dblaikie added inline comments.
================ Comment at: clang/include/clang/AST/Stmt.h:596-597 - // These don't need to be particularly wide, because they're - // strictly limited by the forms of expressions we permit. - unsigned NumSubExprs : 8; - unsigned ResultIndex : 32 - 8 - NumExprBits; + unsigned NumSubExprs : 16; + unsigned ResultIndex : 16; }; ---------------- yronglin wrote: > dblaikie wrote: > > aaron.ballman wrote: > > > yronglin wrote: > > > > dblaikie wrote: > > > > > dblaikie wrote: > > > > > > aaron.ballman wrote: > > > > > > > dblaikie wrote: > > > > > > > > Could/should we add some error checking in the ctor to assert > > > > > > > > that we don't overflow these longer values/just hit the bug > > > > > > > > later on? > > > > > > > > > > > > > > > > (& could we use `unsigned short` here rather than bitfields?) > > > > > > > We've already got them packed in with other bit-fields from the > > > > > > > expression bits, so I think it's reasonable to continue the > > > > > > > pattern of using bit-fields (that way we don't accidentally end > > > > > > > up with padding between the unnamed bits at the start and the > > > > > > > named bits in this object). > > > > > > > > > > > > > > I think adding some assertions would not be a bad idea as a > > > > > > > follow-up. > > > > > > Maybe some unconditional (rather than only in asserts builds) error > > > > > > handling? (report_fatal_error, if this is low priority enough to > > > > > > not have an elegant failure mode, but something where we don't just > > > > > > overflow and carry on would be good... ) > > > > > Ping on this? I worry this code has just punted the same bug further > > > > > down, but not plugged the hole/ensured we don't overflow on > > > > > novel/larger inputs. > > > > Sorry for the late reply, I was looking through the emails and found > > > > this. I agree add some assertions to check the value is a good idea, > > > > It's easy to help people catch bugs, at least with when > > > > `-DLLVM_ENABLE_ASSERTIONS=ON`, and I'm glad to work on it, but one > > > > thing that worries me is that, in ASTReader, we access this field > > > > directly, not through the constructor or accessor, and we have to add > > > > assertions everywhere. > > > > https://github.com/llvm/llvm-project/blob/05b4310c8aec7a050574277ced08a0ab86b27681/clang/lib/Serialization/ASTReaderStmt.cpp#L1382 > > > I don't think we have to add too many assertions. As best I can tell, > > > we'll need one in each of the `PseudoObjectExpr` constructors and one in > > > `ASTStmtReader::VisitPseudoObjectExpr()`, but those are the only places > > > we assign a value into the bit-field. Three assertions isn't a lot, but > > > if we're worried, we could add a setter method that does the assertion > > > and use the setter in all three places. > > My concern wasn't (well, wasn't entirely) about adding more assertions - > > but about having a reliable error here. The patch only makes the sizes > > larger, but doesn't have a hard-stop in case those sizes are exceeded again > > (which, admittedly, is much harder to do - maybe it's totally unreachable > > now, for all practical purposes?) > > > > I suspect with more carefully constructed recursive inputs could still > > reach the higher limit & I think it'd be good to fail hard in that case in > > some way? (it's probably rare enough that a report_fatal_error would be > > not-the-worst-thing-ever) > > > > But good assertions would be nice too (the old code only failed when you > > hit /exactly/ on just the overflow value, and any more than that the > > wraparound would not crash/fail, but misbehave) - I did add the necessary > > assertion to ArrayRef (begin <= end) which would've helped detect this more > > reliably, but some assert checking for overflow in the ctor would be good > > too (with all the usual nuance/care in checking for overflow) - unless > > we're going to make that into a fatal or other real error. > Sorry for the very late reply. I have no preference between assertion and > `llvm_unreachable`, if error then fail fast is looks good. I have a patch > D158296 to add assertion. Thanks for the assertions - though they still haven't met my main concern that this should have a hard failure even in a non-assertions build. I know we don't have a perfect plan/policy for these sort of "run out of resources/hit a representational limit" issues (at least I don't think we do... do we, @aaron.ballman ? I know we have some limits (recursion, template expansion, etc) but they're fairly specific/aren't about every possible case of integer overflow in some representational element, etc) but we've seen this one is pretty reachable. Here's a test case that would still trigger the assertion, and trigger UB in a non-assertions build: ``` truct t1 { }; template<typename T1> struct templ { T1 v1; T1 v2; T1 v3; T1 v4; }; struct t2 { templ<templ<templ<templ<templ<templ<t1>>>>>> c0; templ<templ<templ<templ<templ<templ<t1>>>>>> c1; templ<templ<templ<templ<templ<templ<t1>>>>>> c2; }; void aj(...); void f1(t2 w) { __builtin_dump_struct(&w, aj); } ``` (used templates to pack this a bit more densely than the original test case) - the `sizeof` the struct is certainly a bit outlandish (~12kbytes) bit not, I think, totally unreasonable? Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D154784/new/ https://reviews.llvm.org/D154784 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
