It seems that LLVM's linker does not correctly handle variable assignments
involving section positions that are updated during the SECTIONS
parsing. Commit aa69fb62be ("arm64/efi: Mark __efistub_stext_offset as
an absolute symbol explicitly") ran into this too, but found a different
workaround.
However, this was not enough, as other variables were also miscalculated
which manifested as boot failures under UEFI where __efistub__end was
not taking the correct _end value (they should be the same):
$ ld.lld -EL -maarch64elf --no-undefined -X -shared \
-Bsymbolic -z notext -z norelro --no-apply-dynamic-relocs \
-o vmlinux.lld -T poc.lds --whole-archive vmlinux.o && \
readelf -Ws vmlinux.lld | egrep '\b(__efistub_|)_end\b'
368272: ffff000002218000 0 NOTYPE LOCAL HIDDEN 38 __efistub__end
368322: ffff000012318000 0 NOTYPE GLOBAL DEFAULT 38 _end
$ aarch64-linux-gnu-ld.bfd -EL -maarch64elf --no-undefined -X -shared \
-Bsymbolic -z notext -z norelro --no-apply-dynamic-relocs \
-o vmlinux.bfd -T poc.lds --whole-archive vmlinux.o && \
readelf -Ws vmlinux.bfd | egrep '\b(__efistub_|)_end\b'
338124: ffff000012318000 0 NOTYPE LOCAL DEFAULT ABS __efistub__end
383812: ffff000012318000 0 NOTYPE GLOBAL DEFAULT 15325 _end
To work around this, all of the __efistub_-prefixed variable assignments
need to be moved after the linker script's SECTIONS entry. As it turns
out, this also solves the problem fixed in commit aa69fb62be, so those
changes are reverted here.
Link: https://github.com/ClangBuiltLinux/linux/issues/634
Link: https://bugs.llvm.org/show_bug.cgi?id=42990
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Will Deacon <will@kernel.org>
Kees Cook