The month Zack Brown reports on…

The month Zack Brown reports on wonky typecasting.

Wonky Typecasting

Tinkering with strange forces, Steven Rostedt felt that a particular struct in the kernel should be tucked away into a private file, and the parts of the kernel using that struct should only be given a pointer to it. However, some (but not all) compilers started to complain about those in-kernel users trying to dereference the pointer (i.e., to see what it pointed to, which Steven had rendered hidden and inaccessible).

However, it was a little more complicated than that. The code that dereferenced the pointer wasn’t targeting this specific pointer; it would simply dereference any pointer that was handed to it. But in this case, the struct’s pointer was a secret hidden struct and was thus different from the types of pointers that code was usually handed, so … boom.

To fix this, Steven went through a big pile of kernel code and changed a lot of pointer declarations so that the dereferencing code wouldn’t complain anymore. He didn’t change it to be more accurate; he just changed it so the dereferencing process itself wouldn’t care what was at the end of its request.

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