Converting Object Capabilities to Symbol Capabilities

Ideally, the compiler can generate objects that are identified with symbol capabilities. If the compiler can not create symbol capabilities, the link-editor offers a solution.

A relocatable object that defines object capabilities can be transformed into a relocatable object that defines symbol capabilities using the link-editor. Using the link-editor -z symbolcap option, any capability data section is converted to define symbol capabilities. All global functions within the object are converted into local functions, and are associated with symbol capabilities. All global initialized data items are converted to local data items, and are associated with symbol capabilities. These transformed symbols are appended with any capability identifier specified as part of the object capabilities group. If a capability identifier is not defined, a default group name is appended.

For each original global function or initialized data item, a global reference is created. This reference is associated to any relocation requirements, and provides for binding to a default, global symbol when this object is finally combined to create a dynamic executable or shared object.

The -z symbolcap option applies to objects that contain an object capabilities section. The option has no affect upon relocatable objects that already contain symbol capabilities, or relocatable objects that contain both object and symbol capabilities. This design allows multiple objects to be combined by the link-editor, with only those objects that contain object capabilities being affected by the option.

In the following example, a x86 relocatable object contains two global functions foo() and bar(). This object has been compiled to require the MMX and SSE hardware capabilities. In these examples, the capabilities group has been named with a capabilities identifier entry. This identifier name is appended to the transformed symbol names. Without this explicit identifier, the link-editor appends a default capabilities group name.

$ elfdump -H foo.o

Capabilities Section:  .SUNW_cap

 Object Capabilities:
     index  tag               value
       [0]  CA_SUNW_ID        sse,mmx
       [1]  CA_SUNW_HW_1      0x840  [ SSE MMX ]

$ elfdump -s foo.o | egrep "foo|bar"
      [25]  0x00000000 0x00000021  FUNC GLOB  D    0 .text    foo
      [26]  0x00000024 0x0000001e  FUNC GLOB  D    0 .text    bar

$ elfdump -r foo.o | fgrep foo
  R_386_PLT32                0x38       .rel.text             foo

This relocatable object can now be transformed into a symbols capabilities relocatable object.

$ ld -r -o foo.1.o -z symbolcap foo.o
$ elfdump -H foo.1.o

Capabilities Section:  .SUNW_cap

 Symbol Capabilities:
     index  tag               value
       [1]  CA_SUNW_ID        sse,mmx
       [2]  CA_SUNW_HW_1      0x840  [ SSE MMX ]

  Symbols:
     index    value      size      type bind oth ver shndx    name
      [25]  0x00000000 0x00000021  FUNC LOCL  D    0 .text    foo%sse,mmx
      [26]  0x00000024 0x0000001e  FUNC LOCL  D    0 .text    bar%sse,mmx

$ elfdump -s foo.1.o | egrep "foo|bar"
      [25]  0x00000000 0x00000021  FUNC LOCL  D    0 .text    foo%sse,mmx
      [26]  0x00000024 0x0000001e  FUNC LOCL  D    0 .text    bar%sse,mmx
      [37]  0x00000000 0x00000000  FUNC GLOB  D    0 UNDEF    foo
      [38]  0x00000000 0x00000000  FUNC GLOB  D    0 UNDEF    bar

$ elfdump -r foo.1.o | fgrep foo
  R_386_PLT32                0x38       .rel.text             foo

This object can now be combined with other objects containing instances of the same functions, associated with different symbol capabilities, to produce an executable or shared object. In addition, a default instance of each function, one that is not associated with any symbol capabilities, should be provided to lead each capabilities family. This default instance provides for all external references, and ensures that an instance of the function is available on any system.

At runtime, any references to foo() and bar() are directed to the lead instances. However, the runtime linker selects the best symbol capabilities instance if the system accommodates the appropriate capabilities.