This is elftin, a possibly-growing selection of scripts/tools which do 
wonderful and strange things with ELF binaries.

Currently there are the following scripts/tools:

- custom-phdrs: lets you relink a binary to include a custom ELF program 
header, with whatever content you like. You could read my blog post.
<https://humprog.org/~stephen/blog/2017/02/14#custom-elf-phdrs>

- embed-file: extends custom-phdrs so that the added program header is 
accompanied by a binary blob that is inserted into the file, with the 
program header marking its extent

- embed-loadable: extends embed-file to allow for the embedded binary 
blob to be itself an ELF file -- whose offset fields have been fixed up 
(by a utility program, included) such that they are relative to the 
loaded file. This allows the embedded file to be loaded from a file 
descriptor ponting at the beginning of the blob (its ELF header) within 
the larger file, even though it is not necessarily at the beginning of 
that file. This only works if you have a ld.so or libdl that exposes a 
"dlopen from file descriptor". Glibc's ld.so doesn't do that; ask me 
<srk31@srcf.ucam.org> how to make a hacked version that does. I believe 
FreeBSD's (and others') do expose this.

- normrelocs: update an ELF file, in place, to prefer non-section 
symbols in any relocations that reference the symbol 'externally' (e.g. 
to call it) rather than internally (e.g. to reference a point within its 
code, e.g. from debugging information). This is sometimes useful after 
globalizing a symbol using objcopy, since the assembler may have chosen 
section-relative relocs against local symbols.

- abs2und, sym2und: update an ELF file, in place, turning some symbols
into UND symbols. This is part of a bigger recipe for symbol wrapping,
which is documented on my blog.
<https://www.humprog.org/%7Estephen/blog/2022/08/03#elf-symbol-wrapping-via-replacement>

- sym2dyn: following use of 'objcopy' that updates the (static) symbol
table, propagate changes to the dynamic symbol table. This requires
librunt headers at build time. Currently it is very limited, e.g. cannot
add new strings to dynstr (so renames must use a subsequence of an
existing symbol name), cannot regenerate GNU symbol hash table (so input
binaries must use only the SysV table).

- xwrap-ldplugin: a linker plugin for the GNU bfd/gold linkers, doing
extended wrapping ('xwrap'), overcoming some of the problems with the
standard ld --wrap feature. The core technique is documented under the
'sym2und' link above, and the plugin specifically is documented in a
separate blog post.
<https://www.humprog.org/%7Estephen/blog/2022/10/06#elf-symbol-wrapping-plugin>

- base-ldplugin: utility code used by xwrap-ldplugin, but usable by
other GNU linker plugins. It includes features for enumerating input
files and (by a hacky self-restart mechanism) modifying the linker
command line.