An executable file that participates in dynamic linking shall have one PT_INTERP program header element. During exec(base operating system), the system retrieves a path name from the PT_INTERP segment and creates the initial process image from the interpreter file's segments. That is, instead of using the original executable file's segment images, the system composes a memory image for the interpreter. It then is the interpreter's responsibility to receive control from the system and provide an environment for the application program.
As Process Initialization in Chapter 32 of the processor supplement mentions, the interpreter receives control in one of two ways. First, it may receive a file descriptor to read the executable file, positioned at the beginning. It can use this file descriptor to read and/or map the executable file's segments into memory. Second, depending on the executable file format, the system may load the executable file into memory instead of giving the interpreter an open file descriptor. With the possible exception of the file descriptor, the interpreter's initial process state matches what the executable file would have received. The interpreter itself may not require a second interpreter. An interpreter may be either a shared object or an executable file.
When building an executable file that uses dynamic linking, the link editor adds a program header element of type PT_INTERP to an executable file, telling the system to invoke the dynamic linker as the program interpreter.
NOTE: The locations of the system provided dynamic linkers are processor specific.
Exec(base operating system) and the dynamic linker cooperate to create the process image for the program, which entails the following actions:
The link editor also constructs various data that assist the dynamic linker for executable and shared object files. As shown above in Program Header, this data resides in loadable segments, making them available during execution. (Once again, recall the exact segment contents are processor-specific. See the processor supplement for complete information).
Because every ABI-conforming program imports the basic system services from a shared object library [See System Library in Chapter 32], the dynamic linker participates in every ABI-conforming program execution.
As Program Loading explains in the processor supplement, shared objects may occupy virtual memory addresses that are different from the addresses recorded in the file's program header table. The dynamic linker relocates the memory image, updating absolute addresses before the application gains control. Although the absolute address values would be correct if the library were loaded at the addresses specified in the program header table, this normally is not the case.
If the process environment [see exec(base operating system)] contains a variable named LD_BIND_NOW with a non-null value, the dynamic linker processes all relocations before transferring control to the program. For example, all the following environment entries would specify this behavior.
Otherwise, LD_BIND_NOW either does not occur in the environment or has a null value. The dynamic linker is permitted to evaluate procedure linkage table entries lazily, thus avoiding symbol resolution and relocation overhead for functions that are not called. See the Procedure Linkage Table in this chapter of the processor supplement for more information.
If an object file participates in dynamic linking, its program header table will have an element of type PT_DYNAMIC. This segment contains the .dynamic section. A special symbol, _DYNAMIC, labels the section, which contains an array of the following structures.
Dynamic Structure
typedef struct { Elf32_Sword d_tag; union { Elf32_Word d_val; Elf32_Addr d_ptr; } d_un; } Elf32_Dyn; extern Elf32_Dyn _DYNAMIC[]; typedef struct { Elf64_Sxword d_tag; union { Elf64_Xword d_val; Elf64_Addr d_ptr; } d_un; } Elf64_Dyn; extern Elf64_Dyn _DYNAMIC[];
For each object with this type, d_tag controls the interpretation of d_un.
For consistency, files do not contain relocation entries to correct addresses in the dynamic structure.
To make it simpler for tools to interpret the contents of dynamic section entries, the value of each tag, except for those in two special compatibility ranges, will determine the interpretation of the d_un union. A tag whose value is an even number indicates a dynamic section entry that uses d_ptr. A tag whose value is an odd number indicates a dynamic section entry that uses d_val or that uses neither d_ptr nor d_val. Tags whose values are less than the special value DT_ENCODING and tags whose values fall between DT_HIOS and DT_LOPROC do not follow these rules.
The following table summarizes the tag requirements for executable and shared object files. If a tag is marked mandatory, the dynamic linking array for an ABI-conforming file must have an entry of that type. Likewise, optional means an entry for the tag may appear but is not required.
Dynamic Array Tags, d_tag
Name | Value | d_un | Executable | Shared Object |
---|---|---|---|---|
DT_NULL | 0 | ignored | mandatory | mandatory |
DT_NEEDED | 1 | d_val | optional | optional |
DT_PLTRELSZ | 2 | d_val | optional | optional |
DT_PLTGOT | 3 | d_ptr | optional | optional |
DT_HASH | 4 | d_ptr | mandatory | mandatory |
DT_STRTAB | 5 | d_ptr | mandatory | mandatory |
DT_SYMTAB | 6 | d_ptr | mandatory | mandatory |
DT_RELA | 7 | d_ptr | mandatory | optional |
DT_RELASZ | 8 | d_val | mandatory | optional |
DT_RELAENT | 9 | d_val | mandatory | optional |
DT_STRSZ | 10 | d_val | mandatory | mandatory |
DT_SYMENT | 11 | d_val | mandatory | mandatory |
DT_INIT | 12 | d_ptr | optional | optional |
DT_FINI | 13 | d_ptr | optional | optional |
DT_SONAME | 14 | d_val | ignored | optional |
DT_RPATH* | 15 | d_val | optional | ignored |
DT_SYMBOLIC* | 16 | ignored | ignored | optional |
DT_REL | 17 | d_ptr | mandatory | optional |
DT_RELSZ | 18 | d_val | mandatory | optional |
DT_RELENT | 19 | d_val | mandatory | optional |
DT_PLTREL | 20 | d_val | optional | optional |
DT_DEBUG | 21 | d_ptr | optional | ignored |
DT_TEXTREL* | 22 | ignored | optional | optional |
DT_JMPREL | 23 | d_ptr | optional | optional |
DT_BIND_NOW* | 24 | ignored | optional | optional |
DT_INIT_ARRAY | 25 | d_ptr | optional | optional |
DT_FINI_ARRAY | 26 | d_ptr | optional | optional |
DT_INIT_ARRAYSZ | 27 | d_val | optional | optional |
DT_FINI_ARRAYSZ | 28 | d_val | optional | optional |
DT_RUNPATH | 29 | d_val | optional | optional |
DT_FLAGS | 30 | d_val | optional | optional |
DT_ENCODING | 32 | unspecified | unspecified | unspecified |
DT_PREINIT_ARRAY | 32 | d_ptr | optional | ignored |
DT_PREINIT_ARRAYSZ | 33 | d_val | optional | ignored |
DT_LOOS | 0x6000000D | unspecified | unspecified | unspecified |
DT_HIOS | 0x6ffff000 | unspecified | unspecified | unspecified |
DT_LOPROC | 0x70000000 | unspecified | unspecified | unspecified |
DT_HIPROC | 0x7fffffff | unspecified | unspecified | unspecified |
* Signifies an entry that is at level 2.
Except for the DT_NULL element at the end of the array, and the relative order of DT_NEEDED elements, entries may appear in any order. Tag values not appearing in the table are reserved.
Name | Value |
---|---|
DF_ORIGIN | 0x1 |
DF_SYMBOLIC | 0x2 |
DF_TEXTREL | 0x4 |
DF_BIND_NOW | 0x8 |
When the link editor processes an archive library, it extracts library members and copies them into the output object file. These statically linked services are available during execution without involving the dynamic linker. Shared objects also provide services, and the dynamic linker must attach the proper shared object files to the process image for execution.
When the dynamic linker creates the memory segments for an object file, the dependencies (recorded in DT_NEEDED entries of the dynamic structure) tell what shared objects are needed to supply the program's services. By repeatedly connecting referenced shared objects and their dependencies, the dynamic linker builds a complete process image. When resolving symbolic references, the dynamic linker examines the symbol tables with a breadth-first search. That is, it first looks at the symbol table of the executable program itself, then at the symbol tables of the DT_NEEDED entries (in order), and then at the second level DT_NEEDED entries, and so on. Shared object files must be readable by the process; other permissions are not required.
NOTE: Even when a shared object is referenced multiple times in the dependency list, the dynamic linker will connect the object only once to the process.
Names in the dependency list are copies either of the DT_SONAME strings or the path names of the shared objects used to build the object file. For example, if the link editor builds an executable file using one shared object with a DT_SONAME entry of lib1 and another shared object library with the path name /usr/lib/lib2, the executable file will contain lib1 and /usr/lib/lib2 in its dependency list.
If a shared object name has one or more slash (/) characters anywhere in the name, such as /usr/lib/lib2 or directory/file, the dynamic linker uses that string directly as the path name. If the name has no slashes, such as lib1, three facilities specify shared object path searching.
The set of directories specified by a given DT_RUNPATH entry is used to find only the immediate dependencies of the executable or shared object containing the DT_RUNPATH entry. That is, it is used only for those dependencies contained in the DT_NEEDED entries of the dynamic structure containing the DT_RUNPATH entry, itself. One object's DT_RUNPATH entry does not affect the search for any other object's dependencies.
The following values would be equivalent to the previous example:
Although some programs (such as the link editor) treat the lists before and after the semicolon differently, the dynamic linker does not. Nevertheless, the dynamic linker accepts the semicolon notation, with the semantics described previously.
All LD_LIBRARY_PATH directories are searched before those from DT_RUNPATH.
When the dynamic linker is searching for shared objects, it is not a fatal error if an ELF file with the wrong attributes is encountered in the search. Instead, the dynamic linker shall exhaust the search of all paths before determining that a matching object could not be found. For this determination, the relevant attributes are contained in the following ELF header fields:
e_ident[EI_DATA], e_ident[EI_CLASS], e_ident[EI_OSABI], e_ident[EI_ABIVERSION], e_machine, e_type, e_flags and e_version.
NOTE: For security, the dynamic linker ignores LD_LIBRARY_PATH for set-user and set-group ID programs. It does, however, search DT_RUNPATH directories and the default directories. The same restriction may be applied to processes that have more than minimal privileges on systems with installed extended security mechanisms.
NOTE: A fourth search facility, the dynamic array tag DT_RPATH, has been moved to level 2 in the ABI. It provides a colon-separated list of directories to search. Directories specified by DT_RPATH are searched before directories specified by LD_LIBRARY_PATH.
If both DT_RPATH and DT_RUNPATH entries appear in a single object's dynamic array, the dynamic linker processes only the DT_RUNPATH entry.
Within a string provided by dynamic array entries with the DT_NEEDED or DT_RUNPATH tags and in pathnames passed as parameters to the dlopen() routine, a dollar sign ($) introduces a substitution sequence. This sequence consists of the dollar sign immediately followed by either the longest name sequence or a name contained within left and right braces ({) and (}). A name is a sequence of bytes that start with either a letter or an underscore followed by zero or more letters, digits or underscores. If a dollar sign is not immediately followed by a name or a brace-enclosed name, the behavior of the dynamic linker is unspecified.
If the name is ORIGIN, then the substitution sequence is replaced by the dynamic linker with the absolute pathname of the directory in which the object containing the substitution sequence originated. Moreover, the pathname will contain no symbolic links or use of . or .. components. Otherwise (when the name is not ORIGIN) the behavior of the dynamic linker is unspecified.
When the dynamic linker loads an object that uses $ORIGIN, it must calculate the pathname of the directory containing the object. Because this calculation can be computationally expensive, implementations may want to avoid the calculation for objects that do not use $ORIGIN. If an object calls dlopen() with a string containing $ORIGIN and does not use $ORIGIN in one if its dynamic array entries, the dynamic linker may not have calculated the pathname for the object until the dlopen() actually occurs. Since the application may have changed its current working directory before the dlopen() call, the calculation may not yield the correct result. To avoid this possibility, an object may signal its intention to reference $ORIGIN by setting the DF_ORIGIN flag. An implementation may reject an attempt to use $ORIGIN within a dlopen() call from an object that did not set the DF_ORIGIN flag and did not use $ORIGIN within its dynamic array.
NOTE: For security, the dynamic linker does not allow use of $ORIGIN substitution sequences for set-user and set-group ID programs. For such sequences that appear within strings specified by DT_RUNPATH dynamic array entries, the specific search path containing the $ORIGIN sequence is ignored (though other search paths in the same string are processed). $ORIGIN sequences within a DT_NEEDED entry or path passed as a parameter to dlopen() are treated as errors. The same restrictions may be applied to processes that have more than minimal privileges on systems with installed extended security mechanisms.
NOTE: This section requires processor-specific information. The System V Application Binary Interface supplement for the desired processor describes the details.
NOTE: This section requires processor-specific information. The System V Application Binary Interface supplement for the desired processor describes the details.
A hash table of Elf32_Word objects supports symbol table access. The same table layout is used for both the 32-bit and 64-bit file class. Labels appear below to help explain the hash table organization, but they are not part of the specification.
Symbol Hash
nbucket |
nchain |
bucket[0]
. . . bucket[nbucket-1] |
chain[0]
. . . chain[nchain-1] |
The bucket array contains nbucket entries, and the chain array contains nchain entries; indexes start at 0. Both bucket and chain hold symbol table indexes.
Chain table entries parallel the symbol table. The number of symbol table entries should equal nchain; so symbol table indexes also select chain table entries. A hashing function (shown below) accepts a symbol name and returns a value that may be used to compute a bucket index.
Consequently, if the hashing function returns the value x for some name, bucket[x%nbucket] gives an index, y, into both the symbol table and the chain table.
If the symbol table entry is not the one desired, chain[y] gives the next symbol table entry with the same hash value.
One can follow the chain links until either the selected symbol
table entry holds the desired name or the chain entry contains the
value STN_UNDEF.
Hashing Function
unsigned long elf_hash(const unsigned char *name) { unsigned long h = 0, g; while (*name) { h = (h << 4) + *name++; if (g = h & 0xf0000000) h ^= g >> 24; h &= ~g; } return h; } |
After the dynamic linker has built the process image and performed the relocations, each shared object and the executable file get the opportunity to execute some initialization functions. All shared object initializations happen before the executable file gains control.
Before the initialization functions for any object A is called, the initialization functions for any other objects that object A depends on are called. For these purposes, an object A depends on another object B, if B appears in A's list of needed objects (recorded in the DT_NEEDED entries of the dynamic structure). The order of initialization for circular dependencies is undefined.
The initialization of objects occurs by recursing through the needed entries of each object. The initialization functions for an object are invoked after the needed entries for that object have been processed. The order of processing among the entries of a particular list of needed objects is unspecified.
NOTE: Each processor supplement may optionally further restrict the algorithm used to determine the order of initialization. Any such restriction, however, may not conflict with the rules described by this specification.
The following example illustrates two of the possible correct orderings which can be generated for the example NEEDED lists. In this example the a.out is dependent on b, d, and e. b is dependent on d and f, while d is dependent on e and g. From this information a dependency graph can be drawn. The above algorithm on initialization will then allow the following specified initialization orderings among others.
Initialization Ordering Example Similarly, shared objects and executable files may have termination functions, which are executed with the atexit(base operating system) mechanism after the base process begins its termination sequence. The termination functions for any object A must be called before the termination functions for any other objects that object A depends on. For these purposes, an object A depends on another object B, if B appears in A's list of needed objects (recorded in the DT_NEEDED entries of the dynamic structure). The order of termination for circular dependencies is undefined.
Finally, an executable file may have pre-initialization functions. These functions are executed after the dynamic linker has built the process image and performed relocations but before any shared object initialization functions. Pre-initialization functions are not permitted in shared objects.
NOTE: Complete initialization of system libraries may not have occurred when pre-initializations are executed, so some features of the system may not be available to pre-initialization code. In general, use of pre-initialization code can be considered portable only if it has no dependencies on system libraries.
The dynamic linker ensures that it will not execute any initialization, pre-initialization, or termination functions more than once.
Shared objects designate their initialization and termination code in one of two ways. First, they may specify the address of a function to execute via the DT_INIT and DT_FINI entries in the dynamic structure, described in Dynamic Section above.
Shared objects may also (or instead) specify the address and size of an array of function pointers. Each element of this array is a pointer to a function to be executed by the dynamic linker. Each array element is the size of a pointer in the programming model followed by the object containing the array. The address of the array of initialization function pointers is specified by the DT_INIT_ARRAY entry in the dynamic structure. Similarly, the address of the array of pre-initialization functions is specified by DT_PREINIT_ARRAY and the address of the array of termination functions is specified by DT_FINI_ARRAY. The size of each array is specified by the DT_INIT_ARRAYSZ, DT_PREINIT_ARRAYSZ, and DT_FINI_ARRAYSZ entries.
The functions whose addresses are contained in the arrays specified by DT_INIT_ARRAY and by DT_PREINIT_ARRAY are executed by the dynamic linker in the same order in which their addresses appear in the array; those specified by DT_FINI_ARRAY are executed in reverse order.
If an object contains both DT_INIT and DT_INIT_ARRAY entries, the function referenced by the DT_INIT entry is processed before those referenced by the DT_INIT_ARRAY entry for that object. If an object contains both DT_FINI and DT_FINI_ARRAY entries, the functions referenced by the DT_FINI_ARRAY entry are processed before the one referenced by the DT_FINI entry for that object.
NOTE: Although the atexit(base operating system) termination processing normally will be done, it is not guaranteed to have executed upon process death. In particular, the process will not execute the termination processing if it calls _exit [see exit(base operating system)] or if the process dies because it received a signal that it neither caught nor ignored.
The processor supplement for each processor specifies whether the dynamic linker is responsible for calling the executable file's initialization function or registering the executable file's termination function with atexit(base operating system). Termination functions specified by users via the atexit(base operating system) mechanism must be executed before any termination functions of shared objects.