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

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

Hi all,

I am building an application which utilizes elfutils to add NOTE records to ELF files .. 
the purpose of this application is to provide a form of 'internal inventory control' over 
executables being used internally in our environment, and for that purpose a NOTE record 
is added to ELF files containing an MD5 sum of various parts of the ELF files themselves.

All has been working fine with this utility program so far, but I have run into some 
problems with the handling of the string table - namely, where I was once realloc()'ing 
the string table data buffer and appending my new ".note.MD5" string to the list, and this 
isn't a very safe way of doing things - segfaults galore.  Its not stable, alas, which is 
why I ended up implementing things this way only to discover on larger string tables that 
there was a problem, then tracing it back (valgrind to the rescue) to the fact that libelf 
doesn't like it when you realloc() its jiggly bits^W^Wstruct fields ..

So my next approach has been to, as I'm iterating through the original ELF file, make a 
new string table out of the headers for each section, and upon completion (calculation of 
the to-be-inserted md5 string, construction of a proper NOTE record, attachment of the 
NOTE to the ELF file, etc.) I then burn the old string table and attach the new one.

However, I'm really not sure if this is the safe way to do things, or if there is any 
other way that I could utilize - its a bit 'dodgy' to me at first blush, because what if 
there are cases out there in our ELF collection where I somehow mess up the string table, 
or what about the unspoken cases where a string table is expected to be something that 
libebl doesn't reproduce - I've already noted that some string tables created through 
libebl don't exactly 100% match the original ones (such as the ones in my redhat machines 
libc) due to internal optimization of the string table itself (finding overlaps and not 
inserting duplicate string values and so on..)

My question to the wise developers on this list is whether or not there is another way to 
safely add an entry to an existing string table without doing too much 
create-new/destroy-old processing?  I've added the use of libebl to my project, and I'm 
under way with completing my implementation as described above, but I'd really like some 
dialog with other ELF hackers on what they think about this method?

BTW, I plan to release my "ELFNoter" app as GPL when its working/debugged .. I wonder if 
there is interest in including a working app to attach ELF Notes in the elfutils package? 
I'd be happy were that the case, to provide this app for review.

-- 
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Jay Vaughan
jay...@th...