[LLVMdev] [lld] Verifying the Architecture of files read

[Nick Kledzik]

On Oct 4, 2013, at 8:50 PM, Shankar Easwaran <shankare at codeaurora.org> wrote:
> It is needed that lld verifies the input to the linker.
> 
> For example : a x86 ELF file can be given to lld when the target is x86_64. Similiarly with other flavors.
> 
> I was thinking to have a varargs function in the LinkingContext that would be overridden by each of the LinkingContexts to verify files after being read.
> 
> The reader would call the varargs function in the LinkingContext and raise an error if the input is not suitable with the current link mode.

Yes.  We need a way to error out if there is an architecture mismatch.  But there are some interesting scenarios we need to support.

* If linking with a static library, you may not know until you actually need to load one of the members if the architecture is wrong, and it may not be an error if the architecture is wrong, but nothing is loaded.

* It might be a warning instead of an error to link against a shared library of the wrong architecture.  That is, the linker may need to ignore (and warn) but continue and try to complete the link without it.

* The mach-o linker also allows you to not specify the architecture on the command line.  Instead the linker infers the architecture by looking at the first object file.  This is mostly used in -r mode.  So, where the check is done to see that the arch is correct, may actually cause the architecture in the LinkingContext to be set.

* mach-o also has “fat” files which can contain multiple architectures.  So, the reader needs to know the arch to even try to parse.  In other words, if the Reader is told the intended arch, the Reader could error out if the file is not of that arch (and for mach-o the Reader would select the right slice in a fat file).

-Nick