Re: [Elftoolchain-developers] A safe way to re-build a string table ..

[Jay V. <jay...@th...>]

Hi Kai - thanks for responding ..

> First of all I think you are posting to the wrong mailling list...
>

Ah, I perhaps should be on elfutils-developers?  I don't mind either way, I just need some
ELF expert(s) who are willing to talk about this stuff .. ;)  Been winging it on my own on
this development lately, so its just good to know people who know ELF.  Please feel free
to tell me to sod off if this is not topical for this list, though. :)

> Second, for your problem: If this .note.MD5 (for example) section does
> not need to be presented to the runtime loader or used during runtime,
> you don't need to roll your own tool and you can just use objcopy(1)
> to handle it:

It does need to be readable during runtime and the loader in our internal system is going
to be expected to use this information along with other online services and dynamic loader
paths, so I'd really like to not use objcopy if it means it will produce a more 'standard
ELF Note' to write an app to do it.

> 
> 1. Calculate the checksum or other stuff of the ELF, construct the Elf Notes
>    and then write all the bits to a temporary file, say '/tmp/foo'.
> 2. Execute objcopy to add a .note.MD5 section to the ELF file:
>    objcopy --add-section '.note.MD5=/tmp/foo' your_executable
>   (objcopy will handle all the string table, section table headers stuff etc)

One thing this method doesn't do is properly update the name/namesz and type fields, I
have found (correct me if I'm wrong) - so for my purposes it appears that the objcopy
technique will not produce an *entirely* valid (to-spec) NOTE record.  What I am doing is
using a struct like this in my elfnoter application:

#define INTW1X_ELF_SECTION      ".note.MD5"
#define ELF_TEXT_NAME           ".text"

#define INTW1X_MD5_NOTE_NAME    "w1x_md5"
#define INTW1X_MD5_TYPE         1

#define MD5_DESC "deadbeefd00df00ddeadbeefd00df00d"
#define MD5_BUF_LEN 33

const struct intw1x_elf_note_md5 local_elf_note_md5
     __attribute__ ((section(INTW1X_ELF_SECTION))) =
{
     .elf_note = {
     	.n_namesz = sizeof(INTW1X_MD5_NOTE_NAME),
	.n_descsz = MD5_BUF_LEN,
	.n_type = INTW1X_MD5_TYPE
     },
     .name = INTW1X_MD5_NOTE_NAME,
     .desc = MD5_DESC
};

.. then, locating this ELF record internally within my elfnoter app programmatically, and
'injecting' it into the target executable after the md5 string is updated, so that I
produce an entirely valid (all fields correct) record in the destination binary.  At least
the thinking is that inlining the definition of this record means that the final object is
going to be as standard as gcc is going to make it and thus all I have to do is modify the
string and glom the object onto my target ELF binary.  This is working, its just a little
delicate when it comes to updating the string table, alas.

If there is a way to get objcopy to do this work, I'd love to know .. perhaps it would be
worthwhile investigating this further and adding code to objcopy for NOTE objects, but
then again I don't know enough about this to say for sure and I appreciate your feedback.

-- 
;
--
Jay Vaughan
jay...@in...