[PATCH] D148355: [analyzer] Fix comparison logic in ArrayBoundCheckerV2

[Balázs Benics via Phabricator via cfe-commits]

steakhal added a comment.

Please mark comments "Done" where applicable.



================
Comment at: clang/lib/StaticAnalyzer/Checkers/ArrayBoundCheckerV2.cpp:173
+  const MemSpaceRegion *SR = rawOffset.getRegion()->getMemorySpace();
+  if (SR->getKind() != MemRegion::UnknownSpaceRegionKind) {
+    // a pointer to UnknownSpaceRegionKind may point to the middle of
----------------
donat.nagy wrote:
> donat.nagy wrote:
> > steakhal wrote:
> > > donat.nagy wrote:
> > > > steakhal wrote:
> > > > > 
> > > > You're completely right, I just blindly copied this test from the needlessly overcomplicated `computeExtentBegin()`.
> > > Hold on. This would only skip the lower bounds check if it's an `UnknownSpaceRegion`.
> > > Shouldn't we early return instead?
> > This behavior is inherited from the code before my commit: the old block `if ( /*... =*/ extentBegin.getAs<NonLoc>() ) { /* ... */ }` is equivalent to `if (llvm::isa<UnknownSpaceRegion>(SR)) { /*...*/ }` and there was no early return connected to //this// NonLocness check. (The old code skipped the upper bound check if the result of `evalBinOpNN()` is unknown, and that's what I changed because I saw no reason to do an early return there.)
> > 
> > After some research into the memory region model, I think that there is no reason to perform an early return -- in fact, the condition of this  `if` seems to be too narrow because we would like to warn about code like
> >   struct foo {
> >     int tag;
> >     int array[5];
> >   };
> >   int f(struct foo *p) {
> >     return p->arr[-1];
> >   }
> > despite the fact that it's indexing into a `FieldRegion` inside a `SymbolicRegion` in `UnknownSpaceRegion`. That is, instead of checking the top-level MemorySpace, the correct logic would be checking the kind of the memory region and/or perhaps its immediate super-region.
> > 
> > As this is a complex topic and completely unrelated to the main goal of this commit; I'd prefer to keep the old (not ideal, but working) logic in this patch, then revisit this question by creating a separate follow-up commit.
> Minor nitpick: your suggested change accidentally negated the conditional :) ... and I said that it's "completely right". I'm glad that I noticed this and inserted the "!" before the `isa` check because otherwise it could've been annoying to debug this...
Agreed.


================
Comment at: clang/lib/StaticAnalyzer/Checkers/ArrayBoundCheckerV2.cpp:173
+  const MemSpaceRegion *SR = rawOffset.getRegion()->getMemorySpace();
+  if (SR->getKind() != MemRegion::UnknownSpaceRegionKind) {
+    // a pointer to UnknownSpaceRegionKind may point to the middle of
----------------
steakhal wrote:
> donat.nagy wrote:
> > donat.nagy wrote:
> > > steakhal wrote:
> > > > donat.nagy wrote:
> > > > > steakhal wrote:
> > > > > > 
> > > > > You're completely right, I just blindly copied this test from the needlessly overcomplicated `computeExtentBegin()`.
> > > > Hold on. This would only skip the lower bounds check if it's an `UnknownSpaceRegion`.
> > > > Shouldn't we early return instead?
> > > This behavior is inherited from the code before my commit: the old block `if ( /*... =*/ extentBegin.getAs<NonLoc>() ) { /* ... */ }` is equivalent to `if (llvm::isa<UnknownSpaceRegion>(SR)) { /*...*/ }` and there was no early return connected to //this// NonLocness check. (The old code skipped the upper bound check if the result of `evalBinOpNN()` is unknown, and that's what I changed because I saw no reason to do an early return there.)
> > > 
> > > After some research into the memory region model, I think that there is no reason to perform an early return -- in fact, the condition of this  `if` seems to be too narrow because we would like to warn about code like
> > >   struct foo {
> > >     int tag;
> > >     int array[5];
> > >   };
> > >   int f(struct foo *p) {
> > >     return p->arr[-1];
> > >   }
> > > despite the fact that it's indexing into a `FieldRegion` inside a `SymbolicRegion` in `UnknownSpaceRegion`. That is, instead of checking the top-level MemorySpace, the correct logic would be checking the kind of the memory region and/or perhaps its immediate super-region.
> > > 
> > > As this is a complex topic and completely unrelated to the main goal of this commit; I'd prefer to keep the old (not ideal, but working) logic in this patch, then revisit this question by creating a separate follow-up commit.
> > Minor nitpick: your suggested change accidentally negated the conditional :) ... and I said that it's "completely right". I'm glad that I noticed this and inserted the "!" before the `isa` check because otherwise it could've been annoying to debug this...
> Agreed.
Sorry about that. Happens


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