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 March 24, 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 . 134.Bl -tag -width ".Ev LD_LIBMAP_DISABLE" 135.It Ev LD_DUMP_REL_POST 136If set, 137.Nm 138will print a table containing all relocations after symbol 139binding and relocation. 140.It Ev LD_DUMP_REL_PRE 141If set, 142.Nm 143will print a table containing all relocations before symbol 144binding and relocation. 145.It Ev LD_LIBMAP 146A library replacement list in the same format as 147.Xr libmap.conf 5 . 148For convenience, the characters 149.Ql = 150and 151.Ql \&, 152can be used instead of a space and a newline. 153This variable is parsed after 154.Xr libmap.conf 5 , 155and will override its entries. 156This variable is unset for set-user-ID and set-group-ID programs. 157.It Ev LD_LIBMAP_DISABLE 158If set, disables the use of 159.Xr libmap.conf 5 160and 161.Ev LD_LIBMAP . 162This variable is unset for set-user-ID and set-group-ID programs. 163.It Ev LD_ELF_HINTS_PATH 164This variable will override the default location of 165.Dq hints 166file. 167This variable is unset for set-user-ID and set-group-ID programs. 168.It Ev LD_LIBRARY_PATH 169A colon separated list of directories, overriding the default search path 170for shared libraries. 171This variable is unset for set-user-ID and set-group-ID programs. 172.It Ev LD_LIBRARY_PATH_RPATH 173If the variable is specified and has a value starting with 174any of \'y\', \'Y\' or \'1\' symbols, the path specified by 175.Ev LD_LIBRARY_PATH 176variable is allowed to override the path from 177.Dv DT_RPATH 178for binaries which does not contain 179.Dv DT_RUNPATH 180tag. 181For such binaries, when the variable 182.Ev LD_LIBRARY_PATH_RPATH 183is set, 184.Dq Fl z Ar nodefaultlib 185link-time option is ignored as well. 186.It Ev LD_PRELOAD 187A list of shared libraries, separated by colons and/or white space, 188to be linked in before any 189other shared libraries. 190If the directory is not specified then 191the directories specified by 192.It Ev LD_PRELOAD_FDS 193A colon separated list of file descriptor numbers for libraries. 194This is intended for preloading libraries in which we already have a file 195descriptor. 196This may optimize the process of loading libraries because we do not have to 197look for them in directories. 198It may also be useful in a capability base system where we do not have access to 199global namespaces such as the filesystem. 200.Ev LD_LIBRARY_PATH 201will be searched first 202followed by the set of built-in standard directories. 203This variable is unset for set-user-ID and set-group-ID programs. 204.It Ev LD_LIBRARY_PATH_FDS 205A colon separated list of file descriptor numbers for library directories. 206This is intended for use within 207.Xr capsicum 4 208sandboxes, when global namespaces such as the filesystem are unavailable. 209It is consulted just after LD_LIBRARY_PATH. 210This variable is unset for set-user-ID and set-group-ID programs. 211.It Ev LD_BIND_NOT 212When set to a nonempty string, prevents modifications of the PLT slots when 213doing bindings. 214As result, each call of the PLT-resolved function is resolved. 215In combination with debug output, this provides complete account of 216all bind actions at runtime. 217This variable is unset for set-user-ID and set-group-ID programs. 218.It Ev LD_BIND_NOW 219When set to a nonempty string, causes 220.Nm 221to relocate all external function calls before starting execution of the 222program. 223Normally, function calls are bound lazily, at the first call 224of each function. 225.Ev LD_BIND_NOW 226increases the start-up time of a program, but it avoids run-time 227surprises caused by unexpectedly undefined functions. 228.It Ev LD_TRACE_LOADED_OBJECTS 229When set to a nonempty string, causes 230.Nm 231to exit after loading the shared objects and printing a summary which includes 232the absolute pathnames of all objects, to standard output. 233.It Ev LD_TRACE_LOADED_OBJECTS_ALL 234When set to a nonempty string, causes 235.Nm 236to expand the summary to indicate which objects caused each object to 237be loaded. 238.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 239.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 240When set, these variables are interpreted as format strings a la 241.Xr printf 3 242to customize the trace output and are used by 243.Xr ldd 1 Ns 's 244.Fl f 245option and allows 246.Xr ldd 1 247to be operated as a filter more conveniently. 248If the dependency name starts with string 249.Pa lib , 250.Ev LD_TRACE_LOADED_OBJECTS_FMT1 251is used, otherwise 252.Ev LD_TRACE_LOADED_OBJECTS_FMT2 253is used. 254The following conversions can be used: 255.Bl -tag -width 4n 256.It Li %a 257The main program's name 258(also known as 259.Dq __progname ) . 260.It Li \&%A 261The value of the environment variable 262.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME . 263Typically used to print both the names of programs and shared libraries 264being inspected using 265.Xr ldd 1 . 266.It Li %o 267The library name. 268.It Li %p 269The full pathname as determined by 270.Nm rtld Ns 's 271library search rules. 272.It Li %x 273The library's load address. 274.El 275.Pp 276Additionally, 277.Ql \en 278and 279.Ql \et 280are recognized and have their usual meaning. 281.It Ev LD_UTRACE 282If set, 283.Nm 284will log events such as the loading and unloading of shared objects via 285.Xr utrace 2 . 286.It Ev LD_LOADFLTR 287If set, 288.Nm 289will process the filtee dependencies of the loaded objects immediately, 290instead of postponing it until required. 291Normally, the filtees are opened at the time of the first symbol resolution 292from the filter object. 293.El 294.Sh DIRECT EXECUTION MODE 295.Nm 296is typically used implicitly, loaded by the kernel as requested by the 297.Dv PT_INTERP 298program header of the executed binary. 299.Fx 300also supports a direct execution mode for the dynamic linker. 301In this mode, the user explicitly executes 302.Nm 303and provides the path of the program to be linked and executed as 304an argument. 305This mode allows use of a non-standard dynamic linker for a program 306activation without changing the binary or without changing 307the installed dynamic linker. 308Execution options may be specified. 309.Pp 310The syntax of the direct invocation is 311.Bd -ragged -offset indent 312.Pa /libexec/ld-elf.so.1 313.Op Fl b Ar exe 314.Op Fl f Ar fd 315.Op Fl p 316.Op Fl - 317.Pa image_path 318.Op Ar image arguments 319.Ed 320.Pp 321The options are: 322.Bl -tag -width indent 323.It Fl b Ar exe 324Use the executable 325.Fa exe 326instead of 327.Fa image_path 328for activation. 329If this option is specified, 330.Ar image_path 331is only used to provide the 332.Va argv[0] 333value to the program. 334.It Fl f Ar fd 335File descriptor 336.Ar fd 337references the binary to be activated by 338.Nm . 339It must already be opened in the process when executing 340.Nm . 341If this option is specified, 342.Ar image_path 343is only used to provide the 344.Va argv[0] 345value to the program. 346.It Fl p 347If the 348.Pa image_path 349argument specifies a name which does not contain a slash 350.Dq Li / 351character, 352.Nm 353uses the search path provided by the environment variable 354.Dv PATH 355to find the binary to execute. 356.It Fl v 357Display information about this run-time linker binary, then exit. 358.It Fl - 359Ends the 360.Nm 361options. 362The argument following 363.Fl - 364is interpreted as the path of the binary to execute. 365.El 366.Pp 367In the direct execution mode, 368.Nm 369emulates verification of the binary execute permission for the 370current user. 371This is done to avoid breaking user expectations in naively restricted 372execution environments. 373The verification only uses Unix 374.Dv DACs , 375ignores 376.Dv ACLs , 377and is naturally prone to race conditions. 378Environments which rely on such restrictions are weak 379and breakable on their own. 380.Sh VERSIONING 381Newer 382.Nm 383might provide some features or changes in runtime behavior that cannot be 384easily detected at runtime by checking of the normal exported symbols. 385Note that it is almost always wrong to verify 386.Dv __FreeBSD_version 387in userspace to detect features, either at compile or at run time, 388because either kernel, or libc, or environment variables could not 389match the running 390.Nm . 391.Pp 392To solve the problem, 393.Nm 394exports some feature indicators in the 395.Fx 396private symbols namespace 397.Dv FBSDprivate_1.0 . 398Symbols start with the 399.Dv _rtld_version 400prefix. 401Current list of defined symbols and corresponding features is: 402.Bl -tag -width indent 403.It Dv _rtld_version__FreeBSD_version 404Symbol exports the value of the 405.Dv __FreeBSD_version 406definition as it was provided during the 407.Nm 408build. 409The symbol is always present since the 410.Dv _rtld_version 411facility was introduced. 412.It Dv _rtld_version_laddr_offset 413The 414.Va l_addr 415member of the 416.Vt link_map 417structure contains the load offset of the shared object. 418Before that, 419.Va l_addr 420contained the base address of the library. 421See 422.Xr dlinfo 3 . 423.Pp 424Also it indicates the presence of 425.Va l_refname 426member of the structure. 427.El 428.Sh FILES 429.Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact 430.It Pa /var/run/ld-elf.so.hints 431Hints file. 432.It Pa /var/run/ld-elf32.so.hints 433Hints file for 32-bit binaries on 64-bit system. 434.It Pa /etc/libmap.conf 435The libmap configuration file. 436.It Pa /etc/libmap32.conf 437The libmap configuration file for 32-bit binaries on 64-bit system. 438.El 439.Sh SEE ALSO 440.Xr ld 1 , 441.Xr ldd 1 , 442.Xr dlinfo 3 , 443.Xr capsicum 4 , 444.Xr elf 5 , 445.Xr libmap.conf 5 , 446.Xr ldconfig 8 447