1.\" Copyright (c) 1995 Paul Kranenburg 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgment: 14.\" This product includes software developed by Paul Kranenburg. 15.\" 3. The name of the author may not be used to endorse or promote products 16.\" derived from this software without specific prior written permission 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 19.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 20.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 21.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 22.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 23.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 24.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 25.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 26.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 27.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 28.\" 29.\" $FreeBSD$ 30.\" 31.Dd August 15, 2021 32.Dt RTLD 1 33.Os 34.Sh NAME 35.Nm ld-elf.so.1 , 36.Nm ld.so , 37.Nm rtld 38.Nd run-time link-editor 39.Sh DESCRIPTION 40The 41.Nm 42utility is a self-contained shared object providing run-time 43support for loading and link-editing shared objects into a process' 44address space. 45It is also commonly known as the dynamic linker. 46It uses the data structures 47contained within dynamically linked programs to determine which shared 48libraries are needed and loads them using the 49.Xr mmap 2 50system call. 51.Pp 52After all shared libraries have been successfully loaded, 53.Nm 54proceeds to resolve external references from both the main program and 55all objects loaded. 56A mechanism is provided for initialization routines 57to be called on a per-object basis, giving a shared object an opportunity 58to perform any extra set-up before execution of the program proper begins. 59This is useful for C++ libraries that contain static constructors. 60.Pp 61When resolving dependencies for the loaded objects, 62.Nm 63translates dynamic token strings in rpath and soname. 64If the 65.Fl "z origin" 66option of the static linker was set when linking the binary, 67the token expansion is performed at the object load time, see 68.Xr ld 1 . 69The following strings are recognized now: 70.Bl -tag -width ".Pa $PLATFORM" 71.It Pa $ORIGIN 72Translated to the full path of the loaded object. 73.It Pa $OSNAME 74Translated to the name of the operating system implementation. 75.It Pa $OSREL 76Translated to the release level of the operating system. 77.It Pa $PLATFORM 78Translated to the machine hardware platform. 79.El 80.Pp 81The 82.Nm 83utility itself is loaded by the kernel together with any dynamically-linked 84program that is to be executed. 85The kernel transfers control to the 86dynamic linker. 87After the dynamic linker has finished loading, 88relocating, and initializing the program and its required shared 89objects, it transfers control to the entry point of the program. 90The following search order is used to locate required shared objects: 91.Pp 92.Bl -enum -offset indent -compact 93.It 94.Dv DT_RPATH 95of the referencing object unless that object also contains a 96.Dv DT_RUNPATH 97tag 98.It 99.Dv DT_RPATH 100of the program unless the referencing object contains a 101.Dv DT_RUNPATH 102tag 103.It 104Path indicated by 105.Ev LD_LIBRARY_PATH 106environment variable 107.It 108.Dv DT_RUNPATH 109of the referencing object 110.It 111Hints file produced by the 112.Xr ldconfig 8 113utility 114.It 115The 116.Pa /lib 117and 118.Pa /usr/lib 119directories, unless the referencing object was linked using the 120.Dq Fl z Ar nodefaultlib 121option 122.El 123.Pp 124The 125.Nm 126utility 127recognizes a number of environment variables that can be used to modify 128its behaviour. 129On 64-bit architectures, the linker for 32-bit objects recognizes 130all the environment variables listed below, but is being prefixed with 131.Ev LD_32_ , 132for example: 133.Ev LD_32_TRACE_LOADED_OBJECTS . 134If the activated image is setuid or setgid, the variables are ignored. 135.Bl -tag -width ".Ev LD_LIBMAP_DISABLE" 136.It Ev LD_DUMP_REL_POST 137If set, 138.Nm 139will print a table containing all relocations after symbol 140binding and relocation. 141.It Ev LD_DUMP_REL_PRE 142If set, 143.Nm 144will print a table containing all relocations before symbol 145binding and relocation. 146.It Ev LD_DYNAMIC_WEAK 147If set, use the ELF standard-compliant symbol lookup behavior: 148resolve to the first found symbol definition. 149.Pp 150By default, 151.Fx 152provides the non-standard symbol lookup behavior: 153when a weak symbol definition is found, remember the definition and 154keep searching in the remaining shared objects for a non-weak definition. 155If found, the non-weak definition is preferred, otherwise the remembered 156weak definition is returned. 157.Pp 158Symbols exported by dynamic linker itself (see 159.Xr dlfcn 3 ) 160are always resolved using 161.Fx 162rules regardless of the presence of the variable. 163This variable is unset for set-user-ID and set-group-ID programs. 164.It Ev LD_LIBMAP 165A library replacement list in the same format as 166.Xr libmap.conf 5 . 167For convenience, the characters 168.Ql = 169and 170.Ql \&, 171can be used instead of a space and a newline. 172This variable is parsed after 173.Xr libmap.conf 5 , 174and will override its entries. 175This variable is unset for set-user-ID and set-group-ID programs. 176.It Ev LD_LIBMAP_DISABLE 177If set, disables the use of 178.Xr libmap.conf 5 179and 180.Ev LD_LIBMAP . 181This variable is unset for set-user-ID and set-group-ID programs. 182.It Ev LD_ELF_HINTS_PATH 183This variable will override the default location of 184.Dq hints 185file. 186This variable is unset for set-user-ID and set-group-ID programs. 187.It Ev LD_LIBRARY_PATH 188A colon separated list of directories, overriding the default search path 189for shared libraries. 190This variable is unset for set-user-ID and set-group-ID programs. 191.It Ev LD_LIBRARY_PATH_RPATH 192If the variable is specified and has a value starting with 193any of \'y\', \'Y\' or \'1\' symbols, the path specified by 194.Ev LD_LIBRARY_PATH 195variable is allowed to override the path from 196.Dv DT_RPATH 197for binaries which does not contain 198.Dv DT_RUNPATH 199tag. 200For such binaries, when the variable 201.Ev LD_LIBRARY_PATH_RPATH 202is set, 203.Dq Fl z Ar nodefaultlib 204link-time option is ignored as well. 205.It Ev LD_PRELOAD 206A list of shared libraries, separated by colons and/or white space, 207to be linked in before any 208other shared libraries. 209If the directory is not specified then 210the directories specified by 211.It Ev LD_PRELOAD_FDS 212A colon separated list of file descriptor numbers for libraries. 213This is intended for preloading libraries in which we already have a file 214descriptor. 215This may optimize the process of loading libraries because we do not have to 216look for them in directories. 217It may also be useful in a capability base system where we do not have access to 218global namespaces such as the filesystem. 219.Ev LD_LIBRARY_PATH 220will be searched first 221followed by the set of built-in standard directories. 222This variable is unset for set-user-ID and set-group-ID programs. 223.It Ev LD_LIBRARY_PATH_FDS 224A colon separated list of file descriptor numbers for library directories. 225This is intended for use within 226.Xr capsicum 4 227sandboxes, when global namespaces such as the filesystem are unavailable. 228It is consulted just after LD_LIBRARY_PATH. 229This variable is unset for set-user-ID and set-group-ID programs. 230.It Ev LD_BIND_NOT 231When set to a nonempty string, prevents modifications of the PLT slots when 232doing bindings. 233As result, each call of the PLT-resolved function is resolved. 234In combination with debug output, this provides complete account of 235all bind actions at runtime. 236This variable is unset for set-user-ID and set-group-ID programs. 237.It Ev LD_BIND_NOW 238When set to a nonempty string, causes 239.Nm 240to relocate all external function calls before starting execution of the 241program. 242Normally, function calls are bound lazily, at the first call 243of each function. 244.Ev LD_BIND_NOW 245increases the start-up time of a program, but it avoids run-time 246surprises caused by unexpectedly undefined functions. 247.It Ev LD_TRACE_LOADED_OBJECTS 248When set to a nonempty string, causes 249.Nm 250to exit after loading the shared objects and printing a summary which includes 251the absolute pathnames of all objects, to standard output. 252.It Ev LD_TRACE_LOADED_OBJECTS_ALL 253When set to a nonempty string, causes 254.Nm 255to expand the summary to indicate which objects caused each object to 256be loaded. 257.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 258.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 259When set, these variables are interpreted as format strings a la 260.Xr printf 3 261to customize the trace output and are used by 262.Xr ldd 1 Ns 's 263.Fl f 264option and allows 265.Xr ldd 1 266to be operated as a filter more conveniently. 267If the dependency name starts with string 268.Pa lib , 269.Ev LD_TRACE_LOADED_OBJECTS_FMT1 270is used, otherwise 271.Ev LD_TRACE_LOADED_OBJECTS_FMT2 272is used. 273The following conversions can be used: 274.Bl -tag -width 4n 275.It Li %a 276The main program's name 277(also known as 278.Dq __progname ) . 279.It Li \&%A 280The value of the environment variable 281.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME . 282Typically used to print both the names of programs and shared libraries 283being inspected using 284.Xr ldd 1 . 285.It Li %o 286The library name. 287.It Li %p 288The full pathname as determined by 289.Nm rtld Ns 's 290library search rules. 291.It Li %x 292The library's load address. 293.El 294.Pp 295Additionally, 296.Ql \en 297and 298.Ql \et 299are recognized and have their usual meaning. 300.It Ev LD_UTRACE 301If set, 302.Nm 303will log events such as the loading and unloading of shared objects via 304.Xr utrace 2 . 305.It Ev LD_LOADFLTR 306If set, 307.Nm 308will process the filtee dependencies of the loaded objects immediately, 309instead of postponing it until required. 310Normally, the filtees are opened at the time of the first symbol resolution 311from the filter object. 312.El 313.Sh DIRECT EXECUTION MODE 314.Nm 315is typically used implicitly, loaded by the kernel as requested by the 316.Dv PT_INTERP 317program header of the executed binary. 318.Fx 319also supports a direct execution mode for the dynamic linker. 320In this mode, the user explicitly executes 321.Nm 322and provides the path of the program to be linked and executed as 323an argument. 324This mode allows use of a non-standard dynamic linker for a program 325activation without changing the binary or without changing 326the installed dynamic linker. 327Execution options may be specified. 328.Pp 329The syntax of the direct invocation is 330.Bd -ragged -offset indent 331.Pa /libexec/ld-elf.so.1 332.Op Fl b Ar exe 333.Op Fl d 334.Op Fl f Ar fd 335.Op Fl p 336.Op Fl u 337.Op Fl v 338.Op Fl - 339.Pa image_path 340.Op Ar image arguments 341.Ed 342.Pp 343The options are: 344.Bl -tag -width indent 345.It Fl b Ar exe 346Use the executable 347.Fa exe 348instead of 349.Fa image_path 350for activation. 351If this option is specified, 352.Ar image_path 353is only used to provide the 354.Va argv[0] 355value to the program. 356.It Fl d 357Turn off the emulation of the binary execute permission. 358.It Fl f Ar fd 359File descriptor 360.Ar fd 361references the binary to be activated by 362.Nm . 363It must already be opened in the process when executing 364.Nm . 365If this option is specified, 366.Ar image_path 367is only used to provide the 368.Va argv[0] 369value to the program. 370.It Fl p 371If the 372.Pa image_path 373argument specifies a name which does not contain a slash 374.Dq Li / 375character, 376.Nm 377uses the search path provided by the environment variable 378.Dv PATH 379to find the binary to execute. 380.It Fl u 381Ignore all 382.Ev LD_ 383environment variables that otherwise affect the dynamic 384linker behavior. 385.It Fl v 386Display information about this run-time linker binary, then exit. 387.It Fl - 388Ends the 389.Nm 390options. 391The argument following 392.Fl - 393is interpreted as the path of the binary to execute. 394.El 395.Pp 396In the direct execution mode, 397.Nm 398emulates verification of the binary execute permission for the 399current user. 400This is done to avoid breaking user expectations in naively restricted 401execution environments. 402The verification only uses Unix 403.Dv DACs , 404ignores 405.Dv ACLs , 406and is naturally prone to race conditions. 407Environments which rely on such restrictions are weak 408and breakable on their own. 409It can be turned off with the 410.Fl d 411option. 412.Sh VERSIONING 413Newer 414.Nm 415might provide some features or changes in runtime behavior that cannot be 416easily detected at runtime by checking of the normal exported symbols. 417Note that it is almost always wrong to verify 418.Dv __FreeBSD_version 419in userspace to detect features, either at compile or at run time, 420because either kernel, or libc, or environment variables could not 421match the running 422.Nm . 423.Pp 424To solve the problem, 425.Nm 426exports some feature indicators in the 427.Fx 428private symbols namespace 429.Dv FBSDprivate_1.0 . 430Symbols start with the 431.Dv _rtld_version 432prefix. 433Current list of defined symbols and corresponding features is: 434.Bl -tag -width indent 435.It Dv _rtld_version__FreeBSD_version 436Symbol exports the value of the 437.Dv __FreeBSD_version 438definition as it was provided during the 439.Nm 440build. 441The symbol is always present since the 442.Dv _rtld_version 443facility was introduced. 444.It Dv _rtld_version_laddr_offset 445The 446.Va l_addr 447member of the 448.Vt link_map 449structure contains the load offset of the shared object. 450Before that, 451.Va l_addr 452contained the base address of the library. 453See 454.Xr dlinfo 3 . 455.Pp 456Also it indicates the presence of 457.Va l_refname 458member of the structure. 459.It Dv _rtld_version_dlpi_tls_data 460The 461.Va dlpi_tls_data 462member of the structure 463.Vt dl_phdr_info 464contains the address of the module TLS segment for the calling thread, 465and not the address of the initialization segment. 466.El 467.Sh FILES 468.Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact 469.It Pa /var/run/ld-elf.so.hints 470Hints file. 471.It Pa /var/run/ld-elf32.so.hints 472Hints file for 32-bit binaries on 64-bit system. 473.It Pa /etc/libmap.conf 474The libmap configuration file. 475.It Pa /etc/libmap32.conf 476The libmap configuration file for 32-bit binaries on 64-bit system. 477.El 478.Sh SEE ALSO 479.Xr ld 1 , 480.Xr ldd 1 , 481.Xr dlinfo 3 , 482.Xr capsicum 4 , 483.Xr elf 5 , 484.Xr libmap.conf 5 , 485.Xr ldconfig 8 486