[PATCH] D64883: Add new warning -Walloca for use of builtin alloca function

Tue Jul 23 10:25:15 PDT 2019

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:
> > > > 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
> I'm not certain the note adds value because it will always be printed on the same line as the warning. A note would make sense if we had a secondary location to annotate though.
SGTM

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  rC Clang

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  https://reviews.llvm.org/D64883/new/

https://reviews.llvm.org/D64883