I will provide links to tutorials etc at the end.
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I decided to spend a day or two last month seeing if I could formally
prove some code inside qemu, and I arbitrarily picked one of the
smallest pieces of code in the "util/" subdirectory:
Frama-C parsed the C code and the formal specification and machine
checked it, and it's correct - the code is bug-free.
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= OVERVIEW OF FRAMA-C ECOSYSTEM =
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= BACK TO RANGE.C =
Going back to what we proved so far:
- Given those assumptions, the code is bug free - you don't need to
write any tests.
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Obviously this is a single line, very trivial function, but I was
quite pleased that I was able to prove it quickly. I kept going on
the range file. The next function is:
return val >= range->lob && val <= range->upb;
}
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The next function is range_make_empty, again easy to prove using the
already existing empty_range predicate. Notice how we declare which
memory locations this function assigns to:
assert(range_is_empty(range));
}
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I'm going to skip forward a few functions to get to an interesting one.
This seems trivial:
$ frama-c -wp -wp-rte snippets/range_size-good.c
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On to the next function. Again this seems very simple, but in fact it
contains a serious problem:
- It probably doesn't make sense for qemu right now though, unless we
could prove more substantial pieces of code.
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= POWER OF 2 =
This is a function from nbdkit:
Essentially bitwise tricks like this are a hard case for automated
theorem proving. I gave up.
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= TIMEVAL DIFFERENCE =
This is another nbdkit function:
understand them. We could contribute these to the Frama-C standard
library.
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= STRING FUNCTIONS =
Uli sent me this function from glibc:
So a proof of the glibc function eludes me.
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There is a set of open source licensed string functions with Frama-C
proofs available:
and this is what the strlen function with proof looks like from that:
- [https://github.com/evdenis/verker/blob/master/src/strlen.c
+ [https://github.com/evdenis/verker/blob/master/src/strlen.c]
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Now you might be asking what happens when you write a function that
uses strlen, for example this trivial function with a working
specification:
strings cannot exist on real computers, but they can exist on
theoretical ones!
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= IN CONCLUSION =
* Frama-C is a real open source tool used by companies to verify