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 June 2, 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_LIBMAP 147A library replacement list in the same format as 148.Xr libmap.conf 5 . 149For convenience, the characters 150.Ql = 151and 152.Ql \&, 153can be used instead of a space and a newline. 154This variable is parsed after 155.Xr libmap.conf 5 , 156and will override its entries. 157This variable is unset for set-user-ID and set-group-ID programs. 158.It Ev LD_LIBMAP_DISABLE 159If set, disables the use of 160.Xr libmap.conf 5 161and 162.Ev LD_LIBMAP . 163This variable is unset for set-user-ID and set-group-ID programs. 164.It Ev LD_ELF_HINTS_PATH 165This variable will override the default location of 166.Dq hints 167file. 168This variable is unset for set-user-ID and set-group-ID programs. 169.It Ev LD_LIBRARY_PATH 170A colon separated list of directories, overriding the default search path 171for shared libraries. 172This variable is unset for set-user-ID and set-group-ID programs. 173.It Ev LD_LIBRARY_PATH_RPATH 174If the variable is specified and has a value starting with 175any of \'y\', \'Y\' or \'1\' symbols, the path specified by 176.Ev LD_LIBRARY_PATH 177variable is allowed to override the path from 178.Dv DT_RPATH 179for binaries which does not contain 180.Dv DT_RUNPATH 181tag. 182For such binaries, when the variable 183.Ev LD_LIBRARY_PATH_RPATH 184is set, 185.Dq Fl z Ar nodefaultlib 186link-time option is ignored as well. 187.It Ev LD_PRELOAD 188A list of shared libraries, separated by colons and/or white space, 189to be linked in before any 190other shared libraries. 191If the directory is not specified then 192the directories specified by 193.Ev LD_LIBRARY_PATH 194will be searched first 195followed by the set of built-in standard directories. 196This variable is unset for set-user-ID and set-group-ID programs. 197.It Ev LD_LIBRARY_PATH_FDS 198A colon separated list of file descriptor numbers for library directories. 199This is intended for use within 200.Xr capsicum 4 201sandboxes, when global namespaces such as the filesystem are unavailable. 202It is consulted just after LD_LIBRARY_PATH. 203This variable is unset for set-user-ID and set-group-ID programs. 204.It Ev LD_BIND_NOT 205When set to a nonempty string, prevents modifications of the PLT slots when 206doing bindings. 207As result, each call of the PLT-resolved function is resolved. 208In combination with debug output, this provides complete account of 209all bind actions at runtime. 210This variable is unset for set-user-ID and set-group-ID programs. 211.It Ev LD_BIND_NOW 212When set to a nonempty string, causes 213.Nm 214to relocate all external function calls before starting execution of the 215program. 216Normally, function calls are bound lazily, at the first call 217of each function. 218.Ev LD_BIND_NOW 219increases the start-up time of a program, but it avoids run-time 220surprises caused by unexpectedly undefined functions. 221.It Ev LD_TRACE_LOADED_OBJECTS 222When set to a nonempty string, causes 223.Nm 224to exit after loading the shared objects and printing a summary which includes 225the absolute pathnames of all objects, to standard output. 226.It Ev LD_TRACE_LOADED_OBJECTS_ALL 227When set to a nonempty string, causes 228.Nm 229to expand the summary to indicate which objects caused each object to 230be loaded. 231.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 232.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 233When set, these variables are interpreted as format strings a la 234.Xr printf 3 235to customize the trace output and are used by 236.Xr ldd 1 Ns 's 237.Fl f 238option and allows 239.Xr ldd 1 240to be operated as a filter more conveniently. 241If the dependency name starts with string 242.Pa lib , 243.Ev LD_TRACE_LOADED_OBJECTS_FMT1 244is used, otherwise 245.Ev LD_TRACE_LOADED_OBJECTS_FMT2 246is used. 247The following conversions can be used: 248.Bl -tag -width 4n 249.It Li %a 250The main program's name 251(also known as 252.Dq __progname ) . 253.It Li \&%A 254The value of the environment variable 255.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME . 256Typically used to print both the names of programs and shared libraries 257being inspected using 258.Xr ldd 1 . 259.It Li %o 260The library name. 261.It Li %p 262The full pathname as determined by 263.Nm rtld Ns 's 264library search rules. 265.It Li %x 266The library's load address. 267.El 268.Pp 269Additionally, 270.Ql \en 271and 272.Ql \et 273are recognized and have their usual meaning. 274.It Ev LD_UTRACE 275If set, 276.Nm 277will log events such as the loading and unloading of shared objects via 278.Xr utrace 2 . 279.It Ev LD_LOADFLTR 280If set, 281.Nm 282will process the filtee dependencies of the loaded objects immediately, 283instead of postponing it until required. 284Normally, the filtees are opened at the time of the first symbol resolution 285from the filter object. 286.El 287.Sh DIRECT EXECUTION MODE 288.Nm 289is typically used implicitly, loaded by the kernel as requested by the 290.Dv PT_INTERP 291program header of the executed binary. 292.Fx 293also supports a direct execution mode for the dynamic linker. 294In this mode, the user explicitly executes 295.Nm 296and provides the path of the program to be linked and executed as 297an argument. 298This mode allows use of a non-standard dynamic linker for a program 299activation without changing the binary or without changing 300the installed dynamic linker. 301Execution options may be specified. 302.Pp 303The syntax of the direct invocation is 304.Bd -ragged -offset indent 305.Pa /libexec/ld-elf.so.1 306.Op Fl b Ar exe 307.Op Fl f Ar fd 308.Op Fl p 309.Op Fl u 310.Op Fl v 311.Op Fl - 312.Pa image_path 313.Op Ar image arguments 314.Ed 315.Pp 316The options are: 317.Bl -tag -width indent 318.It Fl b Ar exe 319Use the executable 320.Fa exe 321instead of 322.Fa image_path 323for activation. 324If this option is specified, 325.Ar image_path 326is only used to provide the 327.Va argv[0] 328value to the program. 329.It Fl f Ar fd 330File descriptor 331.Ar fd 332references the binary to be activated by 333.Nm . 334It must already be opened in the process when executing 335.Nm . 336If this option is specified, 337.Ar image_path 338is only used to provide the 339.Va argv[0] 340value to the program. 341.It Fl p 342If the 343.Pa image_path 344argument specifies a name which does not contain a slash 345.Dq Li / 346character, 347.Nm 348uses the search path provided by the environment variable 349.Dv PATH 350to find the binary to execute. 351.It Fl u 352Ignore all 353.Ev LD_ 354environment variables that otherwise affect the dynamic 355linker behavior. 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.It Dv _rtld_version_dlpi_tls_data 428The 429.Va dlpi_tls_data 430member of the structure 431.Vt dl_phdr_info 432contains the address of the module TLS segment for the calling thread, 433and not the address of the initialization segment. 434.El 435.Sh FILES 436.Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact 437.It Pa /var/run/ld-elf.so.hints 438Hints file. 439.It Pa /var/run/ld-elf32.so.hints 440Hints file for 32-bit binaries on 64-bit system. 441.It Pa /etc/libmap.conf 442The libmap configuration file. 443.It Pa /etc/libmap32.conf 444The libmap configuration file for 32-bit binaries on 64-bit system. 445.El 446.Sh SEE ALSO 447.Xr ld 1 , 448.Xr ldd 1 , 449.Xr dlinfo 3 , 450.Xr capsicum 4 , 451.Xr elf 5 , 452.Xr libmap.conf 5 , 453.Xr ldconfig 8 454