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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 deter- mine 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 pro- ceeds to resolve external references from both the main program and all ob- jects loaded. A mechanism is provided for initialization routines to be called on a per-object basis, giving a shared object an opportunity to per- form 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 per- formed at the object load time, see ld(1). The following strings are rec- ognized 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. $LIB Translated to the system library path component on the platform. It is lib for native binaries, and typically lib32 for compat32 binaries. Other translations might exist for other ABIs sup- ported on the 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 load- ing, relocating, and initializing the program and its required shared ob- jects, it transfers control to the entry point of the program. The follow- ing search order is used to locate required shared objects: 1. DT_RPATH of the referencing object unless that object also con- tains 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. If the activated image is setuid or setgid, the variables are ignored. The run-time linker is able to access the environment provided at process startup. After startup, environment variables are maintained by higher- level libraries and are not accessible by the run-time linker. At run- time, effective settings can be queried using rtld_get_var(3), and some of them can be changed with rtld_set_var(3). 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_DYNAMIC_WEAK If set, use the ELF standard-compliant symbol lookup be- havior: resolve to the first found symbol definition. By default, FreeBSD provides the non-standard symbol lookup behavior: when a weak symbol definition is found, remember the definition and keep searching in the re- maining 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 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 over- ride 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 li- braries. If the directory is not specified then the di- rectories specified by LD_LIBRARY_PATH will be searched first followed by the set of built-in standard directo- ries. This variable is unset for set-user-ID and set- group-ID programs. LD_PRELOAD_FDS A colon separated list of file descriptor numbers for libraries. This is intended for preloading libraries in which we already have a file descriptor. This may opti- mize the process of loading libraries because we do not have to look for them in directories. It may also be useful in a capability base system where we do not have access to global namespaces such as the filesystem. 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 combina- tion 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 re- locate all external function calls before starting exe- cution 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 ob- jects, to standard output. LD_TRACE_LOADED_OBJECTS_ALL When set to a nonempty string, causes ld-elf.so.1 to ex- pand 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 depen- dency 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 li- braries 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. LD_SHOW_AUXV If set, causes ld-elf.so.1 to dump content of the aux vector to standard output, before passing control to any user code. LD_STATIC_TLS_EXTRA If the variable is specified and has a numeric value, ld-elf.so.1 will set the size of the static TLS extra space to the specified number of bytes. The static TLS extra space is used when loading objects compiled for initial-exec TLS code model with dlopen(3). The minimum value that can be specified is '128'. LD_NO_DL_ITERATE_PHDR_AFTER_FORK Allow dl_iterate_phdr(3) to block in callback, without causing deadlock with the fork(2). The drawback is that the image started in this mode cannot use dl_iter- ate_phdr(3) after fork. 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 sup- ports 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 bi- nary or without changing the installed dynamic linker. Execution options may be specified. The syntax of the direct invocation is /libexec/ld-elf.so.1 [-b exe] [-d] [-f fd] [-o OPT=VALUE] [-p] [-u] [-v] [--] image_path [image arguments] The options are: -b exe Use the executable exe instead of image_path for activation. If this option is specified, image_path is only used to provide the argv[0] value to the program. -d Turn off the emulation of the binary execute permission. -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 execut- ing ld-elf.so.1. If this option is specified, image_path is only used to provide the argv[0] value to the program. -o OPT=VALUE Set the OPT configuration variable to the value VALUE. The possi- ble variable names are listed above as LD_ prefixed environment variables, but here are referenced without the LD_ prefix. A con- figuration variable set this way does not leak into the activated image's environment. The option can be repeated as many times as needed to set all con- figuration parameters. The parameters set using this option have priority over the same parameters assigned via environment. -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. -u Ignore all LD_ environment variables and previous command line -o options that otherwise affect the dynamic linker behavior. -v Display information about this run-time linker binary, then exit. -- Ends the ld-elf.so.1 options. The argument following -- is inter- preted as the path of the binary to execute. In the direct execution mode, ld-elf.so.1 emulates verification of the bi- nary execute permission for the current user. This is done to avoid break- ing 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. VERSIONING Newer ld-elf.so.1 might provide some features or changes in runtime behav- ior that cannot be easily detected at runtime by checking of the normal ex- ported 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 Symbol exports the value of the __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_offset The l_addr member of the link_map structure contains the load off- set of the shared object. Before that, l_addr contained the base address of the library. See dlinfo(3). Also it indicates the presence of l_refname member of the struc- ture. _rtld_version_dlpi_tls_data The dlpi_tls_data member of the structure dl_phdr_info contains the address of the module TLS segment for the calling thread, and not the address of the initialization segment. FILES /var/run/ld-elf.so.hints Hints file. /var/run/ld-elf32.so.hints Hints file for 32-bit binaries on 64-bit sys- tem. /etc/libmap.conf The libmap configuration file. /etc/libmap32.conf The libmap configuration file for 32-bit bina- ries on 64-bit system. SEE ALSO ld(1), ldd(1), dlinfo(3), rtld_get_var(3), capsicum(4), elf(5), libmap.conf(5), ldconfig(8) FreeBSD ports 15.quarterly July 24, 2024 RTLD(1)
NAME | DESCRIPTION | DIRECT EXECUTION MODE | VERSIONING | FILES | SEE ALSO
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