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Date: Tue, 25 Jun 2019 15:02:54 +0000
From: Pascal Cuoq <cuoq@...st-in-soft.com>
To: "oss-security@...ts.openwall.com" <oss-security@...ts.openwall.com>
CC: "law@...hat.com" <law@...hat.com>
Subject: Re: Thousands of vulnerabilities, almost no CVEs:
 OSS-Fuzz

Hello,

> On 25 Jun 2019, at 16:33, Jeff Law <law@...hat.com> wrote:
> 
> On 6/25/19 8:14 AM, Matthew Fernandez wrote:
>> 
>> 
>> C/C++ compilers will infer backwards from uninitialized variable reads (undefined behavior in these languages) that preceding code is unreachable. For example, when moving from GCC 6 series to GCC 7 series we found one of our code bases would produce a binary that would only segfault when compiled at >= -O2. We root caused this to exactly the situation you describe: an error handling path that read uninitialized variables. The compiler appeared to infer backwards that the error check itself was a no-op as the true branch led to unconditional UB (this is my interpretation of its actions; I did not delve into the compiler’s internals).
> Well, as a GCC developer, I can say it doesn't use an uninitialized read
> to allow back-propagation of state to eliminate conditionals.  It may
> have looked that way, but there had to be something else going on.

This is tangential to the subject and perhaps we should take this sub-discussion off the list, or at least make a new thread. I'm interested in your opinion of what is going on with Ubuntu's packaged GCC version 4.4.3 in the example under the section “The next example” in this blog post, where this very thing is happening (when invoked with fewer than 3 arguments, the compiled code claims that the result of an unsigned multiplication by 2 is odd):

http://blog.frama-c.com/index.php?post/2013/03/13/indeterminate-undefined

I have not been able to reproduce this with any of the GCC versions hosted at Compiler Explorer, so I believe that this may never have been part of the GCC official tree, but the fact remains that all of Ubuntu 4.4.3 and all source programs that were compiled with Ubuntu 4.4.3's default compiler were compiled with a compiler that did this (surprising, dangerous in some contexts) thing.

Pascal


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