Am Di., 12. Mai 2020 um 13:17 Uhr schrieb Dave Barach (dbarach) < dbar...@cisco.com>:
> Dear Andreas, > > > > Do you have a handy list of places which convert netmasks to lengths? > Regardless of what one might do with min_log2, we ought to clean up those > places in time for the 20.05 release (if possible). > Unfortunately not. This is the code that caused problems for us: https://gerrit.fd.io/r/c/vpp/+/27016 Grepping through the code shows additional places that appear to have the potential to pass a 0 to one of the log2 functions. But we haven't investigated any of them closer. I was trying to add an ASSERT (x != 0) to the log2 functions. But those functions are defined in clib.h, and that header is required by error_bootstrap.h that defines ASSERTs. The resulting include dependency loop is only solvable by moving the log2 functions out of clib.h Andreas > > > Dave > > > > *From:* vpp-dev@lists.fd.io <vpp-dev@lists.fd.io> *On Behalf Of *Andreas > Schultz > *Sent:* Tuesday, May 12, 2020 5:42 AM > *To:* vpp-dev <vpp-dev@lists.fd.io> > *Subject:* [vpp-dev] min_log2 abuse > > > > Hi, > > > > There are few places in VPP (most notable when trying to convert a netmask > into a length) that can pass 0 (zero) into min_log2 and expect to get a > meaningful result. > > > > Obviously log2(0) is undefined. It turns out that this also applies to the > return of min_log2. Under the hood the function uses __builtin_clzl, and > the return of that function is also undefined for input 0. > > > > __builtin_clzl could be replaced with __builtin_ia32_lzcnt_u64 on > supported CPUs to avoid the undefined behaviour. That would still not fix > the problem that passing 0 into a log function is broken by design. > > > > Any comments? > > Andreas > > > > -- > > Andreas Schultz > -- Andreas Schultz -- Principal Engineer t: +49 391 819099-224 ------------------------------- enabling your networks ----------------------------- Travelping GmbH Roentgenstraße 13 39108 Magdeburg Germany t: +49 391 819099-0 f: +49 391 819099-299 e: i...@travelping.com w: https://www.travelping.com/ Company registration: Amtsgericht Stendal Geschaeftsfuehrer: Holger Winkelmann Reg. No.: HRB 10578 VAT ID: DE236673780
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