To those of you with more x86 ISA implementation knowledge than I have:
I've been working through a bug one of our users found (thanks Sanchayan!).
It looks like current versions of ld use the 0x67 instruction prefix
(address size override) as an optimization instead of using a nop. See
https://www.sourceware.org/ml/binutils/2016-05/msg00323.html.
This causes the call instruction to be decoded with with the "address size
override prefix", which is correct, in a sense. From what I can tell, this
is passed to the call instruction via "-env.dataSize" (see call instruction
implementation below).
def macroop CALL_NEAR_I
{
# Make the default data size of calls 64 bits in 64 bit mode
.adjust_env oszIn64Override
.function_call
limm t1, imm
rdip t7
# Check target of call
st t7, ss, [0, t0, rsp], "-env.dataSize"
subi rsp, rsp, ssz
wrip t7, t1
};
Now, the bug is, according to the x86 manual, "For instructions that
implicitly access the stack segment (SS), the address size for stack
accesses is determined by the D (default) bit in the stack-segment
descriptor. In 64-bit mode, the D bit is ignored, and all stack references
have a 64-bit address size." See
https://support.amd.com/TechDocs/24594.pdf page
9.
Thoughts on how to fix this?
Thanks,
Jason
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