ld.so

RTLD(1)                     General Commands Manual                    RTLD(1)

NAME
     ld-elf.so.1, ld.so, rtld — run-time link-editor

DESCRIPTION
     The ld-elf.so.1 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, ld-elf.so.1
     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, ld-elf.so.1
     translates dynamic token strings in rpath and soname.  If the -z origin
     option of the static linker was set when linking the binary, the token
     expansion is performed at the object load time, see 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 ld-elf.so.1 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 ld-elf.so.1 utility recognizes a number of environment variables that
     can be used to modify its behaviour.  On 64-bit architectures, the linker
     for 32-bit objects recognizes all the environment variables listed below,
     but is being prefixed with LD_32_, for example:
     LD_32_TRACE_LOADED_OBJECTS.

     LD_DUMP_REL_POST   If set, ld-elf.so.1 will print a table containing all
                        relocations after symbol binding and relocation.

     LD_DUMP_REL_PRE    If set, ld-elf.so.1 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_LIBRARY_PATH_RPATH
                        If the variable is specified and has a value starting
                        with any of ´y´, ´Y´ or ´1´ symbols, the path
                        specified by LD_LIBRARY_PATH variable is allowed to
                        override the path from DT_RPATH for binaries which
                        does not contain DT_RUNPATH tag.  For such binaries,
                        when the variable LD_LIBRARY_PATH_RPATH is set, “-z
                        nodefaultlib” link-time option is ignored as well.

     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 use within
                        capsicum(4) 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_NOT        When set to a nonempty string, prevents modifications
                        of the PLT slots when doing bindings.  As result, each
                        call of the PLT-resolved function is resolved.  In
                        combination with debug output, this provides complete
                        account of all bind actions at runtime.  This variable
                        is unset for set-user-ID and set-group-ID programs.

     LD_BIND_NOW        When set to a nonempty string, causes ld-elf.so.1 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 ld-elf.so.1 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 ld-elf.so.1 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, ld-elf.so.1 will log events such as the
                        loading and unloading of shared objects via utrace(2).

     LD_LOADFLTR        If set, ld-elf.so.1 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.

DIRECT EXECUTION MODE
     ld-elf.so.1 is typically used implicitly, loaded by the kernel as
     requested by the PT_INTERP program header of the executed binary.
     FreeBSD also supports a direct execution mode for the dynamic linker.  In
     this mode, the user explicitly executes ld-elf.so.1 and provides the path
     of the program to be linked and executed as an argument.  This mode
     allows use of a non-standard dynamic linker for a program activation
     without changing the binary or without changing the installed dynamic
     linker.  Execution options may be specified.

     The syntax of the direct invocation is

           /libexec/ld-elf.so.1 [-f fd] [-p] [--] image_path [image arguments]

     The options are:

     -f fd   File descriptor fd references the binary to be activated by
             ld-elf.so.1.  It must already be opened in the process when
             executing ld-elf.so.1.  If this option is specified, image_path
             is only used to provide the argv[0] value to the program.

     -p      If the image_path argument specifies a name which does not
             contain a slash “/” character, ld-elf.so.1 uses the search path
             provided by the environment variable PATH to find the binary to
             execute.

     --      Ends the ld-elf.so.1 options.  The argument following -- is
             interpreted as the path of the binary to execute.

     In the direct execution mode, ld-elf.so.1 emulates verification of the
     binary execute permission for the current user.  This is done to avoid
     breaking user expectations in naively restricted execution environments.
     The verification only uses Unix DACs, ignores ACLs, and is naturally
     prone to race conditions.  Environments which rely on such restrictions
     are weak and breakable on their own.

FILES
     /var/run/ld-elf.so.hints    Hints file.
     /var/run/ld-elf32.so.hints  Hints file for 32-bit binaries on 64-bit
                                 system.
     /etc/libmap.conf            The libmap configuration file.
     /etc/libmap32.conf          The libmap configuration file for 32-bit
                                 binaries on 64-bit system.

SEE ALSO
     ld(1), ldd(1), capsicum(4), elf(5), libmap.conf(5), ldconfig(8)

                                 May 20, 2017