yronglin 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: > > 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? > Thanks for your example. I have three ways: > 1. use `llvm_unreachable` to emit a hard failure but not an assertion. > 2. extend these two field to 32-bit unsigned, it's may big enough. > 3. limit the functionality of `__builtin_dump_struct`, if there are too many > fields in a struct, the part exceeding the limit will not be output, and > replaced with `...`(maybe). > > WDYT? You guys are expert in clang, and I would like to wait for your > guidance :) We have ever limit the length of c-string value in `__builtin_dump_struct`. https://github.com/llvm/llvm-project/blob/5675f44cebf602931e884595c1d488bcedf6b8f2/clang/lib/Sema/SemaChecking.cpp#L524-L527 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
