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; }; ---------------- dblaikie wrote: > aaron.ballman wrote: > > 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. > > > Yeah, I don't have major/heroic requirements here - just that we don't crash > on these inputs. An error, wider integers, both, etc, it's all good to me. > > just an error seems fine to me - as you say, it's basically a debugging > interface, people shouldn't be making this some loadbearing/totally generic > part of an API or somesuch. Thank you! I'm also fine with going with just an error. 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
