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Re: Linux "NULL pointer dereferece" in the News...



Rahul Sundaram wrote,
On 07/19/2009 11:31 PM, Tom Horsley wrote,
> On Sun, 19 Jul 2009 23:11:52 +0530
> Rahul Sundaram wrote,
>
>> It is not so simple. This is not a compiler bug. I suggest you read
>> through http://lwn.net/Articles/341773/rss to understand why.
>
> I did. It is a compiler bug no matter what a bunch of language lawyer
> holier than thou compiler developers say :-).

The kernel developers claim it is a kernel bug and the compiler
developers claim it is not their bug and this everybody agrees with and yet you don't agree with all of them? Who is being holier than thou here?

Kernel first dereferences a pointer, and after that checks whether it's NULL. It is quite common as a compiler optimization to compile out code
like this.

Rahul


I vaguely remember some of the reasons (mainly the difficulty of untangling the semantics of pointer tests), but, I must admit, after using Java long enough, it seems odd to me that the C compiler would be smart enough to catch the theoretical "don't care" and not smart enough to distinguish between the reasons for the don't care.

I think I remember, way back when, using C compilers that would warn about code that isn't reached and is discarded, and even warn about code that dereferences a pointer before testing it after the pointer is returned by a function call.

So it definitely feels like both a coding error and a (design class?) bug in the way the compiler options interact.

It would be nice if certain combinations of options would at least issue "potential evil compiler switch combination" warnings before a compile started.

Maybe it does already, and the warnings are being swallowed in the make process?


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