Dynamic Section

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. See sys/link.h.

typedef struct {
        Elf32_Sword d_tag;
        union {
                Elf32_Word      d_val;
                Elf32_Addr      d_ptr;
                Elf32_Off       d_off;
        } d_un;
} Elf32_Dyn;

typedef struct {
        Elf64_Xword d_tag;
        union {
                Elf64_Xword     d_val;
                Elf64_Addr      d_ptr;
        } d_un;
} Elf64_Dyn;

For each object with this type, d_tag controls the interpretation of d_un.

d_val

These objects represent integer values with various interpretations.

d_ptr

These objects represent program virtual addresses. A file's virtual addresses might not match the memory virtual addresses during execution. When interpreting addresses contained in the dynamic structure, the runtime linker computes actual addresses, based on the original file value and the memory base address. For consistency, files do not contain relocation entries to correct addresses in the dynamic structure.

In general, the value of each dynamic tag determines the interpretation of the d_un union. This convention provides for simpler interpretation of dynamic tags by third party tools. 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 the tag uses neither d_ptr nor d_val. Tags with values in the following special compatibility ranges do not follow these rules. Third party tools must handle these exception ranges explicitly on an item by item basis.

• Tags whose values are less than the special value DT_ENCODING.

• Tags with values that fall between DT_LOOS and DT_SUNW_ENCODING.

• Tags with values that fall between DT_HIOS and DT_LOPROC.

The following table summarizes the tag requirements for executable and shared object files. If a tag is marked mandatory, then the dynamic linking array must have an entry of that type. Likewise, optional means an entry for the tag can appear but is not required.

Table 13-8 ELF Dynamic Array Tags

DT_NULL

Marks the end of the _DYNAMIC array.

DT_NEEDED

The DT_STRTAB string table offset of a null-terminated string, giving the name of a needed dependency. The dynamic array can contain multiple entries of this type. The relative order of these entries is significant, though their relation to entries of other types is not. See Shared Object Dependencies.

DT_PLTRELSZ

The total size, in bytes, of the relocation entries associated with the procedure linkage table. See Procedure Linkage Table (Processor-Specific).

DT_PLTGOT

An address associated with the procedure linkage table or the global offset table. See Procedure Linkage Table (Processor-Specific) and Global Offset Table (Processor-Specific).

DT_HASH

The address of the symbol hash table. This table refers to the symbol table indicated by the DT_SYMTAB element. See Hash Table Section.

DT_STRTAB

The address of the string table. Symbol names, dependency names, and other strings required by the runtime linker reside in this table. See String Table Section.

DT_SYMTAB

The address of the symbol table. See Symbol Table Section.

DT_RELA

The address of a relocation table. See Relocation Sections.

An object file can have multiple relocation sections. When creating the relocation table for an executable or shared object file, the link-editor catenates those sections to form a single table. Although the sections can remain independent in the object file, the runtime linker sees a single table. When the runtime linker creates the process image for an executable file or adds a shared object to the process image, the runtime linker reads the relocation table and performs the associated actions.

This element requires the DT_RELASZ and DT_RELAENT elements also be present. When relocation is mandatory for a file, either DT_RELA or DT_REL can occur.

DT_RELASZ

The total size, in bytes, of the DT_RELA relocation table.

DT_RELAENT

The size, in bytes, of the DT_RELA relocation entry.

DT_STRSZ

The total size, in bytes, of the DT_STRTAB string table.

DT_SYMENT

The size, in bytes, of the DT_SYMTAB symbol entry.

DT_INIT

The address of an initialization function. See Initialization and Termination Sections.

DT_FINI

The address of a termination function. See Initialization and Termination Sections.

DT_SONAME

The DT_STRTAB string table offset of a null-terminated string, identifying the name of the shared object. See Recording a Shared Object Name.

DT_RPATH

The DT_STRTAB string table offset of a null-terminated library search path string. This element's use has been superseded by DT_RUNPATH. See Directories Searched by the Runtime Linker.

DT_SYMBOLIC

Indicates the object contains symbolic bindings that were applied during its link-edit. This elements use has been superseded by the DF_SYMBOLIC flag. See Using the -B symbolic Option.

DT_REL

Similar to DT_RELA, except its table has implicit addends. This element requires that the DT_RELSZ and DT_RELENT elements also be present.

DT_RELSZ

The total size, in bytes, of the DT_REL relocation table.

DT_RELENT

The size, in bytes, of the DT_REL relocation entry.

DT_PLTREL

Indicates the type of relocation entry to which the procedure linkage table refers, either DT_REL or DT_RELA. All relocations in a procedure linkage table must use the same relocation. See Procedure Linkage Table (Processor-Specific). This element requires a DT_JMPREL element also be present.

DT_DEBUG

Used for debugging.

DT_TEXTREL

Indicates that one or more relocation entries might request modifications to a non-writable segment, and the runtime linker can prepare accordingly. This element's use has been superseded by the DF_TEXTREL flag. See Position-Independent Code.

DT_JMPREL

The address of relocation entries that are associated solely with the procedure linkage table. See Procedure Linkage Table (Processor-Specific). The separation of these relocation entries enables the runtime linker to ignore these entries when the object is loaded with lazy binding enabled. This element requires the DT_PLTRELSZ and DT_PLTREL elements also be present.

DT_POSFLAG_1

Various state flags which are applied to the DT_ element immediately following. See Table 13-11.

DT_BIND_NOW

Indicates that all relocations for this object must be processed before returning control to the program. The presence of this entry takes precedence over a directive to use lazy binding when specified through the environment or by means of dlopen(3C). This element's use has been superseded by the DF_BIND_NOW flag. See When Relocations are Performed.

DT_INIT_ARRAY

The address of an array of pointers to initialization functions. This element requires that a DT_INIT_ARRAYSZ element also be present. See Initialization and Termination Sections.

DT_FINI_ARRAY

The address of an array of pointers to termination functions. This element requires that a DT_FINI_ARRAYSZ element also be present. See Initialization and Termination Sections.

DT_INIT_ARRAYSZ

The total size, in bytes, of the DT_INIT_ARRAY array.

DT_FINI_ARRAYSZ

The total size, in bytes, of the DT_FINI_ARRAY array.

DT_RUNPATH

The DT_STRTAB string table offset of a null-terminated library search path string. See Directories Searched by the Runtime Linker.

DT_FLAGS

Flag values specific to this object. See Table 13-9.

DT_ENCODING

Dynamic tag values that are greater than or equal to DT_ENCODING, and less than or equal to DT_LOOS, follow the rules for the interpretation of the d_un union.

DT_PREINIT_ARRAY

The address of an array of pointers to pre-initialization functions. This element requires that a DT_PREINIT_ARRAYSZ element also be present. This array is processed only in an executable file. This array is ignored if contained in a shared object. See Initialization and Termination Sections.

DT_PREINIT_ARRAYSZ

The total size, in bytes, of the DT_PREINIT_ARRAY array.

DT_MAXPOSTAGS

The number of positive dynamic array tag values.

DT_LOOS - DT_HIOS

Values in this inclusive range are reserved for operating system-specific semantics. All such values follow the rules for the interpretation of the d_un union.

DT_SUNW_AUXILIARY

The DT_STRTAB string table offset of a null-terminated string that names one or more per-symbol, auxiliary filtees. See Generating Auxiliary Filters.

DT_SUNW_RTLDINF

Reserved for internal use by the runtime-linker.

DT_SUNW_FILTER

The DT_STRTAB string table offset of a null-terminated string that names one or more per-symbol, standard filtees. See Generating Standard Filters.

DT_SUNW_CAP

The address of the capabilities section. See Capabilities Section.

DT_SUNW_SYMTAB

The address of the symbol table containing local function symbols that augment the symbols provided by DT_SYMTAB. These symbols are always adjacent to, and immediately precede the symbols provided by DT_SYMTAB. See Symbol Table Section.

DT_SUNW_SYMSZ

The combined size of the symbol tables given by DT_SUNW_SYMTAB and DT_SYMTAB.

DT_SUNW_ENCODING

Dynamic tag values that are greater than or equal to DT_SUNW_ENCODING, and less than or equal to DT_HIOS, follow the rules for the interpretation of the d_un union.

DT_SUNW_SORTENT

The size, in bytes, of the DT_SUNW_SYMSORT and DT_SUNW_TLSSORT symbol sort entries.

DT_SUNW_SYMSORT

The address of the array of symbol table indices that provide sorted access to function and variable symbols in the symbol table referenced by DT_SUNW_SYMTAB. See Symbol Sort Sections.

DT_SUNW_SYMSORTSZ

The total size, in bytes, of the DT_SUNW_SYMSORT array.

DT_SUNW_TLSSORT

The address of the array of symbol table indices that provide sorted access to thread local symbols in the symbol table referenced by DT_SUNW_SYMTAB. See Symbol Sort Sections.

DT_SUNW_TLSSORTSZ

The total size, in bytes, of the DT_SUNW_TLSSORT array.

DT_SUNW_CAPINFO

The address of the array of symbol table indices that provide the association of symbols to their capability requirements. See Capabilities Section.

DT_SUNW_STRPAD

The total size, in bytes, of the unused reserved space at the end of the dynamic string table. If DT_SUNW_STRPAD is not present in an object, no reserved space is available.

DT_SUNW_CAPCHAIN

The address of the array of capability family indices. Each family of indices is terminated with a 0 entry.

DT_SUNW_LDMACH

The machine architecture of the link-editor that produced the object. DT_SUNW_LDMACH uses the same EM_ integer values used for the e_machine field of the ELF header. See ELF Header. DT_SUNW_LDMACH is used to identify the class, 32–bit or 64–bit, and the platform of the link-editor that built the object. This information is not used by the runtime linker, but exists purely for documentation.

DT_SUNW_CAPCHAINENT

The size, in bytes, of the DT_SUNW_CAPCHAIN entries.

DT_SUNW_CAPCHAINSZ

The total size, in bytes, or the DT_SUNW_CAPCHAIN chain.

DT_SUNW_PARENT

The DT_STRTAB string table offset of a null terminated parent object name. The name provided is a basename, containing only a file name without any path component. See Parent Objects.

DT_SUNW_ASLR

Address Space Layout and Randomization (ASLR) flag values specific to this object. See Table 13-12.

DT_SYMINFO

The address of the symbol information table. This element requires that the DT_SYMINENT and DT_SYMINSZ elements also be present. See Syminfo Table Section.

DT_SYMINENT

The size, in bytes, of the DT_SYMINFO information entry.

DT_SYMINSZ

The total size, in bytes, of the DT_SYMINFO table.

DT_VERDEF

The address of the version definition table. Elements within this table contain indexes into the string table DT_STRTAB. This element requires that the DT_VERDEFNUM element also be present. See Version Definition Section.

DT_VERDEFNUM

The number of entries in the DT_VERDEF table.

DT_VERNEED

The address of the version dependency table. Elements within this table contain indexes into the string table DT_STRTAB. This element requires that the DT_VERNEEDNUM element also be present. See Version Dependency Section.

DT_VERNEEDNUM

The number of entries in the DT_VERNEEDNUM table.

DT_RELACOUNT

Indicates the RELATIVE relocation count, which is produced from the concatenation of all Elf32_Rela, or Elf64_Rela relocations. See Combined Relocation Sections.

DT_RELCOUNT

Indicates the RELATIVE relocation count, which is produced from the concatenation of all Elf32_Rel relocations. See Combined Relocation Sections.

DT_AUXILIARY

The DT_STRTAB string table offset of a null-terminated string that names one or more auxiliary filtees. See Generating Auxiliary Filters.

DT_FILTER

The DT_STRTAB string table offset of a null-terminated string that names one or more standard filtees. See Generating Standard Filters.

DT_CHECKSUM

A simple checksum of selected sections of the object. See gelf_checksum(3ELF).

DT_MOVEENT

The size, in bytes, of the DT_MOVETAB move entries.

DT_MOVESZ

The total size, in bytes, of the DT_MOVETAB table.

DT_MOVETAB

The address of a move table. This element requires that the DT_MOVEENT and DT_MOVESZ elements also be present. See Move Section.

DT_CONFIG

The DT_STRTAB string table offset of a null-terminated string defining a configuration file. The configuration file is only meaningful in an executable, and is typically unique to this object. See Configuring the Default Search Paths.

DT_DEPAUDIT

The DT_STRTAB string table offset of a null-terminated string defining one or more audit libraries. See Runtime Linker Auditing Interface.

DT_AUDIT

The DT_STRTAB string table offset of a null-terminated string defining one or more audit libraries. See Runtime Linker Auditing Interface.

DT_FLAGS_1

Flag values specific to this object. See Table 13-10.

DT_VALRNGLO - DT_VALRNGHI

Values in this inclusive range use the d_un.d_val field of the dynamic structure.

DT_ADDRRNGLO - DT_ADDRRNGHI

Values in this inclusive range use the d_un.d_ptr field of the dynamic structure. If any adjustment is made to the ELF object after the object has been built, these entries must be updated accordingly.

DT_SPARC_REGISTER

The index of an STT_SPARC_REGISTER symbol within the DT_SYMTAB symbol table. One dynamic entry exists for every STT_SPARC_REGISTER symbol in the symbol table. See Register Symbols.

DT_LOPROC - DT_HIPROC

Values in this inclusive range are reserved for processor-specific semantics.

Except for the DT_NULL element at the end of the dynamic array and the relative order of DT_NEEDED and DT_POSFLAG_1 elements, entries can appear in any order. Tag values not appearing in the table are reserved.

Table 13-9 ELF Dynamic Flags, DT_FLAGS

DF_ORIGIN

Indicates that the object requires $ORIGIN processing. See Locating Associated Dependencies.

DF_SYMBOLIC

Indicates that the object contains symbolic bindings that were applied during its link-edit. See Using the -B symbolic Option.

DF_TEXTREL

Indicates that one or more relocation entries might request modifications to a non-writable segment, and the runtime linker can prepare accordingly. See Position-Independent Code.

DF_BIND_NOW

Indicates that all relocations for this object must be processed before returning control to the program. The presence of this entry takes precedence over a directive to use lazy binding when specified through the environment or by means of dlopen(3C). See When Relocations are Performed.

DF_STATIC_TLS

Indicates that the object contains code using a static thread-local storage scheme. Static thread-local storage should not be used in objects that are dynamically loaded, either using dlopen(3C), or using lazy loading.

Table 13-10 ELF Dynamic Flags, DT_FLAGS_1

DF_1_NOW

Indicates that all relocations for this object must be processed before returning control to the program. The presence of this flag takes precedence over a directive to use lazy binding when specified through the environment or by means of dlopen(3C). See When Relocations are Performed.

DF_1_GROUP

Indicates that the object is a member of a group. This flag is recorded in the object using the link-editor's -B group option. See Object Hierarchies.

DF_1_NODELETE

Indicates that the object cannot be deleted from a process. If the object is loaded in a process, either directly or as a dependency, with dlopen(3C), the object cannot be unloaded with dlclose(3C). This flag is recorded in the object using the link-editor -z nodelete option.

DF_1_LOADFLTR

Meaningful only for filters. Indicates that all associated filtees be processed immediately. This flag is recorded in the object using the link-editor's -z loadfltr option. See Filtee Processing.

DF_1_INITFIRST

Indicates that this object's initialization section be run before any other objects loaded. This flag is intended for specialized system libraries only, and is recorded in the object using the link-editor's -z initfirst option.

DF_1_NOOPEN

Indicates that the object cannot be added to a running process with dlopen(3C). This flag is recorded in the object using the link-editor's -z nodlopen option.

DF_1_ORIGIN

Indicates that the object requires $ORIGIN processing. See Locating Associated Dependencies.

DF_1_DIRECT

Indicates that the object should use direct binding information. See Chapter 6, Direct Bindings.

DF_1_INTERPOSE

Indicates that the objects symbol table is to interpose before all symbols except the primary load object, which is typically the executable. This flag is recorded with the link-editor's -z interpose option. See Runtime Interposition.

DF_1_NODEFLIB

Indicates that the search for dependencies of this object ignores any default library search paths. This flag is recorded in the object using the link-editor's -z nodefaultlib option. See Directories Searched by the Runtime Linker.

DF_1_NODUMP

Indicates that this object is not dumped by dldump(3C). Candidates for this option include objects with no relocations that might get included when generating alternative objects using crle(1). This flag is recorded in the object using the link-editor's -z nodump option.

DF_1_CONFALT

Identifies this object as a configuration alternative object generated by crle(1). This flag triggers the runtime linker to search for a configuration file $ORIGIN/ld.config.app-name.

DF_1_ENDFILTEE

Meaningful only for filtees. Terminates a filters search for any further filtees. This flag is recorded in the object using the link-editor's -z endfiltee option. See Reducing Filtee Searches.

DF_1_DISPRELDNE

Indicates that this object has displacement relocations applied. The displacement relocation records no longer exist within the object as the records were discarded once the relocation was applied. See Displacement Relocations.

DF_1_DISPRELPND

Indicates that this object has displacement relocations pending. The displacement relocations exits within the object so the relocation can be completed at runtime. See Displacement Relocations.

DF_1_NODIRECT

Indicates that this object contains symbols that can not be directly bound to. See SYMBOL_SCOPE / SYMBOL_VERSION Directives.

DF_1_IGNMULDEF

Reserved for internal use by the kernel runtime-linker.

DF_1_NOKSYMS

Reserved for internal use by the kernel runtime-linker.

DF_1_NOHDR

Reserved for internal use by the kernel runtime-linker.

DF_1_EDITED

Indicates that this object has been edited or has been modified since the objects original construction by the link-editor. This flag serves as a warning to debuggers that an object might have had an arbitrary change made since the object was originally built.

DF_1_NORELOC

Reserved for internal use by the kernel runtime-linker.

DF_1_SYMINTPOSE

Indicates that the object contains individual symbols that should interpose before all symbols except the primary load object, which is typically the executable. This flag is recorded when the object is built using a mapfile and the INTERPOSE keyword. See SYMBOL_SCOPE / SYMBOL_VERSION Directives.

DF_1_GLOBAUDIT

Indicates that the dynamic executable requires global auditing. See Recording Global Auditors.

DF_1_SINGLETON

Indicates that the object defines, or makes reference to singleton symbols. See SYMBOL_SCOPE / SYMBOL_VERSION Directives.

Table 13-11 ELF Dynamic Position Flags, DT_POSFLAG_1

DF_P1_LAZYLOAD

Identifies the following DT_NEEDED entry as an object to be lazy loaded. This flag is recorded in the object using the link-editor's -z lazyload option. See Lazy Loading of Dynamic Dependencies.

DF_P1_GROUPPERM

Identifies the following DT_NEEDED entry as an object to be loaded as a group. This flag is recorded in the object using the link-editor's -z groupperm option. See Isolating a Group.

Table 13-12 ELF ASLR Values, DT_SUNW_ASLR

DV_SUNW_ASLR_DISABLE and DV_SUNW_ASLR_ENABLE are recoded in the object using the link-editor's -z aslr option.