What about the linker?

Posted Jan 8, 2021 7:00 UTC (Fri) by eru (subscriber, #2753)
In reply to: Bootstrappable builds by dvdeug
Parent article: Bootstrappable builds

Should we not also take into account the "ld", "ar" and other bintools? I think the Thompson attack would also work with "ld" and any other tool that is used in the process of generating the final program.

So the bootstrap process should either start with a compiler that directly produces an executable, or also bootstrap the linker without depending on any existing linker.

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What about the linker?

Posted Jan 10, 2021 1:11 UTC (Sun) by JoeBuck (guest, #2330) [Link]

The traditional gcc bootstrapping process can work with all of the binutils tools, built together in the same tree (this was pioneered by the Cygnus folks maybe 30 years ago); everything is built again with the new compiler, linker, and assembler to eliminate dependencies. We can demonstrate that the classic attacks in the Thompson paper either don't exist, or have affected every C compiler since the dawn of the language, by starting with unrelated compilers, doing the bootstraps, maybe going through a number of compiler versions and even throwing in cross-compilers, involving a mix of free and proprietary compilers, and verifying that in the end the binaries are the same (for some systems, timestamps have to be filtered out of object files when doing the comparison, but for most, we wind up with every byte identical).

However, we still inherit dependencies from system libraries, and this can include macros and inline functions. Someone could perhaps sneak an attack into a system library function that needs to be coded in assembler for optimal performance, and have this wind up in the compiler. So efforts like these that start with a tiny compiler and a tiny library can eliminate that threat as well.

But I think efforts like this, while fascinating, are a lot less important than they used to be because the real threat these days is in the microcode, the system under the system.