| LD-ELF.SO.1(1) | FreeBSD General Commands Manual | LD-ELF.SO.1(1) |
ld-elf.so.1, ld.so,
rtld —
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:
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:
DT_RPATH
of the referencing object unless that object also contains a
DT_RUNPATH tagDT_RPATH
of the program unless the referencing object contains a
DT_RUNPATH tagLD_LIBRARY_PATH environment
variableDT_RUNPATH
of the referencing object-z
nodefaultlib” optionThe ld-elf.so.1 utility recognizes a
number of environment variables that can be used to modify its behaviour. If
the activated image is setuid or setgid, the variables are ignored.
LD_DUMP_REL_POSTld-elf.so.1 will print a table containing
all relocations after symbol binding and relocation.LD_DUMP_REL_PREld-elf.so.1 will print a table containing
all relocations before symbol binding and relocation.LD_DYNAMIC_WEAKBy default, FreeBSD provides the non-standard symbol lookup behavior: when a weak symbol definition is found, remember the definition and keep searching in the remaining shared objects for a non-weak definition. If found, the non-weak definition is preferred, otherwise the remembered weak definition is returned.
Symbols exported by dynamic linker itself (see dlfcn(3)) are always resolved using FreeBSD rules regardless of the presence of the variable. This variable is unset for set-user-ID and set-group-ID programs.
LD_LIBMAP=’ 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_DISABLELD_LIBMAP. This variable is unset for set-user-ID
and set-group-ID programs.LD_ELF_HINTS_PATHLD_LIBRARY_PATHLD_LIBRARY_PATH_RPATHLD_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_PRELOADLD_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_PRELOAD_FDSLD_LIBRARY_PATH_FDSLD_BIND_NOTLD_BIND_NOWld-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_OBJECTSld-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_ALLld-elf.so.1
to expand the summary to indicate which objects caused each object to be
loaded.LD_TRACE_LOADED_OBJECTS_FMT1LD_TRACE_LOADED_OBJECTS_FMT2-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%ALD_TRACE_LOADED_OBJECTS_PROGNAME. Typically
used to print both the names of programs and shared libraries being
inspected using ldd(1).%o%prtld's
library search rules.%xAdditionally, ‘\n’ and
‘\t’ are recognized and have their
usual meaning.
LD_UTRACEld-elf.so.1 will log events such as the
loading and unloading of shared objects via
utrace(2).LD_LOADFLTRld-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.LD_SHOW_AUXVld-elf.so.1 to dump content of the
aux vector to standard output, before passing control to any user
code.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
-b exe]
[-d] [-f
fd] [-p]
[-u] [-v]
[--] image_path
[image arguments]The options are:
-b
exe-d-f
fdld-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/”
character, ld-elf.so.1 uses the search path
provided by the environment variable PATH to find
the binary to execute.-uLD_ environment variables that
otherwise affect the dynamic linker behavior.-v--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. It can be turned off with
the -d option.
ld-elf.so.1 might provide some features or changes
in runtime behavior that cannot be easily detected at runtime by checking of
the normal exported symbols. Note that it is almost always wrong to verify
__FreeBSD_version in userspace to detect features,
either at compile or at run time, because either kernel, or libc, or
environment variables could not match the running
ld-elf.so.1.
To solve the problem, ld-elf.so.1 exports
some feature indicators in the FreeBSD private
symbols namespace FBSDprivate_1.0. Symbols start
with the _rtld_version prefix. Current list of
defined symbols and corresponding features is:
_rtld_version__FreeBSD_version__FreeBSD_version
definition as it was provided during the
ld-elf.so.1 build. The symbol is always present
since the _rtld_version facility was
introduced._rtld_version_laddr_offsetAlso it indicates the presence of l_refname member of the structure.
_rtld_version_dlpi_tls_data| July 27, 2023 | FreeBSD 13.1-RELEASE-p2 |