On Thu, 23 Nov 2017, Jason A. Donenfeld wrote:
On older versions of binutils, \sym points to an aligned address. On newer versions of binutils, \sym sometimes points to the unaligned thumb address in certain circumstances. In order to homogenize this behavior, rather than adding 1, we could simply OR in 1, so that already unaligned instructions don't change. While that works, the downside is that we have to add an `orr` instruction to a fast path. The assembler can't do this at assemble time via "|1" because "invalid operands (.text and *ABS* sections) for `|'". A better solution would be to have consistent binutils behavior, but that ship has sailed.
So, this commit adds a detection mechanism, which began as a small thing from Russell King that I then rewrote to use pure bash instead of shelling out, so that it doesn't slow down the build process. The detection mechanism _could_ be used to modify the assembly we generate, but for now it's just being used to catch buggy binutils and abort the build process in that case.
The rest of this commit message contains all of the relevant information about the boot bug when compiled in thumb2 mode.
My tests concerned these versions: broken: GNU ld (Gentoo 2.29.1 p3) 2.29.1 working: GNU ld (GNU Binutils for Ubuntu) 2.26.1
FWIW, this issue stems from this change: https://sourceware.org/bugzilla/show_bug.cgi?id=21458
The same issue caused problems in libavcodec as well, where we chose to work around the issue in this fashion: https://git.libav.org/?p=libav.git%3Ba=commitdiff%3Bh=9dde6ab06c48f9447cd16f...
Related debian bug report, with a different workaround: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=870622
(In libav, we chose the workaround since the .eqv one suggested in the debian bug report didn't really work well with assemblers for other platforms.)
// Martin