fixed a few remaining ubsan warnings in lz4hc #1160
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mostly related to pointer arithmetic involving NULL ptr.
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that were previously hidden due to the
-fsanitize-recover=pointer-overflowflag.The 2 minor instances involve pointer arithmetic with
NULLptr.For the record, I also tried to fix a few remaining
ubsanwarnings inlz4.c,related to pointer overflow arithmetic.
Unfortunately, the fix resulted in a massive drop in performance,
so it's not bundled here.
The issue (in
lz4.c) is that the code uses a virtual pointer addressbaseas a reference point for its indexing scheme.The
basepointer itself is never dereferenced, it's always combined with anindex, in order to reference a valid address.Nevertheless, from a
ubsanperspective, it's an invalid operation.Fixing that requires changing one variable (
base) by 2 variables (prefixStartandprefixIndex) and an arithmetic operation (currentIndex - prefixIndex). It's a small difference (one more register, one more addition), but at the speed oflz4fast mode, it's enough to make a sensible impact on performance.Therefore, removing these last
ubsanwarnings will require finding a good replacement strategy, which doesn't involve a significant speed loss.No such problem in
lz4hc, which compression speed is much lower, and algorithm is more complex. Consequently, this issue has already been fixed (in previous release) without any noticeable drop in performance.