On 16 Dec 2013 16:37, "Michael Hudson-Doyle" michael.hudson@linaro.org wrote:
Michael Hudson-Doyle michael.hudson@linaro.org writes:
Aaah, you might be onto something there. I built myself a cross gcc-4.8 today and it appeared to compile things correctly (I didn't actually get to run it, but the objdump poking looked right) and I got a bit worried that this was all down to some cosmic ray / corruption when I first compiled it. But, the scripts I cargo culted just use compile binutils from git tip, so if the bug is in binutils...
So I still don't know what's going on, exactly, but I have a debug build of binutils now and some clues. It still only happens on real hardware, not cross compiling on my laptop, but I think I have an idea as to why. This might be complete crack, but anyway.
I think it's to do with the order of things within the GOT.
When I cross compile, sort the relocations by address, then count up the number of relocations of each type, it looks like this:
$ objdump -C -R build/linux2/*/mongo/base/counter_test | LC_ALL=C sort |
cut -d' ' -f 2 | uniq -c
4 496 R_AARCH64_GLOB_DAT 1 R_AARCH64_TLS_TPREL64 103 R_AARCH64_GLOB_DAT 305 R_AARCH64_JUMP_SLOT 12 R_AARCH64_COPY 1 RELOCATION 2In this case, the code and the relocation agree on where the thread local variable is.
When I compile natively, it looks like this:
(t-mwhudson)ubuntu@arm64:~/src/mongo$ objdump -C -R
build/linux2/*/mongo/base/counter_test | LC_ALL=C sort | cut -d' ' -f 2 | uniq -c
4 295 R_AARCH64_JUMP_SLOT 496 R_AARCH64_GLOB_DAT 1 R_AARCH64_TLS_TPREL64 104 R_AARCH64_GLOB_DAT 12 R_AARCH64_COPY 1 RELOCATION 2And the code and the relocation disagree on where the thread local variable is -- by 298 * sizeof(void*). Which is almost (but I admit, not exactly) the number of JUMP_SLOTs that are, in this case, before the TLS variable in the GOT. When I compiled in a different way, there were only 160 JUMP_SLOTs before the TLS reloc, and the code and relocation disagreed by 163 slots.
So is it possible somehow that the GOT has these JUMP_SLOTs inserted into it after the relocation for the TLS has been written out?
I guess I really mean "between patching the code and writing the relocs out" here.
I don't really see how but maybe this rings a bell...
Cheers, mwh
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