<div class="gmail_quote">On Thu, Mar 1, 2012 at 9:26 PM, Nico Weber <span dir="ltr"><<a href="mailto:thakis@chromium.org">thakis@chromium.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="HOEnZb"><div class="h5">>> > On Tue, Feb 21, 2012 at 7:17 PM, Nico Weber <<a href="mailto:thakis@chromium.org">thakis@chromium.org</a>> wrote:<br>
>> >><br>
>> >> the attached patch adds -Wstring-plus-int (I'm open to more creative<br>
>> >> names :-) ), which warns on code like |"error code: " + err|. The<br>
>> >> warning found 4 bugs in 2 files in chromium. The bugs it found look<br>
>> >> like this:<br>
>> >><br>
>> >> File 1:<br>
>> >> if (!read_node.InitByIdLookup(it->id)) {<br>
>> >> error_handler()->OnUnrecoverableError(FROM_HERE, "Failed to look<br>
>> >> up "<br>
>> >> " data for received change with id " + it->id);<br>
>> >> return;<br>
>> >> }<br>
>> >><br>
>> >> File 2:<br>
>> >> ResetStream(stream_id, spdy::INVALID_ASSOCIATED_STREAM,<br>
>> >> "Received OnSyn with inactive associated stream " +<br>
>> >> associated_stream_id);<br>
>> >> (Fix:<br>
>> >> <a href="http://codereview.chromium.org/9372076/diff/1/net/spdy/spdy_session.cc" target="_blank">http://codereview.chromium.org/9372076/diff/1/net/spdy/spdy_session.cc</a><br>
>> >> )<br>
>> >><br>
>> >> (A coworker found the bug in the one file, which prompted me writing<br>
>> >> the warning. The warning then found the bug in the other file.)<br>
>> >><br>
>> >><br>
>> >> When building all of chromium and its many dependencies, the warning<br>
>> >> did also find 3 false positives, but they all look alike: Ffmepg<br>
>> >> contains three different functions that all look like this:<br>
>> ><br>
>> ><br>
>> > 4 bugs and 3 false positives doesn't sound great to me.<br>
>><br>
>> True. All of the the 3 false positives are really the same snippet<br>
>> though. (On the other hand, 3 of the 4 bugs are very similar as well.)<br>
>> The bugs this did find are crash bugs, and they were in rarely run<br>
>> error-logging code, so it does find interesting bugs. Maybe you guys<br>
>> can run it on your codebase after it's in, and we can move it out of<br>
>> -Wmost if you consider it too noisy in practice?<br>
><br>
><br>
> Sounds reasonable.<br>
><br>
>><br>
>> > Have you considered<br>
>> > relaxing the warning in cases where the integral summand is a constant<br>
>> > expression and is in-bounds? Would this pattern have matched any of your<br>
>> > real bugs?<br>
>><br>
>> That's a good idea (for my bugs, the int was always a declrefexpr,<br>
>> never a literal). The ffmpeg example roughly looks like this:<br>
>><br>
>> #define A "foo"<br>
>> #define B "bar"<br>
>> consume(A B + sizeof(A) - 1);<br>
>><br>
>> The RHS is just "sizeof(A)" without the "- 1", but since A B has a<br>
>> length of 6, this still makes this warning go away. I added this to<br>
>> the patch. With this change, it's 4 bugs / 0 false positives.<br>
>><br>
>> Note that this suppresses the warning for most enums which start at 0.<br>
>> Or did you mean to do this only for non-enum constant expressions?<br>
><br>
><br>
> I could imagine code legitimately wanting to do something like this:<br>
><br>
> template</*...*/><br>
> struct S {<br>
> enum { Offset = /* ... */ };<br>
> static const char *str = "some string" + Offset;<br>
> };<br>
><br>
> That said, I'm happy for us to warn on such code. Whatever you prefer seems<br>
> fine to me; we can refine this later, if a need arises.<br>
<br>
</div></div>Enums are handled like ints: They warn if the offset is out of bounds,<br>
but don't if it's they are in bounds.<br>
<div><div class="h5"><br>
>> > Onto the patch...<br>
>> ><br>
>> > Index: include/clang/Basic/DiagnosticGroups.td<br>
>> > ===================================================================<br>
>> > --- include/clang/Basic/DiagnosticGroups.td (revision 150418)<br>
>> > +++ include/clang/Basic/DiagnosticGroups.td (working copy)<br>
>> > @@ -320,6 +321,7 @@<br>
>> > ReturnType,<br>
>> > SelfAssignment,<br>
>> > SizeofArrayArgument,<br>
>> > + StringPlusInt,<br>
>> > Trigraphs,<br>
>> > Uninitialized,<br>
>> > UnknownPragmas,<br>
>> ><br>
>> > Given the current level of false positives, I'm not completely convinced<br>
>> > this should go into -Wmost. I imagine we'll know more once we've run<br>
>> > this on<br>
>> > more code.<br>
>> ><br>
>> > Index: include/clang/Basic/DiagnosticSemaKinds.td<br>
>> > ===================================================================<br>
>> > --- include/clang/Basic/DiagnosticSemaKinds.td (revision 150418)<br>
>> > +++ include/clang/Basic/DiagnosticSemaKinds.td (working copy)<br>
>> > @@ -3414,6 +3414,12 @@<br>
>> > "explicitly assigning a variable of type %0 to itself">,<br>
>> > InGroup<SelfAssignment>, DefaultIgnore;<br>
>> ><br>
>> > +def warn_string_plus_int : Warning<<br>
>> > + "Adding an integral to a string does not append to the string">,<br>
>> ><br>
>> > Diagnostics outside the static analyzer should not be capitalized.<br>
>><br>
>> Done.<br>
>><br>
>> > + InGroup<StringPlusInt>;<br>
>> > +def note_string_plus_int_silence : Note<<br>
>> > + "use &(str[index]) if pointer arithmetic is intended">;<br>
>> ><br>
>> > The parentheses here are redundant. I think I'd prefer for the<br>
>> > suggestion to<br>
>> > be provided via fixits (and to reword this as something like "use array<br>
>> > indexing to silence this warning").<br>
>><br>
>> Clang tends to suggest very explicit parentheses (see<br>
>> -Wparentheses-logical-op for example).<br>
><br>
><br>
> Usually only in cases where there is a potential bug. If we saw &"foo"[2],<br>
> we wouldn't be likely to suspect that the user might have really meant<br>
> (&"foo")[2], so my (weak) preference is to suggest the simpler fix.<br>
<br>
</div></div>Removed the parens.<br>
<div class="im"><br>
> (Incidentally, -Warray-bounds should be taught to complain about<br>
> (&"foo")[2]!)<br>
<br>
</div>Patches accepted! :-D<br>
<div><div class="h5"><br>
>> I did change it to a fixit and<br>
>> reworded, it now looks like so:<br>
>><br>
>> test/SemaCXX/string-plus-int.cpp:12:17: warning: adding an integral to<br>
>> a string does not append to the string [-Wstring-plus-int]<br>
>> consume("foo" + 5);<br>
>> ~~~~~~^~~<br>
>> test/SemaCXX/string-plus-int.cpp:12:17: note: use array indexing to<br>
>> silence this warning<br>
>> consume("foo" + 5);<br>
>> ^<br>
>> &( )[ ]<br>
><br>
><br>
> This looks nice ... though if we're going to have parens in the fixit, they<br>
> should contain the [] :-)<br>
><br>
>><br>
>> > Index: lib/Sema/SemaExpr.cpp<br>
>> > ===================================================================<br>
>> > --- lib/Sema/SemaExpr.cpp (revision 150418)<br>
>> > +++ lib/Sema/SemaExpr.cpp (working copy)<br>
>> > @@ -7798,6 +7798,20 @@<br>
>> > ParensRange);<br>
>> > }<br>
>> ><br>
>> > +/// DiagnoseStringPlusInt - Emit a warning when adding an integer to a<br>
>> > string<br>
>> > +/// literal.<br>
>> > +static void DiagnoseStringPlusInt(Sema &Self, SourceLocation OpLoc,<br>
>> > + Expr *LHSExpr, Expr *RHSExpr) {<br>
>> > + bool IsStringPlusInt = dyn_cast<StringLiteral>(LHSExpr) &&<br>
>> > + RHSExpr->getType()->isIntegralType(Self.getASTContext());<br>
>> ><br>
>> > We should probably also check for (unscoped) enumeration types, and for<br>
>> > class types with implicit conversion to int. Doing so will require<br>
>> > moving<br>
>> > this check to checkArithmeticOpPointerOperand (after we've checked for<br>
>> > an<br>
>> > overloaded operator+).<br>
>><br>
>> Added a check for (unscoped) enumeration types. class types with<br>
>> implicit conversion to int feel like overkill to me.<br>
><br>
><br>
> OK. You should still move the check to after we check for an overloaded<br>
> operator+, since operator+ can be overloaded for enums. (Once that's done,<br>
> you may find it's easier to warn on such implicitly-convertible types than<br>
> to not do so.)<br>
<br>
</div></div>Done, added an enum-overloaded operator+ to the test.<br>
<div class="im"><br>
>> > + if (!IsStringPlusInt)<br>
>> > + return;<br>
>> > +<br>
>> > + SourceRange DiagRange(LHSExpr->getLocStart(), RHSExpr->getLocEnd());<br>
>> > + Self.Diag(OpLoc, diag::warn_string_plus_int) << DiagRange;<br>
>> > + Self.Diag(OpLoc, diag::note_string_plus_int_silence);<br>
>> > +}<br>
>> ><br>
>> > We should also catch the case where the integer is on the LHS (and skip<br>
>> > the<br>
>> > fixit in that case).<br>
>><br>
>> I tried that. It found neither additional bugs nor additional false<br>
>> positives, so it's probably not worth it?<br>
><br>
><br>
> Fine, there's no point adding this if it doesn't actually catch any bugs. I<br>
> have only empirical evidence that it might -- I once saw[*] the moral<br>
> equivalent of:<br>
><br>
> int n = /*...*/;<br>
> std::string s = n + " widgets found";<br>
><br>
> [*] a long time ago, in a codebase far away...<br>
<br>
</div>Ok, I added the warning for the int on the left as well (without fixit<br>
in that case, like you suggested upthread).</blockquote><div><br></div><div>Just a few more things:</div><div><br></div><div><div>+def warn_string_plus_int : Warning<</div><div>+ "adding an integral to a string does not append to the string">,</div>
<div>+ InGroup<StringPlusInt>;</div></div><div><br></div><div>Rather than "an integral", can we say something like:</div><div><br></div><div> adding %0 to a string literal does not append to the string</div>
<div><br></div><div>(where %0 is the type)?</div><div><br></div><div><div>+ if (IndexExpr->EvaluateAsInt(index, Self.getASTContext())) {</div><div>+ if (index.isNonNegative() &&</div><div>+ index < llvm::APSInt(llvm::APInt(index.getBitWidth(),</div>
<div>+ StrExpr->getByteLength()),</div></div><div><br></div><div>This should be <=, not <, I think: we want to allow "foo"+4 (a pointer past the null) but warn on "foo"+5 (which has undefined behavior). And getByteLength() should be getLength(), to properly handle wide string literals. We'll get duplicate warnings for the same issue if -Warray-bounds-pointer-arithmetic is enabled and we carry on here, but such code is rare enough that I don't think that's a pressing concern.</div>
<div><br></div><div>Other than that, a test for the fix-it would be great (see test/FixIt for examples).</div><div><br></div><div>Thanks!</div><div>Richard</div></div>