adds x86 stack pointer handling for external calls #955
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So far the stack pointer register wasn't properly handled by primus in case of external calls. The reason is in the asymmetry of the function calling in x86 ABI: the caller side allocates the stack slot for the returning address and callee sise is responsible for clearing it. But in case of external calls, dependless if a lisp stub exists or call remains unresolved, there wasn't any stack restoring. As a result stack pointer didn't point to a proper place. This PR solve this problem by incrementing stack pointer after lisp call returns. It works for unresolved calls as well.
ivg
approved these changes
Jun 14, 2019
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For x86 and x86_64 ABI it is the responsibility of the callee to pop the return address from the stack pointer and increase the stack pointer. Since the code of external functions is not present in the binary, every time an external function is called, the stack grows without being corrected. In case if the base pointer is not used (e.g., in optimized code, or if the no-frame-pointer option was specified) this will also lead to the stack corruption as all variables will shift. This corruption results in an incorrect interpretation of a program and in case of taint analysis will introduce aliasing between neighboring stack variables.
The solution is to appropriately increase the stack pointer every time we return from a call from the Lisp Machine back to the Primus Machine. This will correctly handle three cases:
In the latter will work correctly, because all external functions that are not linked explicitly, are implicitly linked to a special function __primus_linker_unresolved_call, which in turn is overridden by the primus:handle-unresolved-names function, therefore an unlinked external function will still end up with a Lisp machine call. The same is true, if a function is overridden without using the Lisp Machine, directly from the Linker.
Obviously, this patch relies on the presence of the Lisp machine and if the Lisp Machine is not loaded, the stack pointer will not be corrected.
This patch will also misbehave, if a local static function that is not ABI comformant is overridden by a Lisp stub. In that case it is the responsibility of the Lisp stub to properly correct the stack pointer.
A better solution is still being sought, but so far this is the best option we have.
Resolves #879