On 05/11/2017 06:51 AM, Ard Biesheuvel wrote:
[snip]
In my opinion, the correct fix would be to make -fpie (as opposed to
-fpic) imply hidden visibility, given that PIE executables don't
export symbols in the first place, and so the preemption rules do not
apply. It is worth a try whether -fpie works as expected in this case
on Clang, but the last time I tried it on GCC, it behaved exactly like
-fpic.
Thanks a lot for the detailed description and your suggestions!
A clang build with -fpie for the EFI stub succeeds without complaints
about GOT entries. I will send out an updated patch (with -fpie only
for clang) later.
Good! I never liked the visibility hack, which is why I never upstreamed it.
Could you please check how recent GCC behaves?
I tried GCC v4.9.4 and v6.3.1, both build the EFI stub with -fpie
without errors.
Are you suggesting to use -fpie for both clang and GCC? Do you know
what the minimum required GCC version is for building an arm64 kernel?
Yes. Up until now, we have been relying on the position independent
nature of small model code, but it would be better to specify it
explicitly, so if -fpie gives us mostly identical code and does not
need visibility hacks, I would prefer to add it for all compilers and
not have an exception only for Clang. Note that the same applies to
the entire kernel when built in KASLR mode, so it would also be good
to know our options here.
Arnd, Will, what is the oldest GCC version we claim to support for arm64?
Unfortunately, after looking into this a bit more, -fpie by itself
doesn't force clang to disable symbol preeemption. For example when
building the EFI stub from 4.9 with clang, -fpie gives me a stub that
crashes with a synchronous exception inside handle_kernel_image(). The
faulting instruction is a read from __nokaslr that still goes through
the GOT.
Right now you'll get a usable EFI stub with -fpie anyway, since
60f38de7a8d4 ("efi/libstub: Unify command line param parsing") masked
the problem when it moved __nokaslr behind a helper function. But AIUI
there's nothing really preventing a similar problem in the future.
You *can* force clang to disable symbol preemption using "-fpie
-mpie-copy-relocations". That said, I don't know enough about EFI to
say whether this is actually appropriate for building the EFI stub.
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