ld.elf_so(1) - NetBSD Manual Pages

LD.ELF_SO(1)            NetBSD General Commands Manual            LD.ELF_SO(1)


NAME

     ld.elf_so -- run-time link-editor (linker)


DESCRIPTION

     ld.elf_so is a self-contained, position independent program image provid-
     ing run-time support for loading and link-editing shared objects into a
     process' address space.  It uses information stored in data structures
     within the binary (see elf(5)) and environment variables to determine
     which shared objects are needed.  These shared objects are loaded at a
     convenient virtual address using the mmap(2) system call.  After all
     shared objects have been successfully loaded, ld.elf_so proceeds to
     resolve external references from both the main program and all objects
     loaded.  Once all required references are resolved control is passed to
     the program via its entry point.

   Startup
     On the execution of a dynamically linked binary the kernel will load the
     program and its run-time linker as specified in the PT_INTERP section in
     the program header.  At this point, instead of passing control directly
     to the program, the kernel passes control to the specified linker.  An
     auxiliary vector of information is passed that includes the address of
     the program header, the size of each entry in the header, and the number
     of entries.  The entry point of the program and the base address of where
     ld.elf_so is loaded is also supplied.

   Finding objects
     Each elf(5) object file may contain information in its dynamic
     (PT_DYNAMIC) section about which shared objects it requires (often
     referred to as dependencies).  These dependencies are specified in the
     optional DT_NEEDED entry within the dynamic section.  Each DT_NEEDED
     entry refers to a filename string of the shared object that is to be
     searched for.

     The linker will search for libraries in three lists of paths:

     1.   A user defined list of paths as specified in LD_LIBRARY_PATH and
          ld.so.conf(5).

          The use of ld.so.conf should be avoided as the setting of a global
          search path can present a security risk.

     2.   A list of paths specified within a shared object using the DT_RPATH
          and DT_RUNPATH entries in the dynamic section.  This is defined at
          shared object link time.

     3.   The list of default paths which is set to /usr/lib.

     Dynamic loading via dlopen(3) uses the DT_RPATH and DT_RUNPATH entries of
     the main binary, independently of which object the call came from.

     ld.elf_so will expand the following variables if present in the paths:

     $HWCAP     Processor hardware capabilities, for example FPU, MMX, SSE.
                Currently unimplemented.

     $ISALIST   List of instructions sets this processor can execute.  Cur-
                rently unimplemented.

     $ORIGIN    The directory of the main object.

     $OSNAME    The value of the kern.ostype sysctl(3).

     $OSREL     The value of the kern.osrelease sysctl(3).

     $PLATFORM  The value of the hw.machine_arch sysctl(3).

     Both ${VARIABLE} and $VARIABLE are recognized.

     The filename string can be considered free form, however, it will almost
     always take the form lib<name>.so.<number>, where name specifies the
     `library' name and number is conceptually the library's major version
     number.

     This name and another of the form lib<name>.so are normally symbolic
     links to the real shared object which has a filename of the form
     lib<name>.so.<major>.<minor>[.<teeny>].  This naming convention allows a
     versioning scheme similar to a.out(5).

   Relocation
     ld.elf_so will perform all necessary relocations immediately except for
     relocations relating to the Procedure Linkage Table (PLT).  The PLT is
     used as an indirection method for procedure calls to globally defined
     functions.  It allows, through the use of intermediate code, the delayed
     binding of a call to a globally defined function to be performed at pro-
     cedure call time.  This `lazy' method is the default (see LD_BIND_NOW).

   Initialization
     A mechanism is provided for initialization and termination routines to be
     called, on a per-object basis before execution of the program proper
     begins or after the program has completed.  This gives a shared object an
     opportunity to perform any extra set-up or completion work.

     The DT_INIT and DT_FINI entries in the dynamic section specify the
     addresses of the initialization and termination functions, respectively,
     for the shared object.  ld.elf_so arranges for each initialization func-
     tion to be called before control is passed to the program and for the
     termination functions to be called by using atexit(3).

     This mechanism is exploited by the system-supplied constructor initial-
     ization and destructor code located in /usr/lib/crtbeginS.o and
     /usr/lib/crtendS.o.  These files are automatically included by cc(1) and
     c++(1) in the list of object-code files passed to ld(1) when building a
     shared C or C++ object.


ENVIRONMENT

     If the following environment variables exist they will be used by
     ld.elf_so.

     LD_LIBRARY_PATH  A colon separated list of directories, overriding the
                      default search path for shared libraries.

     LD_PRELOAD       A colon or space separated list of shared object file-
                      names to be loaded after the main program but before its
                      shared object dependencies.  Space is allowed as a sepa-
                      rator for backwards compatibility only.  Support may be
                      removed in a future release and should not be relied
                      upon.

     LD_BIND_NOW      If defined immediate binding of Procedure Link Table
                      (PLT) entries is performed instead of the default lazy
                      method.

     LD_DEBUG         If defined a variety of debug information will be writ-
                      ten to the standard error of an dynamically linked exe-
                      cuted when it is run.  This variable is only recognized
                      if ld.elf_so was compiled with debugging support
                      (-DDEBUG).


FILES

     /etc/ld.so.conf  library location hints supplied by the system adminis-
                      trator.


SEE ALSO

     ld(1), ld.aout_so(1), dlfcn(3), elf(5)


HISTORY

     The ELF shared library model employed first appeared in Sys V R4.

     The path expansion variables first appeared in Solaris 10, and in
     NetBSD 5.0.


SECURITY CONSIDERATIONS

     The environment variables LD_LIBRARY_PATH and LD_PRELOAD are not honored
     when executing in a set-user-ID or set-group-ID environment.  This action
     is taken to prevent malicious substitution of shared object dependencies
     or interposition of symbols.

NetBSD 9.3                      April 15, 2019                      NetBSD 9.3