OPENSSL_ia32cap_addr avoids any relocations within the module, at the
cost of a runtime TEXTREL, which causes problems in some cases.
(Notably, if someone links us into a binary which uses the GCC "ifunc"
attribute, the loader crashes.)
We add a OPENSSL_ia32cap_addr_delta symbol (which is reachable
relocation-free from the module) stores the difference between
OPENSSL_ia32cap_P and its own address. Next, reference
OPENSSL_ia32cap_P in code as usual, but always doing LEAQ (or the
equivalent GOTPCREL MOVQ) into a register first. This pattern we can
then transform into a LEAQ and ADDQ on OPENSSL_ia32cap_addr_delta.
ADDQ modifies the FLAGS register, so this is only a safe transformation
if we safe and restore flags first. That, in turn, is only a safe
transformation if code always uses %rsp as a stack pointer (specifically
everything below the stack must be fair game for scribbling over). Linux
delivers signals on %rsp, so this should already be an ABI requirement.
Further, we must clear the red zone (using LEAQ to avoid touching FLAGS)
which signal handlers may not scribble over.
This also fixes the GOTTPOFF logic to clear the red zone.
Change-Id: I4ca6133ab936d5a13d5c8ef265a12ab6bd0073c9
Reviewed-on: https://boringssl-review.googlesource.com/15545
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
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