wiki:Developer/Tracing/Trace_Linker

RTEMS Trace Linker

The RTEMS Trace Linker is a tool that is part of the RTEMS Tool Project and is central in the RTEMS Tracing framework.

The RTEMS Trace Linker is a post link tool that performs a re-link of your application to produce a trace executable. A trace executable has been instrumented by the RTEMS Trace Linker with additional code that implements software tracing. A key requirement of the trace process in RTEMS is to take existing code in a compiled format (ELF) and instrument it without rebuilding that code from source and without annotating that source with trace code. The code being trace may be from a 3rd party or already certified or in the process of being certified and the trace testing is part of that process.

The RTEMS Trace Linker is controlled using configuration files in a flexible way and can be configured to implement a number of tracing schemes.

The RTEMS Source Builder builds and installs the RTEMS Tools package so the RTEMS Trace Linker is available for you to use.

Command Line

The current command line for the trace linker is:

$ rtems-tld -h
rtems-trace-ld [options] objects
Options and arguments:
 -h          : help (also --help)
 -V          : print linker version number and exit (also --version)
 -v          : verbose (trace import parts), can supply multiple times
               to increase verbosity (also --verbose)
 -w          : generate warnings (also --warn)
 -k          : keep temporary files (also --keep)
 -c compiler : target compiler is not standard (also --compiler)
 -l linker   : target linker is not standard (also --linker)
 -E prefix   : the RTEMS tool prefix (also --exec-prefix)
 -f cflags   : C compiler flags (also --cflags)
 -r path     : RTEMS path (also --rtems)
 -B bsp      : RTEMS arch/bsp (also --rtems-bsp)
 -W wrapper  : wrapper file name without ext (also --wrapper)
 -C ini      : user configuration INI file (also --config)
 -P path     : user configuration INI file search path (also --path)

There are two parts to the command line passed to the trace linker. The first part controls the trace linker and provides the various options it needs and the second part is a standard linker command line you would use to link an RTEMS application. The first and second parts are separated by -- the command line option escape sequence.

The trace linker generates code that needs to be compiled and linked your standard executable so it needs to know the target compiler and CFLAGS. There are a couple of ways to do this. The simplest is to provide the path to RTEMS using the -r option and the architecture and BSP name in the standard RTEMS format of arch/bsp. The trace linker will extract the compiler and flags used to build RTEMS and will use them. If you require specific options you can use the -f, -c, -l and -E options to provide them. If the functions you are tracing use types from you code add the include path to the CFLAGS.

The trace linker requires you provide a configuration file using the -C or --config option. This is an INI format file detailed in the Configuration section. You can also provide an INI file search path using the -P option.

If you are working with new configuration files and you want to view the files the trace linker generates add the -k option to keep the temporary files, and -W to specify an explicit wrapper C file name. If you set the dump-on-error option in the configuration options section you will get a dump of the configuration on an error.

Wrapping

The trace linker's main role is to wrap functions in the existing executable with trace code. The trace linker executable does not know about the trace code added. The trace is provided by the generator configuration. The wrapping function uses a GNU linker option called --wrap=symbol. The GNU Ld manual states:

Use a wrapper function for symbol. Any undefined reference to symbol will be resolved to __wrap_symbol. Any undefined reference to __real_symbol will be resolved to symbol.

The trace linker generates C code with a wrapper for each function to be instrumented. The trace code generated is driven by the configuration INI files.

Function Signatures

A function signature is the function's declaration. It is the name of the function, the return value and the arguments. Tracing using function wrappers requires we have accurate function signatures and ideally we would like to determine the function signature from the data held in ELF files. ELF files can contain DWARF data, the ELF debugging data format. In time the trace project would like to support libdwarf so the DWARF data can be accessed and use to determine a function's signature. This work is planned but not scheduled to be done and so in the meantime we explicitly define the function signatures in the configuration files.

Configuration

The trace linker uses the INI file format for configuration files. Users provide a top level configuration that defines the trace executable created. The trace linker comes with a number of standard configurations that provide a range of functionality. A user can use those configurations or they can define a completely new set and produce a localised specific trace executable that meets their needs.

There are 3 types of configurations:

  1. User configurations. These are specific to an application and are typically kept with the application.
  2. Tracer configurations. These are like a library of common or base trace functions that can be referenced by an application. These files tend to hold the details needed to wrap a specific set of functions. Examples provided with the RTEMS Linker are the RTEMS API and Libc.
  3. Generator configurations. These encapsulate a specific method of tracing. The RTEMS Linker provides buffer tracer, printk and printf generators.

The break down into these types of files promotes reuse. You could combine all the files into a single configuration file.

INI Files

INI file format consists of sections with a section name that groups keys. A key has a name and value used as name=value. An example format is:

[section]
name1=value1
name2 = value2

The use of keys depends on the user of the INI file and in our case this is the trace linker. The trace linker can include other INI files using the include key name and a comma separated list of files to include:

include = rtems.ini, rtld-base.ini

The trace linker also uses values in keys to specify other sections. In this example the functions name lists test-trace-funcs and that section contains a headers key that references a further section test-headers:

functions = test-trace-funcs, rtems-api

[test-trace-funcs]
; Parsed via the 'function-set', not parse as a 'trace'.
headers = test-headers

[test-headers] 
header = '#include "test-trace-1.h"'

The ability to include INI files and have key lists reference sections lets the trace linker provide base functionality a user can specialise.

Tracer Section

The top level section is [tracer]. The tracer section can contain the following keys:

name
The name of trace being linked.
options
A list of option sections.
defines
A list of sections containing defines or define record.
define
A list of define string that are single or double quoted.
enables
The list of sections containing enabled functions to trace.
triggers
The list of sections containing enabled functions to trigger trace on.
traces
The list of sections containing function lists to trace.
functions
The list of sections containing function details.
include
The list of files to include.

Options

The following options are available:

dump-on-error
Dump the parsed configuration data on error. The value can be true or false.
verbose
Set the verbose level. The value can be true or a number value.
prefix
The prefix for the tools and an install RTEMS if rtems-path is not set.
cc
The compiler used to compile the generated wrapper code. Overrides the BSP configuration value if a BSP is specified.
ld
The linker used to link the application. The default is the cc value as read from the BSP configuration if specificed. If your application contains C++ code use this setting to the change the linker to g++.
cflags
Set the CFLAGS used to compiler the wrapper. These flags are pre-pended to the BSP read flags if a BSP is specified. This option is used to provide extra include paths to header files in your application that contain types any functions being traced reference.
rtems-path
The path to an install RTEMS if not installed under the prefix.
rtems-bsp
The BSP we are building the trace executable for. The is an arch and bsp pair. For example arm/xilinx_zynq_zc706.

An example tracer section is:

;
; RTEMS Trace Linker Test Configuration.
;
; We must provide a top level trace section.
;
[tracer]
;
; Name of the trace.
;
name = RTEMS Trace Linker Test
;
; The BSP.
;
bsp = sparc/sis
;
; Options can be defined here or on the command line.
;
options = test-options
;
; Functions to trace.
;
traces = test-trace, test-trace-funcs, rtems-api-task
;
; Define the function sets. These are the function's that can be
; added to the trace lists.
;
functions = test-trace-funcs, rtems-api
;
; Include RTEMS Trace support.
;
include = rtems.ini, rtld-base.ini

[test-options]
verbose = true
prefix = /opt/rtems/4.11

Trace Section

A trace section defines how trace wrapper functions are built. To build a trace function that wraps an existing function in an ELF object file or library archive we need to have the function's signature. A signature is the function's declaration with any types used. The the signature has specific types we need access to those types which means the wrapper code needs to include header files that define those types. There may also be specific defines needed to access those types.

generator
The generator defines the type of tracing being used.
headers
List of sections that contain header files keys.
header
A header key. Typically the include code.
defines
List of sections that contain defines.
define
A define key. Typically the define code.
signatures
List of function signature sections.
trace
Functions that are instrumented with trace code.

An example trace section is:

;
; User application trace example.
;
[test-trace]
generator = printf-generator
; Just here for testing.
trace = test_trace_3

[test-trace-funcs]
; Parsed via the 'function-set', not parse as a 'trace'.
headers = test-headers
header = '#include "test-trace-2.h"'
defines = test-defines
define = "#define TEST_TRACE_2 2"
signatures = test-signatures
; Parsed via the 'trace', not parsed as a function-set
trace = test_trace_1, test_trace_2

[test-headers]
header = '#include "test-trace-1.h"'

[test-defines]
define = "#define TEST_TRACE_1 1"

[test-signatures]
test_trace_1 = void, int
test_trace_2 = test_type_2, test_type_1
test_trace_3 = float, float*

A trace section can reference other trace sections of a specific type. This allows a trace sections to build on other trace sections.

Function Sections

Function sections define functions that can be traced. The provide any required defines, header files, and the function signatures. Defining a function so it can be traced does not mean it is traced. The function has to be added to a trace list to be traced.

headers
A list of sections containing headers or header records.
header
A list of include string that are single or double quoted.
defines
A list of sections containing defines or define record.
defines
A list of define string that are single or double quoted.
signatures
A list of section names of signatures.
includes
A list of files to include.

Function signatures are specified with the function name being the key's name and the key's value being the return value and a list of function arguments. You need to provide void if the function uses void. Variable argument list are currently not supported. There is no way to determine statically a variable argument list.

Generators

Generators sections specify how to generate trace wrapping code. The trace linker and generator section must match to work. The trace linker expects a some things to be present when wrapping functions. The section's name specifies the generator and can be listed in a generator key in a tracer or trace section. If the generator is not interested in a specific phase it does not need do not define it and nothing will be generated. For example code to profile specific functions may only provide the entry-trace and exit-trace code where a nano-second time stamp is taken.

The generate code will create an entry and exit calls and the generator code block can be used to allocate buffer space for each with the lock held. The entry call and argument copy is performed with the lock released. The buffer space having been allocated will cause the trace events to be in order. The same goes for the exit call. Space is allocated in separate buffer allocate calls so the blocking calls will have the exit event appear in the correct location in the buffer.

A generator specifies the code generated:

headers
A list of sections containing headers or header records.
header
A list of include string that are single or double quoted.
defines
A list of sections containing defines or define record.
define
A list of define string that are single or double quoted.
entry-trace
The wrapper call made on a function's entry. Returns bool where true` is the function is being traced. This call is made without the lock being held if a lock is defined.
arg-trace
The wrapper call made for each argument to the trace function if the function is being traced. This call is made without the lock being held if a lock is defined.
exit-trace
The wrapper call made after a function's exit. Returns bool where true` is the function is being traced. This call is made without the lock being held if a lock is defined.
ret-trace
The wrapper call made to log the return value if the function is being traced. This call is made without the lock being held if a lock is defined.
lock-local
The wrapper code to declare a local lock variable.
lock-acquire
The wrapper code to acquire the lock.
lock-release
The wrapper code to release the lock.
buffer-local
The wrapper code to declare a buffer index local variable.
buffer-alloc
The wrapper call made with a lock held if defined to allocate buffer space to hold the trace data. A suitable 32bit buffer index is returned. If there is no space an invalid index is returned. The generator must handle any overhead space needed. the generator needs to make sure the space is available before making the alloc all.
code-blocks
A list of code block section names.
code
A code block in <<CODE ... CODE (without the single quote).
includes
A list of files to include.

The following macros can be used in specific wrapper calls. The lists of where you can use them is listed before. The macros are:

@FUNC_NAME@
The trace function name as a quote C string.
@FUNC_INDEX@
The trace function index as a held in the sorted list of trace functions by teh trace linker. It can be used to index the names, enables and triggers data.
@FUNC_LABEL@
The trace function name as a C label that can be referenced. You can take the address of the label.
@FUNC_DATA_SIZE@
The size of the data in bytes.
@FUNC_DATA_ENTRY_SIZE@
The size of the entry data in bytes.
@FUNC_DATA_RET_SIZE@
The size of the return data in bytes.
@ARG_NUM@
The argument number to the trace function.
@ARG_TYPE@
The type of the argument as a C string.
@ARG_SIZE@
The size of the type of the argument in bytes.
@ARG_LABEL@
The argument as a C label that can be referenced.
@RET_TYPE@
The type of the return value as a C string.
@RET_SIZE@
The size of the type of the return value in bytes.
@RET_LABEL@
The return value as a C label that can be referenced.

The buffer-alloc, entry-trace and exit-trace can be transformed using the following macros:

  • @FUNC_NAME@
  • @FUNC_INDEX@
  • @FUNC_LABEL@
  • @FUNC_DATA_SZIE@
  • @FUNC_DATA_ENTRY_SZIE@
  • @FUNC_DATA_EXIT_SZIE@

The arg-trace can be transformed using the following macros:

  • @ARG_NUM@
  • @ARG_TYPE@
  • @ARG_SIZE@
  • @ARG_LABEL@

The ret-trace can be transformed using the following macros:

  • @RET_TYPE@
  • @RET_SIZE@
  • @RET_LABEL@

An example generator section is:

[printk-generator]
headers = printk-generator-headers
entry-trace = "rtld_pg_printk_entry(@FUNC_NAME@, (void*) &@FUNC_LABEL@);"
arg-trace = "rtld_pg_printk_arg(@ARG_NUM@, @ARG_TYPE@, @ARG_SIZE@, (void*) &@ARG_LABEL@);"
exit-trace = "rtld_pg_printk_exit(@FUNC_NAME@, (void*) &@FUNC_LABEL@);"
ret-trace = "rtld_pg_printk_ret(@RET_TYPE@, @RET_SIZE@, (void*) &@RET_LABEL@);"
code = <<<CODE
static inline void rtld_pg_printk_entry(const char* func_name,
                                        void*       func_addr)
{
  printk (">>> %s (0x%08x)\n", func_name, func_addr);
}
static inline void rtld_pg_printk_arg(int         arg_num,
                                     const char* arg_type,
                                     int         arg_size,
                                     void*       arg)
{
  const unsigned char* p = arg;
  int   i;
  printk (" %2d] %s(%d) = ", arg_num, arg_type, arg_size);
  for (i = 0; i < arg_size; ++i, ++p) printk ("%02x", (unsigned int) *p);
  printk ("\n");
}
static inline void rtld_pg_printk_exit(const char* func_name,
                                       void*       func_addr)
{
  printk ("<<< %s (0x%08x)\n", func_name, func_addr);
}
static inline void rtld_pg_printk_ret(const char* ret_type,
                                      int         ret_size,
                                      void*       ret)
{
  const unsigned char* p = ret;
  int   i;
  printk (" rt] %s(%d) = ", ret_type, ret_size);
  for (i = 0; i < ret_size; ++i, ++p) printk ("%02x", (unsigned int) *p);
  printk ("\n");
}
CODE

[printk-generator-headers]
header = "#include <stdio.h>"

Limitation

The trace linker and software tracing in this way currently has some limitations. The are:

  • Function signatures are held in configuration file and so a change in a signature will not be automatically handled.
  • Variable argument lists cannot be wrapped.
  • C++ is not supported. In time we would like to add C++ support via the demangler support in the RTEMS Toolkit, however there are C++ constructs that make wrapping difficult such as templates. C++ that results in explicit calls should be able to be wrapped.
  • Functions must have external linkage to allow the linker to wrap the symbol.
  • The application being trace cannot crash. This may change if GDB Python support is provided.
Last modified on Mar 29, 2015 at 8:00:44 AM Last modified on Mar 29, 2015, 8:00:44 AM