[cfe-commits] StringRef: undefined behavior issues with null pointers

[Argyrios Kyrtzidis]

(cc'ing cfe-commits)

On May 20, 2011, at 11:04 AM, Matthieu Monrocq wrote:

> Hello,
> 
> here is a second iteration of the patch
> 
> 2011/5/19 Argyrios Kyrtzidis <kyrtzidis at apple.com>
> (moved to cfe-commits)
> 
>>      /// Construct a string ref from a cstring.
>>      /*implicit*/ StringRef(const char *Str)
>> -      : Data(Str), Length(::strlen(Str)) {}
>> +      : Data(Str), Length() {
>> +        assert(Str && "StringRef cannot be built from a NULL argument");
>> +        Length = ::strlen(Str); // invoking strlen(NULL) is undefined behavior
>> +      }
>>  
> 
> 
> "Length()" is not necessary.
> Could you also add an assert in the  StringRef(const char *data, size_t length) constructor asserting that data is not null or length is 0 ?
> 
> 
> Removed and Done. 
>> +    
>> +    // Workaround memcmp issue with null pointers (undefined behavior)
>> +    // by providing a specialized version
>> +    static int memcmp(const char *Lhs, const char *Rhs, size_t Length) {
>> +      if (Length == 0) { return 0; }
>> +      assert(Lhs && "memcmp - Lhs should be non-null when Length is not 0");
>> +      assert(Rhs && "memcmp - Rhs should be non-null when Length is not 0");
>> +      return ::memcmp(Lhs,Rhs,Length);
>> +    }
>> +    
> 
> 
> Is this really necessary ? With the 2 asserts in the constructors we are making sure that StringRefs point to non-null or their length is zero, and calling memcmp with zero length is defined, no ?
> 
>  
> I removed the two asserts since we now guarantee that Length is 0 if Data is null.
> 
> I am afraid the check might be necessary, from n869 (a Draft of C99)
> 
> > [7.21.1  String function conventions]
> > [#2] [...] Unless  explicitly  stated otherwise  in  the  description  of a particular function in this subclause, pointer arguments on such a call shall still have valid values, as described in 7.1.4. [...]
> 
> [7.21.4.1  The memcmp function] does not state otherwise in any of its subclauses.
> 
> I've hit the bug on Suse with the memcpy function (on a memcpy(NULL, NULL, 0) call) and am now paranoid about it.

Ugh, that is good know.
But could you please rename 'memcmp' to something else (e.g. 'compareMemory') ? I understand it was the choice with the least amount of changes, but it is confusing, in general, to have member functions with the same name as standard library functions.

-Argyrios

> 
> 
> Thanks for reviewing this, I'll go working on this DiagnosticIDs.cpp issue.
> 
> -- Matthieu
> <llvm_stringref_undefined_behavior.diff>

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