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Full Discussion: Editing or Adding to ELF string tables
Top Forums Programming Editing or Adding to ELF string tables Post 302255963 by otheus on Friday 7th of November 2008 12:56:01 PM
Old 11-07-2008
Lightbulb Using ld to modify an executable
You can use the GNU linker (ld) to create a new executable that's identical as the old one, but additionally contains the section from a new file. Well, in theory. So far I haven't gotten it to work as hoped. Attached are the files you can use for playing around.

First, you take a C source file that contains only static strings:
Code:
static char* __version = "sample 1.0";
static char* __author = "otheus@gmail.com";

Second, you compile it as usual with a C compiler to an object file.
Code:
gcc -o version_tmp.o -c version.c

Third, create a ld script file that will be used by ldto create an object file containing the static strings in your source, but in a ".comment" section.
Code:
SECTIONS
{
  .comment       0 : { *(.rodata) }
  /DISCARD/        : { *(.text .bss .data .comment .note.GNU-stack) }
}

Fourth, use the linker with this special ld script file ("commentsonly.lds") on your object file (created above with the gcc command). Now, instead of the strings being in the ".rodata" section, they are stored in the ".comment" section. Use the -r option so an object file is created.
Code:
ld -r -T commentsonly.lds -o version.o version_tmp.o

Finally, relink the original program with the new object file:
Code:
ld -o new_executable original_executable version.o

Now you can confirm the data is stored in this file using readelf or objdump:
Code:
objdump -s -j .comment new_executable

Here you should see the data you stored in your .c program.

So what happens. When I tried it, I got a segmentation fault. I suspect you have to do some address juggling to get this to work right. Let me know if you get anywhere?
 

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LEARN ABOUT OPENDARWIN

mcs

mcs(1)								   User Commands							    mcs(1)

NAME

       mcs - manipulate the comment section of an object file

SYNOPSIS

       mcs [-cdpVz] [-a string] [-n name] file...

DESCRIPTION

       The  mcs  command  is  used  to manipulate a section, by default the .comment section, in an ELF object file. It is used to add to, delete,
       print, and compress the contents of a section in an ELF object file, and print only the contents of a section in a COFF	object	file.  mcs
       cannot add, delete, or compress the contents of a section that is contained within a segment.

       If  the	input  file is an archive (see ar.h(3HEAD)), the archive is treated as a set of individual files. For example, if the -a option is
       specified, the string is appended to the comment section of each ELF object file in the archive; if the archive member is not an ELF object
       file, then it is left unchanged.

       mcs must be given one or more of the options described below. It applies, in order, each of the specified options to each file.

OPTIONS

       The following options are supported:

       -a string       Appends	string	to the comment section of the ELF object files. If string contains embedded blanks, it must be enclosed in
		       quotation marks.

       -c	       Compresses the contents of the comment section of the ELF object files. All duplicate entries are removed. The ordering	of
		       the remaining entries is not disturbed.

       -d	       Deletes	the  contents of the comment section from the ELF object files. The section header for the comment section is also
		       removed.

       -n name	       Specifies the name of the comment section to access if other than .comment. By default, mcs deals with  the  section  named
		       .comment.  This	option can be used to specify another section. mcs can take multiple -n options to allow for specification
		       of  multiple section comments.

       -p	       Prints the contents of the comment section on the standard output. Each section printed is tagged by the name of  the  file
		       from which it was extracted, using the format file[member_name]: for archive files and file: for other files.

       -V	       Prints on standard error the version number of mcs.

       -z	       Replaces any SHT_PROGBITS sections with zeros while retaining the original attributes of the sections.

EXAMPLES

       Example 1: Printing a file's comment section

       The following entry

       example% mcs -p elf.file

       prints the comment section of the file elf.file.

       Example 2: Appending a string to a comment section

       The following entry

       example% mcs -a	xyz elf.file

       appends string xyz to elf.file's comment section.

FILES

       /tmp/mcs*       temporary files

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWbtool 		   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Stable			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       ar(1), as(1), ld(1), ar.h(3HEAD), elf(3ELF), tmpnam(3C), a.out(4), attributes(5)

NOTES

       When  mcs  deletes  a section using the -d option, it tries to bind together sections of type SHT_REL and target sections pointed to by the
       sh_info section header field. If one is to be deleted, mcs attempts to delete the other of the pair.

       By using the -z option, it is possible to make an object file by removing the contents of SHT_PROGBITS sections while retaining the  object
       file's  original  structure  as	an  ELF  file. The need for use of the -z option is limited. However, the option can be used to deliver an
       object file when the contents of SHT_PROGBITS sections are not relevant.

SunOS 5.10							    1 Apr 2004								    mcs(1)
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