[Linux-ha-dev] Bind/stat Auth Patch

[Alan Robertson]

Lars Marowsky-Bree wrote:
> On 2004-10-06T23:03:46, Andrew Beekhof <lists at beekhof.net> wrote:
> 
> 
>>This SUN_LEN macro looks evil to me, mostly because I dont understand 
>>it, particularly the dereferencing 0 bit.
>>
>>Can anyone (in particular Alan or lmb) verify it is safe?
>>
>>Alternatively, Matthew also found this variant:
>>
>> #define SUN_LEN(su) (sizeof(*(su)) - sizeof((su)->sun_path) + 
>>strlen((su)->sun_path))

This macro assumes two things:
	1) that sun_path is last thing in the structure
	2) That sun_path does not require any more stringent
		alignment than the thing just in front of it

> Dereferencing NULL doesn't sound safe, but it's an awkward place to
> check for NULL - what should the macro do, return 0? Better check the
> pointer beforehand...

It isn't dereferencing it BECAUSE sun_path is an array.
The meaning of -> when the thing after the operator is an array is more 
like &(((struct foo*)0)->sun_path), which keeps it from actually doing the 
indirection on 0.

So, what this construct is doing is computing the offset of sun_path in the 
structure.

I think the following code for computing structure member offset is much 
clearer, and works on every platform, regardless of the data type of 
sun_path, or it's position in the array.


	static ssize_t sunpathoffset = -1;
	if (sunpathoffset < 0) {
	        struct foo x;
	        sunpathoffset = ((char*)(&(x.sun_path))) - ((char*) &x);
	}

This is how to properly compute the offset of a structure element.  It is 
perfectly clear, making neither programmers nor program verifiers sick to 
their stomachs.

What they've done in the macro is eliminated the variable, and replaced it 
with a pointer to zero, which works fine, because they're just computing 
relative addresses, and not actually indirecting (though it looks like they 
are because of the data type of sun_path).  But, all the discussion here 
points out it's big flaw:  it's unclear.

-- 
     Alan Robertson <alanr at unix.sh>

"Openness is the foundation and preservative of friendship...  Let me claim 
from you at all times your undisguised opinions." - William Wilberforce