Reducing Symbol Scope
Symbol definitions that are defined to have local scope within a mapfile can be used to reduce the symbol's eventual binding. This mechanism removes the symbol's visibility to future link-edits which use the generated file as part of their input. In fact, this mechanism can provide for the precise definition of a file's interface, and so restrict the functionality made available to others.
For example, say you want to generate a simple shared object from the files foo.c and bar.c. The file foo.c contains the global symbol foo, which provides the service that you want to make available to others. The file bar.c contains the symbols bar and str, which provide the underlying implementation of the shared object. A shared object created with these files, typically results in the creation of three symbols with global scope.
$ cat foo.c
extern const char *bar();
const char *foo()
{
return (bar());
}
$ cat bar.c
const char *str = "returned from bar.c";
const char *bar()
{
return (str);
}
$ cc -o libfoo.so.1 -G foo.c bar.c
$ elfdump -sN.symtab libfoo.so.1 | egrep 'foo$|bar$|str$'
[41] 0x00000560 0x00000018 FUNC GLOB D 0 .text bar
[44] 0x00000520 0x0000002c FUNC GLOB D 0 .text foo
[45] 0x000106b8 0x00000004 OBJT GLOB D 0 .data str
|
You can now use the functionality offered by libfoo.so.1 as part of the link-edit of another application. References to the symbol foo are bound to the implementation provided by the shared object.
Because of their global binding, direct reference to the symbols bar and str is also possible. This visibility can have dangerous consequences, as you might later change the implementation that underlies the function foo. In so doing, you could unintentionally cause an existing application that had bound to bar or str to fail or misbehave.
Another consequence of the global binding of the symbols bar and str is that these symbols can be interposed upon by symbols of the same name. The interposition of symbols within shared objects is covered in section Simple Resolutions. This interposition can be intentional and be used as a means of circumventing the intended functionality offered by the shared object. On the other hand, this interposition can be unintentional, the result of the same common symbol name used for both the application and the shared object.
When developing the shared object, you can protect against these scenarios by reducing the scope of the symbols bar and str to a local binding. In the following example, the symbols bar and str are no longer available as part of the shared object's interface. Thus, these symbols cannot be referenced, or interposed upon, by an external object. You have effectively defined an interface for the shared object. This interface can be managed while hiding the details of the underlying implementation.
$ cat mapfile
$mapfile_version 2
SYMBOL_SCOPE {
local:
bar;
str;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ elfdump -sN.symtab libfoo.so.1 | egrep 'foo$|bar$|str$'
[24] 0x00000548 0x00000018 FUNC LOCL H 0 .text bar
[25] 0x000106a0 0x00000004 OBJT LOCL H 0 .data str
[45] 0x00000508 0x0000002c FUNC GLOB D 0 .text foo
|
This symbol scope reduction has an additional performance advantage. The symbolic relocations against the symbols bar and str that would have been necessary at runtime are now reduced to relative relocations. See When Relocations are Performed for details of symbolic relocation overhead.
As the number of symbols that are processed during a link-edit increases, defining local scope reduction within a mapfile becomes harder to maintain. An alternative and more flexible mechanism enables you to define the shared object's interface in terms of the global symbols that should be maintained. Global symbol definitions allow the link-editor to reduce all other symbols to local binding. This mechanism is achieved using the special auto-reduction directive “*”. For example, the previous mapfile definition can be rewritten to define foo as the only global symbol required in the output file generated.
$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION ISV_1.1 {
global:
foo;
local:
*;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ elfdump -sN.symtab libfoo.so.1 | egrep 'foo$|bar$|str$'
[26] 0x00000570 0x00000018 FUNC LOCL H 0 .text bar
[27] 0x000106d8 0x00000004 OBJT LOCL H 0 .data str
[50] 0x00000530 0x0000002c FUNC GLOB D 0 .text foo
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This example also defines a version name, libfoo.so.1.1, as part of the mapfile directive. This version name establishes an internal version definition that defines the file's symbolic interface. The creation of a version definition is recommended. The definition forms the foundation of an internal versioning mechanism that can be used throughout the evolution of the file. See Chapter 5, Application Binary Interfaces and Versioning.
Note –
If a version name is not supplied, the output file name is used to label the version definition. The versioning information that is created within the output file can be suppressed using the link-editor's -z noversion option.
Whenever a version name is specified, all global symbols must be assigned to a version definition. If any global symbols remain unassigned to a version definition, the link-editor generates a fatal error condition.
$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION ISV_1.1 {
global:
foo;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
Undefined first referenced
symbol in file
str bar.o (symbol has no version assigned)
bar bar.o (symbol has no version assigned)
ld: fatal: Symbol referencing errors. No output written to libfoo.so.1
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The -B local option can be used to assert the auto-reduction directive “*” from the command line. The previous example an be compiled successfully as follows.
$ cc -o libfoo.so.1 -M mapfile -B local -G foo.c bar.c |
When generating an executable or shared object, any symbol reduction results in the recording of version definitions within the output image. When generating a relocatable object, the version definitions are created but the symbol reductions are not processed. The result is that the symbol entries for any symbol reductions still remain global. For example, using the previous mapfile with the auto-reduction directive and associated relocatable objects, an intermediate relocatable object is created with no symbol reduction.
$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION ISV_1.1 {
global:
foo;
local:
*;
};
$ ld -o libfoo.o -M mapfile -r foo.o bar.o
$ elfdump -s libfoo.o | egrep 'foo$|bar$|str$'
[28] 0x00000050 0x00000018 FUNC GLOB H 0 .text bar
[29] 0x00000010 0x0000002c FUNC GLOB D 2 .text foo
[30] 0x00000000 0x00000004 OBJT GLOB H 0 .data str
|
The version definitions created within this image show that symbol reductions are required. When the relocatable object is used eventually to generate an executable or shared object, the symbol reductions occur. In other words, the link-editor reads and interprets symbol reduction information that is contained in the relocatable objects in the same manner as versioning data is processed from a mapfile.
Thus, the intermediate relocatable object produced in the previous example can now be used to generate a shared object.
$ ld -o libfoo.so.1 -G libfoo.o
$ elfdump -sN.symtab libfoo.so.1 | egrep 'foo$|bar$|str$'
[24] 0x00000508 0x00000018 FUNC LOCL H 0 .text bar
[25] 0x00010644 0x00000004 OBJT LOCL H 0 .data str
[42] 0x000004c8 0x0000002c FUNC GLOB D 0 .text foo
|
Symbol reduction at the point at which an executable or shared object is created is typically the most common requirement. However, symbol reductions can be forced to occur when creating a relocatable object by using the link-editor's -B reduce option.
$ ld -o libfoo.o -M mapfile -B reduce -r foo.o bar.o
$ elfdump -sN.symtab libfoo.o | egrep 'foo$|bar$|str$'
[20] 0x00000050 0x00000018 FUNC LOCL H 0 .text bar
[21] 0x00000000 0x00000004 OBJT LOCL H 0 .data str
[30] 0x00000010 0x0000002c FUNC GLOB D 2 .text foo
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Symbol Elimination
An extension to symbol reduction is the elimination of a symbol entry from an object's symbol table. Local symbols are only maintained in an object's .symtab symbol table. This entire table can be removed from the object by using the link-editor's -s option, or strip(1). On occasion, you might want to maintain the .symtab symbol table but remove selected local symbol definitions.
Symbol elimination can be carried out using the mapfile keyword ELIMINATE. As with the local directive, symbols can be individually defined, or the symbol name can be defined as the special auto-elimination directive “*”. The following example shows the elimination of the symbol bar for the previous symbol reduction example.
$ cat mapfile
$mapfile_version 2
SYMBOL_VERSION ISV_1.1 {
global:
foo;
local:
str;
eliminate:
*;
};
$ cc -o libfoo.so.1 -M mapfile -G foo.c bar.c
$ elfdump -sN.symtab libfoo.so.1 | egrep 'foo$|bar$|str$'
[26] 0x00010690 0x00000004 OBJT LOCL H 0 .data str
[44] 0x000004e8 0x0000002c FUNC GLOB D 0 .text foo
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The -B eliminate option can be used to assert the auto-elimination directive “*” from the command line.
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