[PATCH] D67122: [UBSan][clang][compiler-rt] Applying non-zero offset to nullptr is undefined behaviour

Thu Sep 5 15:11:08 PDT 2019

lebedev.ri added a comment.

In D67122#1659882 <https://reviews.llvm.org/D67122#1659882>, @rupprecht wrote:

> > Still think this looks good. Have you tried running this on the llvm test suite, or some other interesting corpus? Would be curious to see any pre/post patch numbers.
>
> I finally had time this morning to patch this in and give it a shot. (Sorry for the delay... it's been a real busy week :( )

> First, starting off with the good news: I reverted all the fixes I made, and now all the tests fail when running w/ ubsan. Yay!
>  The error we see in each case is `UndefinedBehaviorSanitizer: nullptr-with-nonzero-offset` with the logs containing `runtime error: applying non-zero offset <non-zero> to null pointer`. Which catches the two places where we were adding some non-zero offset to nullptr,

That is good :)

> but doesn't seem to catch the nullptr-after-nonzero-offset case in https://github.com/google/filament/pull/1566 -- instead, it fails later, when the pointer with a value of nullptr is incremented. (Or... maybe this is actually a separate bug. Hmm. Needs some more testing...)

Well yeah, there are quite a few cases in clang codegen where we create `gep inbounds` without sanitization, so it may or may not be one of those cases.

> At any rate, I have some more tests to run to get some idea of what % of code this would flag as being bad.

Repository:
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  https://reviews.llvm.org/D67122/new/

https://reviews.llvm.org/D67122