On Fri, Dec 04, 2015 at 12:44:09PM -0800, Kostya Serebryany wrote: > On Fri, Dec 4, 2015 at 12:35 PM, Alexei Starovoitov < > alexei.starovoi...@gmail.com> wrote: > > > On Fri, Dec 04, 2015 at 08:48:57PM +0100, Dmitry Vyukov wrote: > > > > > > For example, a compiler can assume that result of left shift is larger > > > or equal to first operand, which in turn can allow it to elide some > > > bounds check in code, which in turn can lead to an exploit. I am not > > > saying that this particular pattern is present in the code, what I > > > want to say is that such undefined behaviors can lead to very > > > unpredictable and unexpected consequences. > > > > Within bpf it cannot. > > shift is not used in any memory or bounds operations. > > so reg <<= 1234 cannot be exploited. > > > > I afraid this is not that simple. > In C, undefined behavior applies to the entire program, not just to a > single instruction. > My favorite example: > http://stackoverflow.com/questions/7682477/why-does-integer-overflow-on-x86-with-gcc-cause-an-infinite-loop > Here an undefined behavior in one instruction causes *other* instructions > to misbehave.
that's actually not related example. There compiler takes advantage of undefined behavior which is very typical for compiler to do. for(int i = 0; i < 100; i++) and for(unsigned int i = 0; i < 100; i++) are very different loops from compiler point of view. but that is not applicable in bpf world. there are no loops in bpf in the first place. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
