elffile: ELF Specific File Identification Utility
By Ali Bahrami on Nov 11, 2011
The impetus to add elffile to Solaris came from the effort to extend the format of Solaris archives so that they could grow beyond their previous 32-bit file limits. That work introduced a new archive symbol table format. Now that there was more than one possible format, I thought it would be useful if the file utility could identify which format a given archive is using, leading me to extend the file utility:
In turn, this caused me to think about all the things that I would like the file utility to be able to tell me about an archive. In particular, I'd like to be able to know what's inside without having to unpack it. The end result of that train of thought was elffile.% cc -c ~/hello.c % ar r foo.a hello.o % file foo.a foo.a: current ar archive, 32-bit symbol table % ar r -S foo.a hello.o % file foo.a foo.a: current ar archive, 64-bit symbol table
Much of the discussion in this article is adapted from the PSARC case I filed for elffile in December 2010:
PSARC 2010/432 elffile
Why file Is No Good For Archives And Yet Should Not Be FixedThe standard /usr/bin/file utility is not very useful when applied to archives. When identifying an archive, a user typically wants to know 2 things:
- Is this an archive?
- Presupposing that the archive contains objects, which is by far the most common use for archives, what platform are the objects for? Are they for sparc or x86? 32 or 64-bit? Some confusing combination from varying platforms?
The file utility provides a quick answer to question (1), as it identifies all archives as "current ar archive". It does nothing to answer the more interesting question (2). To answer that question, requires a multi-step process:
- Extract all archive members
- Use the file utility on the extracted files, examine the output for each file in turn, and compare the results to generate a suitable summary description.
- Remove the extracted files
It should be easier and more efficient to answer such an obvious question.
It would be reasonable to extend the file utility to examine archive contents in place and produce a description. However, there are several reasons why I decided not to do so:
- The correct design for this feature within the file utility would have file examine each archive member in turn, applying its full abilities to each member. This would be elegant, but also represents a rather dramatic redesign and re-implementation of file. Archives nearly always contain nothing but ELF objects for a single platform, so such generality in the file utility would be of little practical benefit.
- It is best to avoid adding new options to standard utilities for which other implementations of interest exist. In the case of the file utility, one concern is that we might add an option which later appears in the GNU version of file with a different and incompatible meaning. Indeed, there have been discussions about replacing the Solaris file with the GNU version in the past. This may or may not be desirable, and may or may not ever happen. Either way, I don't want to preclude it.
- Examining archive members is an O(n) operation, and can be relatively slow with large archives. The file utility is supposed to be a very fast operation.
I decided that extending file in this way is overkill, and that an investment in the file utility for better archive support would not be worth the cost. A solution that is more narrowly focused on ELF and other linker related files is really all that we need. The necessary code for doing this already exists within libelf. All that is missing is a small user-level wrapper to make that functionality available at the command line.
In that vein, I considered adding an option for this to the elfdump utility. I examined elfdump carefully, and even wrote a prototype implementation. The added code is small and simple, but the conceptual fit with the rest of elfdump is poor. The result complicates elfdump syntax and documentation, definite signs that this functionality does not belong there.
And so, I added this functionality as a new user level command.
The elffile CommandThe syntax for this new command is
elffile [-s basic | detail | summary] filename...
Please see the elffile(1) manpage for additional details.
To demonstrate how output from elffile looks, I will use the following files:
The file utility identifies these files as follows:
File Description config A runtime linker configuration file produced with crle dwarf.o An ELF object /etc/passwd A text file mixed.a Archive containing a mixture of ELF and non-ELF members mixed_elf.a Archive containing ELF objects for different machines not_elf.a Archive containing no ELF objects same_elf.a Archive containing a collection of ELF objects for the same machine. This is the most common type of archive.
By default, elffile uses its "summary" output style. This output differs from the output from the file utility in 2 significant ways:% file config dwarf.o /etc/passwd mixed.a mixed_elf.a not_elf.a same_elf.a config: Runtime Linking Configuration 64-bit MSB SPARCV9 dwarf.o: ELF 64-bit LSB relocatable AMD64 Version 1 /etc/passwd: ascii text mixed.a: current ar archive, 32-bit symbol table mixed_elf.a: current ar archive, 32-bit symbol table not_elf.a: current ar archive same_elf.a: current ar archive, 32-bit symbol table
- Files that are not an ELF object, archive, or runtime linker configuration file are identified as "non-ELF", whereas the file utility attempts further identification for such files.
- When applied to an archive, the elffile output includes a description of the archive's contents, without requiring member extraction or other additional steps.
The output for same_elf.a is of particular interest: The vast majority of archives contain only ELF objects for a single platform, and in this case, the default output from elffile answers both of the questions about archives posed at the beginning of this discussion, in a single efficient step. This makes elffile considerably more useful than file, within the realm of linker-related files.% elffile config dwarf.o /etc/passwd mixed.a mixed_elf.a not_elf.a same_elf.a config: Runtime Linking Configuration 64-bit MSB SPARCV9 dwarf.o: ELF 64-bit LSB relocatable AMD64 Version 1 /etc/passwd: non-ELF mixed.a: current ar archive, 32-bit symbol table, mixed ELF and non-ELF content mixed_elf.a: current ar archive, 32-bit symbol table, mixed ELF content not_elf.a: current ar archive, non-ELF content same_elf.a: current ar archive, 32-bit symbol table, ELF 64-bit LSB relocatable AMD64 Version 1
elffile can produce output in two other styles, "basic", and "detail". The basic style produces output that is the same as that from 'file', for linker-related files. The detail style produces per-member identification of archive contents. This can be useful when the archive contents are not homogeneous ELF object, and more information is desired than the summary output provides:
% elffile -s detail mixed.a mixed.a: current ar archive, 32-bit symbol table mixed.a(dwarf.o): ELF 32-bit LSB relocatable 80386 Version 1 mixed.a(main.c): non-ELF content mixed.a(main.o): ELF 64-bit LSB relocatable AMD64 Version 1 [SSE]