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 ORC unwinder
for most code. Both of these are especially important to support reliable
stack traces which are in turn necessary for Kernel live patching.
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?
Macros Description¶
The new macros are prefixed with the SYM_
prefix and can be divided into
three main groups:
SYM_FUNC_*
– to annotate C-like functions. This means functions with standard C calling conventions. For example, on x86, this means that 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.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.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.
objtool
requires that all code must be contained in an ELF symbol. Symbol
names that have a .L
prefix do not emit symbol table entries. .L
prefixed symbols can be used within a code region, but should be avoided for
denoting a range of code via SYM_*_START/END
annotations.
SYM_FUNC_START
andSYM_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
andSYM_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
andENDPROC
macros.SYM_FUNC_ALIAS
,SYM_FUNC_ALIAS_LOCAL
, andSYM_FUNC_ALIAS_WEAK
can be used to define multiple names for a function. The typical use is:SYM_FUNC_START(__memset) ... asm insns ... SYN_FUNC_END(__memset) SYM_FUNC_ALIAS(memset, __memset)
In this example, one can call
__memset
ormemset
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
andSYM_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 theFUNC
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 bySYM_CODE_END
.To some extent, this category corresponds to deprecated
ENTRY
andEND
. ExceptEND
had several other meanings too.SYM_INNER_LABEL*
is used to denote a label inside someSYM_{CODE,FUNC}_START
andSYM_{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
andSYM_DATA_START_LOCAL
mark the start of some data and shall be used in conjunction with eitherSYM_DATA_END
, orSYM_DATA_END_LABEL
. The latter adds also a label to the end, so that people can uselstack
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
andSYM_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
withSYM_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.