RE: [PATCH 01/33] libsepol: basic serilization support

[Karl MacMillan <kmacmillan@mentalrootkit.com>]

On Wed, 2007-04-25 at 10:14 -0400, Joshua Brindle wrote:
> > From: Karl MacMillan [mailto:kmacmillan@mentalrootkit.com] 

<snip>

> > > 
> > > SEPOL_SERIAL_SIZE_T is always 64 bits. We should be checking if it 
> > > tries to serialize into a smaller type, if not that's a bug.
> > > 
> > 
> > How can that be? On a 64bit platform I can represent a size 
> > that can't be represented by a 32bit size_t. How can I 
> > transfer this data to a 32bit system? I can't put it in a 
> > data structure with size_t - it would have to be unint64_t. 
> > So why not just use that?
> > 
> 
> That's why I said "We should be checking if it tries to serialize into a
> smaller type, if not that's a bug"
> 
> IIRC we needed something that could serialize/unserialize size_t because
> its used in some components that we serialize. We could always cast to
> uint64_t and serialize it that way (which is pretty much what we are
> doing anyway). 
> 

Something is going wrong in our communication here. You _cannot_
reliably transfer size_t from a 64bit system to a 32bit system (assuming
that size_t is 64 and 32 bits respectively). That means that you cannot
reliably serialize data structures that use size_t - you must change the
data structures. Otherwise the serialized data will not be valid on all
systems.

There is no way to check "if it tries to serialize into a smaller type"
- you will always hit this condition at some point.

As for the size, C99 says:

"The value of the result [of sizeof] is implementation-defined, and its
type (an unsigned integer type) is size_t,"

Later it says:

"The types used for size_t and ptrdiff_t should not have an integer
conversion rank greater than that of signed long int unless the
implementation supports objects large enough to make this necessary."

I read that to say that the width of size_t is implementation defined
and could be larger that 64 bits.

Karl


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