george.burgess.iv 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; ---------------- aaron.ballman wrote: > 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? Yeah, GCC has a similar `-Walloca-larger-than=N` that does roughly what you described. The icky part is exactly what you said. GCC will warn about unknown values, but considers control flow when doing so: https://godbolt.org/z/J3pGiT It looks like it's the same "we apply optimizations and then see what happens" behavior as similar diagnostics: https://godbolt.org/z/lLyteP WRT the diag we emit here, maybe we could use notes to break it up and imply things? e.g. warning: use of function %0 is discouraged, due to its security implications note: 'malloc' or 'new' are suggested alternatives, since they have well-defined behavior on failure ...not sold on the idea, but it's a thought. If we don't want to break it to pieces, I'm fine with your suggestion 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
