Date: Tue, 25 Jun 2019 08:33:42 -0600 From: Jeff Law <law@...hat.com> To: oss-security@...ts.openwall.com, Matthew Fernandez <matthew.fernandez@...il.com> Subject: Re: Thousands of vulnerabilities, almost no CVEs: OSS-Fuzz On 6/25/19 8:14 AM, Matthew Fernandez wrote: > >> On Jun 25, 2019, at 06:41, Bob Friesenhahn <bfriesen@...ple.dallas.tx.us> wrote: >> >> * Consumption of uninitialized data (e.g. image data) which is not >> used to make important decisions. This is usually due to unhandled >> cases or error handling which does not quit immediately. > > 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. Jeff
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