aaron.ballman 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: > 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 > 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. Correct, we don't have a general mechanism for handling resource limits, we mostly play whack-a-mole when we run into them. So having a general utility that can work for other bit-fields would be really nice. However, `__builtin_dump_struct` is a debugging interface and not something we expect users to call particularly often, so some sharp edges with the interface are not the end of the world IMO. > 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). I think this is basically what @dblaikie was asking for -- if there are too many fields in the structure, either give a diagnostic that the structure is too complex for us to dump, or cut off the output somewhere sensible, etc. @dblaikie, do you have a preference for diagnostic vs prettier output? Ideally, I lean towards prettier output, but at the same time, that might be a bigger ask than what you were after. 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
