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 May 20, 2017 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.Ev LD_LIBRARY_PATH 193will be searched first 194followed by the set of built-in standard directories. 195This variable is unset for set-user-ID and set-group-ID programs. 196.It Ev LD_LIBRARY_PATH_FDS 197A colon separated list of file descriptor numbers for library directories. 198This is intended for use within 199.Xr capsicum 4 200sandboxes, when global namespaces such as the filesystem are unavailable. 201It is consulted just after LD_LIBRARY_PATH. 202This variable is unset for set-user-ID and set-group-ID programs. 203.It Ev LD_BIND_NOT 204When set to a nonempty string, prevents modifications of the PLT slots when 205doing bindings. 206As result, each call of the PLT-resolved function is resolved. 207In combination with debug output, this provides complete account of 208all bind actions at runtime. 209This variable is unset for set-user-ID and set-group-ID programs. 210.It Ev LD_BIND_NOW 211When set to a nonempty string, causes 212.Nm 213to relocate all external function calls before starting execution of the 214program. 215Normally, function calls are bound lazily, at the first call 216of each function. 217.Ev LD_BIND_NOW 218increases the start-up time of a program, but it avoids run-time 219surprises caused by unexpectedly undefined functions. 220.It Ev LD_TRACE_LOADED_OBJECTS 221When set to a nonempty string, causes 222.Nm 223to exit after loading the shared objects and printing a summary which includes 224the absolute pathnames of all objects, to standard output. 225.It Ev LD_TRACE_LOADED_OBJECTS_ALL 226When set to a nonempty string, causes 227.Nm 228to expand the summary to indicate which objects caused each object to 229be loaded. 230.It Ev LD_TRACE_LOADED_OBJECTS_FMT1 231.It Ev LD_TRACE_LOADED_OBJECTS_FMT2 232When set, these variables are interpreted as format strings a la 233.Xr printf 3 234to customize the trace output and are used by 235.Xr ldd 1 Ns 's 236.Fl f 237option and allows 238.Xr ldd 1 239to be operated as a filter more conveniently. 240If the dependency name starts with string 241.Pa lib , 242.Ev LD_TRACE_LOADED_OBJECTS_FMT1 243is used, otherwise 244.Ev LD_TRACE_LOADED_OBJECTS_FMT2 245is used. 246The following conversions can be used: 247.Bl -tag -width 4n 248.It Li %a 249The main program's name 250(also known as 251.Dq __progname ) . 252.It Li \&%A 253The value of the environment variable 254.Ev LD_TRACE_LOADED_OBJECTS_PROGNAME . 255Typically used to print both the names of programs and shared libraries 256being inspected using 257.Xr ldd 1 . 258.It Li %o 259The library name. 260.It Li %p 261The full pathname as determined by 262.Nm rtld Ns 's 263library search rules. 264.It Li %x 265The library's load address. 266.El 267.Pp 268Additionally, 269.Ql \en 270and 271.Ql \et 272are recognized and have their usual meaning. 273.It Ev LD_UTRACE 274If set, 275.Nm 276will log events such as the loading and unloading of shared objects via 277.Xr utrace 2 . 278.It Ev LD_LOADFLTR 279If set, 280.Nm 281will process the filtee dependencies of the loaded objects immediately, 282instead of postponing it until required. 283Normally, the filtees are opened at the time of the first symbol resolution 284from the filter object. 285.El 286.Sh DIRECT EXECUTION MODE 287.Nm 288is typically used implicitly, loaded by the kernel as requested by the 289.Dv PT_INTERP 290program header of the executed binary. 291.Fx 292also supports a direct execution mode for the dynamic linker. 293In this mode, the user explicitly executes 294.Nm 295and provides the path of the program to be linked and executed as 296an argument. 297This mode allows use of a non-standard dynamic linker for a program 298activation without changing the binary or without changing 299the installed dynamic linker. 300Execution options may be specified. 301.Pp 302The syntax of the direct invocation is 303.Bd -ragged -offset indent 304.Pa /libexec/ld-elf.so.1 305.Op Fl f Ar fd 306.Op Fl p 307.Op Fl - 308.Pa image_path 309.Op Ar image arguments 310.Ed 311.Pp 312The options are: 313.Bl -tag -width indent 314.It Fl f Ar fd 315File descriptor 316.Ar fd 317references the binary to be activated by 318.Nm . 319It must already be opened in the process when executing 320.Nm . 321If this option is specified, 322.Ar image_path 323is only used to provide the 324.Va argv[0] 325value to the program. 326.It Fl p 327If the 328.Pa image_path 329argument specifies a name which does not contain a slash 330.Dq Li / 331character, 332.Nm 333uses the search path provided by the environment variable 334.Dv PATH 335to find the binary to execute. 336.It Fl - 337Ends the 338.Nm 339options. 340The argument following 341.Fl - 342is interpreted as the path of the binary to execute. 343.El 344.Pp 345In the direct execution mode, 346.Nm 347emulates verification of the binary execute permission for the 348current user. 349This is done to avoid breaking user expectations in naively restricted 350execution environments. 351The verification only uses Unix 352.Dv DACs , 353ignores 354.Dv ACLs , 355and is naturally prone to race conditions. 356Environments which rely on such restrictions are weak 357and breakable on their own. 358.Sh FILES 359.Bl -tag -width ".Pa /var/run/ld-elf32.so.hints" -compact 360.It Pa /var/run/ld-elf.so.hints 361Hints file. 362.It Pa /var/run/ld-elf32.so.hints 363Hints file for 32-bit binaries on 64-bit system. 364.It Pa /etc/libmap.conf 365The libmap configuration file. 366.It Pa /etc/libmap32.conf 367The libmap configuration file for 32-bit binaries on 64-bit system. 368.El 369.Sh SEE ALSO 370.Xr ld 1 , 371.Xr ldd 1 , 372.Xr capsicum 4 , 373.Xr elf 5 , 374.Xr libmap.conf 5 , 375.Xr ldconfig 8 376