[U-Boot] [PATCH V2 2/3] lib: uuid: add functions to generate UUID version 4

[Wolfgang Denk]

Dear Tom,

In message <20140313191814.GB16360 at bill-the-cat> you wrote:
> 
> > > +struct uuid {
> > > +	unsigned int time_low;
> > > +	unsigned short time_mid;
> > > +	unsigned short time_hi_and_version;
> > > +	unsigned char clock_seq_hi_and_reserved;
> > > +	unsigned char clock_seq_low;
> > > +	unsigned char node[6];
> > > +};
> > 
> > This struct starts with an uint, so it requires alignment on a 32 bit
> > boundary (i. e. an address that is a multiple of 4).
>
> And this needs to be marked as packed since we're using this as a direct
> representation of things on-disk.

Not really.  The arrangement of the elemnts makes sure that there are
no inter-element gaps, and even if we had arrays of such structs
(which we don;t have) it would work as the struct adds up to a
multiple of 32 bits.

> > > +void gen_rand_uuid(unsigned char *uuid_bin)
> > > +{
> > > +	struct uuid *uuid = (struct uuid *)uuid_bin;
> > 
> > Here you cast a pointer to the (unaligned) character buffer to a
> > struct buffer, which requires alignment.
>
> Potentially unaligned buffer.  There's only one caller thus far, and it
> will be aligned there.  We do need to comment that the pointer needs to
> be aligned.

No.  We should rather fix the code such that it works with any
alignment.  It is trivial here to use a struct uuid on the stack and
then use memcpy() to cpy the data from the struct to the buffer - no
casting needed anywhere, and no alignment concerns either.

> With the struct not packed, it'll be padded out so this works.  But
> looking at how we later use this as I say above, we do need to pack it,
> and then this will not be safe.  Some looping of strncpy into the char
> buffer, as a char so we whack the rand data in?

I can't see why we would not simply initialize the elements step by
step.  It's just such a small struct.


Best regards,

Wolfgang Denk

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