aaron.ballman added inline comments.
================ Comment at: clang/include/clang/Basic/DiagnosticSemaKinds.td:2776 +def warn_alloca : Warning< + "use of builtin function %0">, + InGroup<DiagGroup<"alloca">>, DefaultIgnore; ---------------- george.burgess.iv wrote: > aaron.ballman wrote: > > george.burgess.iv wrote: > > > nit: I'd just say "use of function '%0'" here; "builtin" doesn't really > > > add much. > > > > > > I also wonder if we should be saying anything more than "we found a use > > > of this function." Looks like GCC doesn't (https://godbolt.org/z/sYs_8G), > > > but since this warning is sort of opinionated in itself, might it be > > > better to expand this to "use of '%0' is discouraged"? > > > > > > WDYT, Aaron? > > What is the purpose to this diagnostic, aside from GCC compatibility? What > > does it protect against? > > > > If there's a reason users should not use alloc(), it would be better for > > the diagnostic to spell it out. > > > > Btw, I'm okay with this being default-off because the GCC warning is as > > well. I'm mostly hoping we can do better with our diagnostic wording. > > I'm mostly hoping we can do better with our diagnostic wording > > +1 > > > What is the purpose to this diagnostic? > > I think the intent boils down to that `alloca` is easily misused, and leads > to e.g., https://www.qualys.com/2017/06/19/stack-clash/stack-clash.txt . > Since its use often boils down to nothing but a tiny micro-optimization, some > parties would like to discourage its use. > > Both glibc and bionic recommend against the use of `alloca` in their > documentation, though glibc's docs are less assertive than bionic's, and > explicitly call out "[alloca's] use can improve efficiency compared to the > use of malloc plus free." > > Greping a codebase and investigating the first 15 results: > - all of them look like microoptimizations; many of them also sit close to > other `malloc`/`new` ops, so allocating on these paths presumably isn't > prohibitively expensive > - all but two of the uses of `alloca` have no logic to fall back to the heap > `malloc` if the array they want to allocate passes a certain threshold. Some > of the uses make it look *really* easy for the array to grow very large. > - one of the uses compares the result of `alloca` to `NULL`. Since `alloca`'s > behavior is undefined if it fails, ... > > I'm having trouble putting this into a concise and actionable diagnostic > message, though. The best I can come up with at the moment is something along > the lines of "use of function %0 is subtle; consider using heap allocation > instead." Okay, that's along the lines of what I was thinking. Part of me thinks that this should not diagnose calls to `alloca()` that are given a constant value that we can test for sanity at compile time. e.g., calling `alloca(10)` is highly unlikely to be a problem, but calling `alloca(1000000)` certainly could be, while `alloca(x)` is a different class of problem without good static analysis. That said, perhaps we could get away with `use of function %0 is discouraged; there is no way to check for failure but failure may still occur, resulting in a possibly exploitable security vulnerability` or something along those lines? ================ Comment at: clang/lib/Sema/SemaChecking.cpp:1175 + Diag(TheCall->getBeginLoc(), diag::warn_alloca) + << Context.BuiltinInfo.getName(BuiltinID); break; ---------------- I think this should see how the user spelled the builtin call. It would be a bit strange for a user who wrote `alloca()` in their code to get a diagnostic about not calling `__builtin_alloca()`. Repository: rC Clang CHANGES SINCE LAST ACTION https://reviews.llvm.org/D64883/new/ https://reviews.llvm.org/D64883 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
