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diff --git a/share/man/man5/link.5 b/share/man/man5/link.5 new file mode 100644 index 0000000..ca3e811 --- /dev/null +++ b/share/man/man5/link.5 @@ -0,0 +1,590 @@ +.\" Copyright (c) 1993 Paul Kranenburg +.\" All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by Paul Kranenburg. +.\" 3. The name of the author may not be used to endorse or promote products +.\" derived from this software without specific prior written permission +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR +.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES +.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. +.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, +.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT +.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, +.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY +.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT +.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF +.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. +.\" +.\" $FreeBSD$ +.\" +.Dd October 23, 1993 +.Dt LINK 5 +.Os +.Sh NAME +.Nm link +.Nd dynamic loader and link editor interface +.Sh SYNOPSIS +.In sys/types.h +.In nlist.h +.In link.h +.Sh DESCRIPTION +The include file +.In link.h +declares several structures that are present in dynamically linked +programs and libraries. +The structures define the interface between several components of the +link-editor and loader mechanism. +The layout of a number of these +structures within the binaries resembles the a.out format in many places +as it serves such similar functions as symbol definitions (including the +accompanying string table) and relocation records needed to resolve +references to external entities. +It also records a number of data structures +unique to the dynamic loading and linking process. +These include references +to other objects that are required to complete the link-editing process and +indirection tables to facilitate +.Em Position Independent Code +(PIC for short) to improve sharing of code pages among different processes. +The collection of data structures described here will be referred to as the +.Em Run-time Relocation Section (RRS) +and is embedded in the standard text and data segments of the dynamically +linked program or shared object image as the existing +.Xr a.out 5 +format offers no room for it elsewhere. +.Pp +Several utilities cooperate to ensure that the task of getting a program +ready to run can complete successfully in a way that optimizes the use +of system resources. +The compiler emits PIC code from which shared libraries +can be built by +.Xr ld 1 . +The compiler also includes size information of any initialized data items +through the .size assembler directive. +PIC code differs from conventional code +in that it accesses data variables through an indirection table, the +Global Offset Table, by convention accessible by the reserved name +.Dv _GLOBAL_OFFSET_TABLE_ . +The exact mechanism used for this is machine dependent, usually a machine +register is reserved for the purpose. +The rational behind this construct +is to generate code that is independent of the actual load address. +Only +the values contained in the Global Offset Table may need updating at run-time +depending on the load addresses of the various shared objects in the address +space. +.Pp +Likewise, procedure calls to globally defined functions are redirected through +the Procedure Linkage Table (PLT) residing in the data segment of the core +image. +Again, this is done to avoid run-time modifications to the text segment. +.Pp +The linker-editor allocates the Global Offset Table and Procedure Linkage Table +when combining PIC object files into an image suitable for mapping into the +process address space. +It also collects all symbols that may be needed by the +run-time link-editor and stores these along with the image's text and data bits. +Another reserved symbol, +.Em _DYNAMIC +is used to indicate the presence of the run-time linker structures. +Whenever +_DYNAMIC is relocated to 0, there is no need to invoke the run-time +link-editor. +If this symbol is non-zero, it points at a data structure from +which the location of the necessary relocation- and symbol information can +be derived. +This is most notably used by the start-up module, +.Em crt0 . +The _DYNAMIC structure is conventionally located at the start of the data +segment of the image to which it pertains. +.Sh DATA STRUCTURES +The data structures supporting dynamic linking and run-time relocation +reside both in the text and data segments of the image they apply to. +The text segments contain read-only data such as symbols descriptions and +names, while the data segments contain the tables that need to be modified by +during the relocation process. +.Pp +The _DYNAMIC symbol references a +.Fa _dynamic +structure: +.Bd -literal -offset indent +struct _dynamic { + int d_version; + struct so_debug *d_debug; + union { + struct section_dispatch_table *d_sdt; + } d_un; + struct ld_entry *d_entry; +}; +.Ed +.Bl -tag -width d_version +.It Fa d_version +This field provides for different versions of the dynamic linking +implementation. +The current version numbers understood by +.Xr ld 1 +and +.Xr ld.so 1 +are +.Em LD_VERSION_SUN (3) , +which is used by the +.Tn SunOS +4.x releases, and +.Em LD_VERSION_BSD (8) , +which has been in use since +.Fx 1.1 . +.It Fa d_un +Refers to a +.Em d_version +dependent data structure. +.It Fa so_debug +this field provides debuggers with a hook to access symbol tables of shared +objects loaded as a result of the actions of the run-time link-editor. +.El +.Pp +The +.Fa section_dispatch_table +structure is the main +.Dq dispatcher +table, containing offsets into the image's segments where various symbol +and relocation information is located. +.Bd -literal -offset indent +struct section_dispatch_table { + struct so_map *sdt_loaded; + long sdt_sods; + long sdt_filler1; + long sdt_got; + long sdt_plt; + long sdt_rel; + long sdt_hash; + long sdt_nzlist; + long sdt_filler2; + long sdt_buckets; + long sdt_strings; + long sdt_str_sz; + long sdt_text_sz; + long sdt_plt_sz; +}; +.Ed +.Bl -tag -width sdt_filler1 +.It Fa sdt_loaded +A pointer to the first link map loaded (see below). +This field is set by +.Nm ld.so +.It Fa sdt_sods +The start of a (linked) list of shared object descriptors needed by +.Em this +object. +.It Fa sdt_filler1 +Deprecated (used by SunOS to specify library search rules). +.It Fa sdt_got +The location of the Global Offset Table within this image. +.It Fa sdt_plt +The location of the Procedure Linkage Table within this image. +.It Fa sdt_rel +The location of an array of +.Fa relocation_info +structures +(see +.Xr a.out 5 ) +specifying run-time relocations. +.It Fa sdt_hash +The location of the hash table for fast symbol lookup in this object's +symbol table. +.It Fa sdt_nzlist +The location of the symbol table. +.It Fa sdt_filler2 +Currently unused. +.It Fa sdt_buckets +The number of buckets in +.Fa sdt_hash +.It Fa sdt_strings +The location of the symbol string table that goes with +.Fa sdt_nzlist . +.It Fa sdt_str_sz +The size of the string table. +.It Fa sdt_text_sz +The size of the object's text segment. +.It Fa sdt_plt_sz +The size of the Procedure Linkage Table. +.El +.Pp +A +.Fa sod +structure describes a shared object that is needed +to complete the link edit process of the object containing it. +A list of such objects +(chained through +.Fa sod_next ) +is pointed at +by the +.Fa sdt_sods +in the section_dispatch_table structure. +.Bd -literal -offset indent +struct sod { + long sod_name; + u_int sod_library : 1, + sod_reserved : 31; + short sod_major; + short sod_minor; + long sod_next; +}; +.Ed +.Bl -tag -width sod_library +.It Fa sod_name +The offset in the text segment of a string describing this link object. +.It Fa sod_library +If set, +.Fa sod_name +specifies a library that is to be searched for by +.Nm ld.so . +The path name +is obtained by searching a set of directories +(see also +.Xr ldconfig 8 ) +for a shared object matching +.Em lib\&<sod_name>\&.so.n.m . +If not set, +.Fa sod_name +should point at a full path name for the desired shared object. +.It Fa sod_major +Specifies the major version number of the shared object to load. +.It Fa sod_minor +Specifies the preferred minor version number of the shared object to load. +.El +.Pp +The run-time link-editor maintains a list of structures called +.Em link maps +to keep track of all shared objects loaded into a process' address space. +These structures are only used at run-time and do not occur within +the text or data segment of an executable or shared library. +.Bd -literal -offset indent +struct so_map { + caddr_t som_addr; + char *som_path; + struct so_map *som_next; + struct sod *som_sod; + caddr_t som_sodbase; + u_int som_write : 1; + struct _dynamic *som_dynamic; + caddr_t som_spd; +}; +.Ed +.Bl -tag -width som_dynamic +.It Fa som_addr +The address at which the shared object associated with this link map has +been loaded. +.It Fa som_path +The full path name of the loaded object. +.It Fa som_next +Pointer to the next link map. +.It Fa som_sod +The +.Fa sod +structure that was responsible for loading this shared object. +.It Fa som_sodbase +Tossed out in later versions of the run-time linker. +.It Fa som_write +Set if (some portion of) this object's text segment is currently writable. +.It Fa som_dynamic +Pointer to this object's +.Fa _dynamic +structure. +.It Fa som_spd +Hook for attaching private data maintained by the run-time link-editor. +.El +.Pp +Symbol description with size. +This is simply an +.Fa nlist +structure with one field +.Pq Fa nz_size +added. +Used to convey size information on items in the data segment +of shared objects. +An array of these lives in the shared object's +text segment and is addressed by the +.Fa sdt_nzlist +field of +.Fa section_dispatch_table . +.Bd -literal -offset indent +struct nzlist { + struct nlist nlist; + u_long nz_size; +#define nz_un nlist.n_un +#define nz_strx nlist.n_un.n_strx +#define nz_name nlist.n_un.n_name +#define nz_type nlist.n_type +#define nz_value nlist.n_value +#define nz_desc nlist.n_desc +#define nz_other nlist.n_other +}; +.Ed +.Bl -tag -width nz_size +.It Fa nlist +(see +.Xr nlist 3 ) . +.It Fa nz_size +The size of the data represented by this symbol. +.El +.Pp +A hash table is included within the text segment of shared object +to facilitate quick lookup of symbols during run-time link-editing. +The +.Fa sdt_hash +field of the +.Fa section_dispatch_table +structure points at an array of +.Fa rrs_hash +structures: +.Bd -literal -offset indent +struct rrs_hash { + int rh_symbolnum; /* symbol number */ + int rh_next; /* next hash entry */ +}; +.Ed +.Bl -tag -width rh_symbolnum +.It Fa rh_symbolnum +The index of the symbol in the shared object's symbol table (as given by the +.Fa ld_symbols +field). +.It Fa rh_next +In case of collisions, this field is the offset of the next entry in this +hash table bucket. +It is zero for the last bucket element. +.El +The +.Fa rt_symbol +structure is used to keep track of run-time allocated commons +and data items copied from shared objects. +These items are kept on linked list +and is exported through the +.Fa dd_cc +field in the +.Fa so_debug +structure (see below) for use by debuggers. +.Bd -literal -offset indent +struct rt_symbol { + struct nzlist *rt_sp; + struct rt_symbol *rt_next; + struct rt_symbol *rt_link; + caddr_t rt_srcaddr; + struct so_map *rt_smp; +}; +.Ed +.Bl -tag -width rt_scraddr +.It Fa rt_sp +The symbol description. +.It Fa rt_next +Virtual address of next rt_symbol. +.It Fa rt_link +Next in hash bucket. +Used internally by +.Nm ld.so . +.It Fa rt_srcaddr +Location of the source of initialized data within a shared object. +.It Fa rt_smp +The shared object which is the original source of the data that this +run-time symbol describes. +.El +.Pp +The +.Fa so_debug +structure is used by debuggers to gain knowledge of any shared objects +that have been loaded in the process's address space as a result of run-time +link-editing. +Since the run-time link-editor runs as a part of process +initialization, a debugger that wishes to access symbols from shared objects +can only do so after the link-editor has been called from crt0. +A dynamically linked binary contains a +.Fa so_debug +structure which can be located by means of the +.Fa d_debug +field in +.Fa _dynamic . +.Bd -literal -offset indent +struct so_debug { + int dd_version; + int dd_in_debugger; + int dd_sym_loaded; + char *dd_bpt_addr; + int dd_bpt_shadow; + struct rt_symbol *dd_cc; +}; +.Ed +.Bl -tag -width dd_in_debugger +.It Fa dd_version +Version number of this interface. +.It Fa dd_in_debugger +Set by the debugger to indicate to the run-time linker that the program is +run under control of a debugger. +.It Fa dd_sym_loaded +Set by the run-time linker whenever it adds symbols by loading shared objects. +.It Fa dd_bpt_addr +The address where a breakpoint will be set by the run-time linker to +divert control to the debugger. +This address is determined by the start-up +module, +.Pa crt0.o , +to be some convenient place before the call to _main. +.It Fa dd_bpt_shadow +Contains the original instruction that was at +.Fa dd_bpt_addr . +The debugger is expected to put this instruction back before continuing the +program. +.It Fa dd_cc +A pointer to the linked list of run-time allocated symbols that the debugger +may be interested in. +.El +.Pp +The +.Em ld_entry +structure defines a set of service routines within +.Nm ld.so . +.\" See +.\" .Xr libdl.a +.\" for more information. +.Bd -literal -offset indent +struct ld_entry { + void *(*dlopen)(char *, int); + int (*dlclose)(void *); + void *(*dlsym)(void *, char *); + char *(*dlerror)(void); +}; +.Ed +.Pp +The +.Fa crt_ldso +structure defines the interface between the start-up code in crt0 and +.Nm ld.so . +.Bd -literal -offset indent +struct crt_ldso { + int crt_ba; + int crt_dzfd; + int crt_ldfd; + struct _dynamic *crt_dp; + char **crt_ep; + caddr_t crt_bp; + char *crt_prog; + char *crt_ldso; + struct ld_entry *crt_ldentry; +}; +#define CRT_VERSION_SUN 1 +#define CRT_VERSION_BSD_2 2 +#define CRT_VERSION_BSD_3 3 +#define CRT_VERSION_BSD_4 4 +.Ed +.Bl -tag -width crt_dzfd +.It Fa crt_ba +The virtual address at which +.Nm ld.so +was loaded by crt0. +.It Fa crt_dzfd +On SunOS systems, this field contains an open file descriptor to +.Dq Pa /dev/zero +used to get demand paged zeroed pages. +On +.Fx +systems it contains -1. +.It Fa crt_ldfd +Contains an open file descriptor that was used by crt0 to load +.Nm ld.so . +.It Fa crt_dp +A pointer to main's +.Fa _dynamic +structure. +.It Fa crt_ep +A pointer to the environment strings. +.It Fa crt_bp +The address at which a breakpoint will be placed by the run-time linker +if the main program is run by a debugger. +See +.Fa so_debug +.It Fa crt_prog +The name of the main program as determined by crt0 (CRT_VERSION_BSD3 only). +.It Fa crt_ldso +The path of the run-time linker as mapped by crt0 (CRT_VERSION_BSD4 only). +.El +.Pp +The +.Fa hints_header +and +.Fa hints_bucket +structures define the layout of the library hints, normally found in +.Dq Pa /var/run/ld.so.hints , +which is used by +.Nm ld.so +to quickly locate the shared object images in the +file system. +The organization of the hints file is not unlike that of an +.Dq a.out +object file, in that it contains a header determining the offset and size +of a table of fixed sized hash buckets and a common string pool. +.Bd -literal -offset indent +struct hints_header { + long hh_magic; +#define HH_MAGIC 011421044151 + long hh_version; +#define LD_HINTS_VERSION_1 1 + long hh_hashtab; + long hh_nbucket; + long hh_strtab; + long hh_strtab_sz; + long hh_ehints; +}; +.Ed +.Bl -tag -width hh_strtab_sz +.It Fa hh_magic +Hints file magic number. +.It Fa hh_version +Interface version number. +.It Fa hh_hashtab +Offset of hash table. +.It Fa hh_strtab +Offset of string table. +.It Fa hh_strtab_sz +Size of strings. +.It Fa hh_ehints +Maximum usable offset in hints file. +.El +.Bd -literal -offset indent +/* + * Hash table element in hints file. + */ +struct hints_bucket { + int hi_namex; + int hi_pathx; + int hi_dewey[MAXDEWEY]; + int hi_ndewey; +#define hi_major hi_dewey[0] +#define hi_minor hi_dewey[1] + int hi_next; +}; +.Ed +.Bl -tag -width hi_ndewey +.It Fa hi_namex +Index of the string identifying the library. +.It Fa hi_pathx +Index of the string representing the full path name of the library. +.It Fa hi_dewey +The version numbers of the shared library. +.It Fa hi_ndewey +The number of valid entries in +.Fa hi_dewey . +.It Fa hi_next +Next bucket in case of hashing collisions. +.El +.Sh CAVEATS +Only the (GNU) C compiler currently supports the creation of shared libraries. +Other programming languages cannot be used. |
