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Date: Sat, 27 May 2023 11:52:23 +0200
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
Subject: crypt: dependency needed between main and test run?
Hi all,
continuing my theme of not providing patches but rather asking
questions, I noticed something about all of the crypt() backends: They
all use a self test, and have no dependency between main and test calls
to their respective backends. E.g. in __crypt_sha512():
|p = sha512crypt(key, setting, output);
|/* self test and stack cleanup */
|q = sha512crypt(testkey, testsetting, testbuf);
The backend is itself a pure function. For any given input, it will
always make the same output. So I don't see a reason why the compiler
can't reorder these calls. The backend is a static function (and only
calling static functions) defined in the same file, so all of this is
available for the compiler to see.
Indeed the compiler might inline the second call, then keep constant
folding and loop unrolling until it is entirely evaluated at compile
time, and only the result remains. Constant folding and loop unrolling
aren't new ideas, but sha512 (and actually all of the hash functions)
would require the compiler to be a lot more agressive about them than
it normally is at this time. But that doesn't mean it cannot change in
future.
I'd propose to add an explicit dependency from the second to the first
call. Something like this:
p = sha512crypt(key, setting, output);
__asm__("" : "=x"(q) : "0"(testsetting), "x"(p));
q = sha512crypt(testkey, q, testbuf);
The asm statement basically tells the compiler that
q = f(testsetting, p);
where f is some unknown pure function. The fact that that function
happens to be the identity is unknown to the compiler.
Doing this has two purposes: One, it makes the second call depend on the
result of the first, and two, it obscures to the compiler that all
inputs to the second call are constant, and thus makes it extremely
unlikely that the test backend call would be entirely eliminated.
What do you guys think?
Ciao,
Markus
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