symbols/ > ``END`` used to be the end of data or end s/used to be/used to mark/ > +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 to generate a nice s/to // > +object with both sizes and types set correctly. For example the result of comma: 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 we have properly > +marked objects like this, we can run tools on them and let the tools generate > +more useful information. In particular, on properly marked objects, we can run > +``objtool`` and let it check and fix the object if needed. Passive formulation pls, i.e., get rid of the "we". Pls check and fix the whole text. > Currently, it can > +report missing frame pointer setup/destruction in functions. It can also > +automatically generate annotations for *ORC unwinder* (cf. > +<Documentation/x86/orc-unwinder.txt>) for most code. Both of these are > +especially important to support reliable stack traces which are in turn > +necessary for *Kernel live patching* (see > +<Documentation/livepatch/livepatch.txt>). > + > +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 > + > +We had a discussion_ and instead of extending the current ``ENTRY/END*`` > +macros, it was decided that we should introduce brand new macros 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@xxxxxxx > + > +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. > + > + We offer also ``SYM_FUNC_START_WEAK`` and ``SYM_FUNC_START_WEAK_NOALIGN`` > + marks as an assembler counterpart of the *weak* attribute known from C. s/marks/markings/ ... counterpart to the *weak*... > + > + 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 start and end of a > + function. I'd change that to "... find exact function boundaries." > + > + So in most cases, developers should write something like in the following > + example, having more instructions in between the macros, of course:: > + > + SYM_FUNC_START(function_hook) > + retq > + SYM_FUNC_END(function_hook) Just use "...asm insns ..." in there so that you don't have to say "having more 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 now deprecated ``ENTRY`` and > + ``ENDPROC``. "... 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) > + ... > + 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. One sentence: "... with the same result, except for the debugging information which is generated in the object file only once - for the non-aliasing function." > + > +* ``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) > + movl $-EFAULT,%eax > + EXIT ... asm insns ... > + SYM_CODE_END(bad_put_user) > + > + Again, every ``SYM_CODE_START*`` **shall** be coupled by ``SYM_CODE_END``. Btw, this coupling: I haven't gone through the whole patchset but do we have automatic checking which makes sure a starting macro is coupled with the correct ending macro? > + > + 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, we have a couple of macros to describe data in the > +assembly. > Again, they help debuggers to understand the layout of the resulting > +object files. No need for that sentence - I think it is clear by now what they're for. :-) > + > +* ``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 "Other architectures... " > +``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. Thx. -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
