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rebind is a small utility that allows one to modify the binding
strength and/or visibility of a global symbol in an ELF object file.
There are two different types of global symbol bindings. The first is
STB_GLOBAL. A global symbol can appear in multiple ELF files, but it
must be an undefined reference in all but one place. The second type
of binding is STB_WEAK. A weak symbol acts identically to a global
symbol only as long as there exists no global symbol with the same
name. If there is, however, then the weak symbol is ignored in favor
of the global symbol, and the condition is not treated as a name
collision error.
Note that rebind does not permit you to make local symbols global, or
vice versa. The dynamic symbol table requires that all local symbols
be listed before global symbols, so making these sort of changes
requires that the symbol table be reordered.
rebind also permits you to change a symbol's visibility. With the
default visibility, a global symbol is visible everywhere. A symbol
with hidden visibility, however, is only available to the module it
was built with -- in other words, the symbol can be accessed by the
static linker but not by the dynamic linker. An symbol with internal
visibility cannot be accessed from outside its module (not even via a
pointer). A symbol with protected visibility is not hidden, but
internally it cannot have its definition overridden by another module
containing a symbol with the same name.
The GNU C compiler permits the user to specify the binding and/or
visibility of a symbol via the __attribute__ extension. The GNU
assembler uses directives such as .weak, .hidden, et al, to accomplish
the same thing. Some other tools, however, offer no way to specify a
global symbol's binding or visibility, so this program can be seen as
providing an alternate method for doing this. This program can also be
used to directly see how changing a symbol's binding can affect its
behavior. And, hopefully, it also provides a template for writing
other specialized programs for making small, in-place modifications to
ELF files.
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