RTLD(1)

NAME
rtld, ld-elf.so.2, _rtld_functrace - run-time link-editor

SYNOPSIS
int
_rtld_functrace(const char *callerso, const char *calleeso, const char *calleefun, void *stack);

DESCRIPTION
The rtld utility is a self-contained shared object providing run-time support for loading and link-editing shared objects into a process' address space. It is also commonly known as the dynamic linker. It uses the data structures contained within dynamically linked programs to determine which shared libraries are needed and loads them using the mmap(2) system call.

After all shared libraries have been successfully loaded, rtld proceeds to resolve external references from both the main program and all objects loaded. A mechanism is provided for initialization routines to be called on a per-object basis, giving a shared object an opportunity to perform any extra set-up before execution of the program proper begins. This is useful for C++ libraries that contain static constructors.

When resolving dependencies for the loaded objects, rtld may be allowed to translate dynamic token strings in rpath and soname by setting -z origin option of the static linker ld(1). The following strings are recognized now:

$ORIGIN
    Translated to the full path of the loaded object.

$OSNAME
    Translated to the name of the operating system implementation.

$OSREL
    Translated to the release level of the operating system.

$PLATFORM
    Translated to the machine hardware platform.

The rtld utility itself is loaded by the kernel together with any dynamically-linked program that is to be executed. The kernel transfers control to the dynamic linker. After the dynamic linker has finished loading, relocating, and initializing the program and its required shared objects, it transfers control to the entry point of the program. The following search order is used to locate required shared objects:

1.  DT_RPATH of the referencing object unless that object also contains a DT_RUNPATH tag
2.  DT_RPATH of the program unless the referencing object contains a DT_RUNPATH tag
3.  Path indicated by LD_LIBRARY_PATH environment variable
4.  DT_RUNPATH of the referencing object
5.  Hints file produced by the ldconfig(8) utility
6.  The /lib and /usr/lib directories, unless the referencing object was linked using the “-z nodefaultlib” option

The rtld utility recognizes a number of environment variables that can be used to modify its behaviour. for example:

LD_DUMP_REL_POST
    If set, rtld will print a table containing all relocations after symbol binding and relocation.

LD_DUMP_REL_PRE
    If set, rtld will print a table containing all relocations before symbol binding and relocation.

LD_LIBMAP
    A library replacement list in the same format as libmap.conf(5). For convenience, the characters ‘=’ and ‘’, can be used instead of a space and a newline. This variable is parsed after libmap.conf(5), and will override its entries. This variable is unset for set-user-ID and set-group-ID programs.

LD_LIBMAP_DISABLE
    If set, disables the use of libmap.conf(5) and LD_LIBMAP. This variable is unset for set-user-ID and set-group-ID programs.

LD_ELF_HINTS_PATH
    This variable will override the default location of “hints” file. This variable is unset for set-user-ID and set-group-ID programs.

LD_LIBRARY_PATH
    A colon separated list of directories, overriding the default search path for shared libraries. This variable is unset for set-user-ID and set-group-ID programs.

LD_PRELOAD
    A list of shared libraries, separated by colons and/or white space, to be linked in before any other shared libraries. If the directory is not specified then the directories specified by LD_LIBRARY_PATH will be searched first followed by the set of built-in standard directories. This variable is unset for set-user-ID and set-group-ID programs. LD_LIBRARY_PATH_FDS A colon separated list of file descriptor numbers for library directories. This is intended for future use within capsicum sandboxes, when global namespaces such as the filesystem are unavailable. It is consulted just after LD_LIBRARY_PATH. This variable is unset for set-user-ID and set-group-ID programs.

LD_BIND_NOW
    When set to a nonempty string, causes rtld to relocate all external function calls before starting execution of the program. Normally, function calls are bound lazily, at the first call of each function. LD_BIND_NOW increases the start-up time of a program, but it avoids run-time surprises caused by unexpectedly undefined functions.

LD_TRACE_LOADED_OBJECTS
    When set to a nonempty string, causes rtld to exit after loading the shared objects and printing a summary which includes the absolute pathnames of all objects, to standard output.

LD_TRACE_LOADED_OBJECTS_ALL
    When set to a nonempty string, causes rtld to expand the summary to indicate which objects caused each object to be loaded.

LD_TRACE_LOADED_OBJECTS_FMT1

LD_TRACE_LOADED_OBJECTS_FMT2
    When set, these variables are interpreted as format strings a la printf(3) to customize the trace output and are used by ldd(1)'s -f option and allows ldd(1) to be operated as a filter more conveniently. If the dependency name starts with string lib, LD_TRACE_LOADED_OBJECTS_FMT1 is used, otherwise LD_TRACE_LOADED_OBJECTS_FMT2 is used. The following conversions can be used:

    %a  The main program's name (also known as “__progname”).

    %A  The value of the environment variable LD_TRACE_LOADED_OBJECTS_PROGNAME. Typically used to print both the names of programs and shared libraries being inspected using ldd(1).

    %o  The library name.

    %p  The full pathname as determined by rtld's library search rules.

    %x  The library's load address.

    Additionally, ‘\n’ and ‘\t’ are recognized and have their usual meaning.

LD_UTRACE
    If set, rtld will log events such as the loading and unloading of shared objects via utrace(2).

LD_LOADFLTR
    If set, rtld will process the filtee dependencies of the loaded objects immediately, instead of postponing it until required. Normally, the filtees are opened at the time of the first symbol resolution from the filter object.

If a shared object preloaded by the LD_PRELOAD mechanism contains a public symbol “_rtld_functrace”, rtld will transfer control to this function each time it needs to resolve an unbound function symbol. By returning a non-zero value, _rtld_functrace() can advise the linker to keep tracing the specified combination of caller shared object and called function; returning 0 will prevent _rtld_functrace() to be called for this combination again.

When implementing a custom _rtld_functrace() function, be aware of the possibility that _rtld_functrace() might be called for functions called on its behalf, or that multiple threads could enter _rtld_functrace() at the same time.

DIFFERENCES BETWEEN .1 and .2
ABI changes have been made to support TLS allocation and initialization and to give threading libraries a chance to complete initialization of the TCB prior to the calling of the _init() functions for the dynamically loaded libraries.

FILES
/var/run/ld-elf.so.hints
    Hints file.
/etc/libmap.conf
    The libmap configuration file.

EXAMPLES
To set up an _rtld_functrace() for printing out the functions as they are called, this code can be used:

    #include <string.h>
    #include <unistd.h>
    static int nl = 10;
    int
    _rtld_functrace(const char *callerso, const char *calleeso,
        const char *calleefun, void *stack)
    {
    	write(2, "calling ", 8);
    	write(2, calleefun, strlen(calleefun));
    	write(2, &nl, 1);
    	return 1;
    }

If put in a file named ft.c and compiled with

    $ cc -shared -fPIC ft.c -o ft.so

setting LD_PRELOAD to the path of ft.so will activate it.

SEE ALSO
ld(1), ldd(1), elf(5), libmap.conf(5), ldconfig(8)