Secure Dereference of NULL-Pointer when using list.h

[Hendrik Post <hendrik@xxxxxxxxxxxx>]

Hi,

While studying the Infiniband drivers if found an interesting line of 
code within the preprocessed files from kernel 2.6.14.1:

// 85 "drivers/infiniband/core/mad.c"
entry = (struct ib_mad_port_private *) ((char *)__mptr - (unsigned int) 
(&((struct ib_mad_port_private *)0) ->port_list) );

This line is the result of a macro expansion from list.h. I wonder why 
it is safe to dereference the NULL-pointer in the second part of the 
right side:

(& ((struct ib_mad_port_private *)0) ->port_list) )

GCC translates the expression into the offset of the field port_list as 
intended, but doing this involves dereferencing a NULL-pointer 
(evaluates to 0). The expression can only be correct if one assumes that 
the dereference will be optimized away - why is it safe to assume that?

Any hints would be appreciated!

Regards,

Hendrik

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