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> it's >> 99.9% true that this will just crash even though it's acshually UB, nasal demons and so forth. Is it though? Linux saw enough bugs from that kind of
by lmm 4mo ago
> it's >> 99.9% true that this will just crash even though it's acshually UB, nasal demons and so forth.
Is it though? Linux saw enough bugs from that kind of issue that they now build with -fno-delete-null-pointer-checks and accept the (supposed) performance penalty.
- uecker 4mo agoThe kernel is perhaps bit special. In the past they had bugs such as first derferencing and then checking for null and weird possibilities to map the zero page. But today I am not convinced this is really needed. In general on a system where you trap when accessing the zero page, this optimization should be safe and a null pointer dereferences should (safely) trap.
- lmm 4mo ago> In general on a system where you trap when accessing the zero page, this optimization should be safe and a null pointer dereferences should (safely) trap. If you mean that C compiler writers "should" prioritise sanity over high scores on microbenchmarks, then I agree. However in practice they do not and this optimization is not remotely safe.
- uecker 4mo agoDo you have any evidence for this? On GCC it should be safe. (EDIT: what is not safe is indexing into a null pointer. For this you need to be safe you need -fsanitize=null)
- lmm 4mo agoI don't understand your comment - dereferencing a null pointer is unsafe, in the sense that it does not reliably crash but may do other things, as we saw in the kernel case we're talking about. Yes that particular case was only exploitable if you mapped the zero page, but given how all-bets-are-off a situation it created (where extremely experienced programmers thought they knew what the code did, thought it was safe, and were wrong), I would not want to count on all cases not being exploitable without mapping the zero page.
- jstimpfle 4mo agoMay. If. If. If. In case. We are talking about an extremely simple straightforward API with an obvious contract. It's good enough for this function to reliably surface almost all wrong uses with a segfault immediately. Wrong use will result in segfaults and otherwise bugs and crashes. The goal is not to work when used wrong but to work when used right. You cannot save the world from scratch in every little function. You still have a job to get done, and you have to move on.
- lmm 4mo ago> You cannot save the world from scratch in every little function. You still have a job to get done, and you have to move on. Or you can take all of 10 minutes to put sanity-check assertions at the start of all your public-facing API functions, eliminating a source of security bugs, get on with your life, and worry about the performance implications as and when it becomes a problem (hint: it's never going to become a problem).
- dwattttt 4mo agoIt takes a lot longer to figure out if it'll be a problem than to just add the check. And you don't have to ponder whether it's possible for a null to get there, because now it's fine if it does.
- jstimpfle 4mo agoAre you talking about extending the API contract to allow for NULL? That is often the path to madness, especially if it requires complicating the signature (return value etc). Better to just assert/crash.
- dwattttt 4mo agoNo. I'm talking about adding the check to reject NULL. Then you don't have to spend time justifying or figuring out why a NULL can't turn up here.