.. _phk_cheri_6:
How Varnish met CHERI 6/N
=========================
Varnish Socket Addresses
------------------------
Socket addresses are a bit of a mess, in particular because nobody
dared shake up all the IPv4 legacy code when IPv6 was introduced.
In varnish we encapsulate all that ugliness in a ``struct suckaddr``,
so named because it sucks that we have to spend time and code on this.
In a case like this, it makes sense to make the internals strictly
read-only, to ensure nobody gets sneaky ideas:
.. code-block:: none
struct suckaddr *
VSA_Build(void *d, const void *s, unsigned sal)
{
struct suckaddr *sua;
[¡ lots of ugly stuff ¡]
return (RO(sua));
}
It would then seem logical to use C's ``const`` to signal this fact,
but since the current VSA api is currently defined such that users
call ``free(3)`` on the suckaddrs when they are done with them, that does
not work, because the prototype for ``free(3)`` is:
.. code-block:: none
void free(void?*ptr);
So you cannot call it with a ``const`` pointer.
All hail the ISO-C standards committee!
This brings me to a soft point with CHERI: Allocators.
How to free things with CHERI
-----------------------------
A very common design-pattern in encapsulating classes look something
like this:
.. code-block:: none
struct real_foo {
struct foo foo;
[some metadata about foo]
};
const struct foo *
Make_Foo([arguments])
{
struct real_foo *rf;
rf = calloc(1, sizeof *rf);
if (rf == NULL)
return (rf);
[fill in rf]
return (&rf->foo);
}
void
Free_Foo(const struct foo **ptr)
{
const struct foo *fp;
struct real_foo *rfp;
assert(ptr != NULL);
fp = *ptr;
assert(fp != NULL);
*ptr = NULL;
rfp = (struct real_foo*)((uintptr_t)fp);
[clean stuff up]
}
We pass a ``**ptr`` to ``Free_Foo()``, another varnish style-pattern,
so we can NULL out the holding variable in the calling function,
to avoid a dangling pointer to the now destroyed object from
causing any kind of trouble later.
In the calling function this looks like:
.. code-block:: none
const struct foo *foo_ptr;
[¡]
Free_Foo(&foo_ptr);
If we use CHERI to make the foo truly ``const`` for the users of
the API, we cannot, as above, wash the ``const`` away with a trip through
``uintptr_t`` and then write to the metadata.
The CHERI C/C++ manual, a terse academic tome, laconically mention that:
*?Therefore, some additional work may be required to derive
a pointer to the allocation¡¯s metadata via another global capability,
rather than the one that has been passed to free().?*
Despite the understatement, I am very much in favour of this, because
this is *precisely* why my own
`phkmalloc `_
became a big hit twenty years ago: By firmly separating the metadata
from the allocated space, several new classes of mistakes using the
``malloc(3)`` API could, and were, detected.
But this *is* going to be an embuggerance for CHERI users, because
with CHERI getting from one pointer to different one is actual work.
The only "proper" solution is to build some kind of datastructure:
List, tree, hash, DB2 database, pick any poison you prefer, and
search out the metadata pointer using the impotent pointer as key.
Given that CHERI pointers are huge, it may be a better idea to embed
a numeric index in the object and use that as the key.
An important benefit of this ?additional work? is that if your
free-function gets passed a pointer to something else, you will
find out, because it is not in your data-structure.
It would be a good idea if CHERI came with a quality implementation
of "Find my other pointer", so that nobody is forced to crack The
Art of Computer Programming open for this.
When the API is "fire-and-forget" like VSA, in the sense that there
is no metadata to clean up, we can leave the hard work to the
``malloc(3)`` implementation.
Ever since ``phkmalloc`` no relevant implementation of ``malloc(3)``
has dereferenced the freed pointer, in order to find the metadata
for the allocation. Despite its non-const C prototype ``free(3)``,
will therefore happily handle a ``const`` or even CHERIed read-only
pointer.
But you *will* have to scrub the ``const`` off with a ``uintptr_t``
to satisfy the C-compiler:
.. code-block:: none
void
VSA_free(const struct suckaddr **vsap)
{
const struct suckaddr *vsa;
AN(vsap);
vsa = *vsap;
*vsap = NULL;
free((char *)(uintptr_t)vsa);
}
Or in varnish style:
.. code-block:: none
void
VSA_free(const struct suckaddr **vsap)
{
const struct suckaddr *vsa;
TAKE_OBJ_NOTNULL(vsa, vsap, SUCKADDR_MAGIC);
free(TRUST_ME(vsa));
}
Having been all over this code now, I have decided to constify ``struct
suckaddr`` in varnish, even without CHERI, it is sounder that way.
It is not bug, but CHERI made it a lot simpler and faster for me
to explore the consequences of this change, so I will forfeit
a score of "half a bug" to CHERI at this point.
*/phk*
Henceforth, whatever our philosopher says about Matter will apply to extension and to extension alone. It cannot be apprehended by sight, nor by hearing, nor by smell, nor by taste, for it is neither colour, nor sound, nor odour, nor juice. Neither can it be touched, for it is not a body, but it becomes corporeal on being blended with sensible qualities. And, in a later essay, he describes it as receiving all things and letting them depart again without retaining the slightest trace of their presence.483 Why then, it may be asked, if Plotinus meant extension, could he not say so at once, and save us all this trouble in hunting out his meaning? There were very good reasons why he should not. In the first place, he wished to express himself, so far as possible, in Aristotelian phraseology, and this was incompatible with the reduction of Matter to extension. In the next place, the idea of an infinite void had been already appropriated by the Epicureans, to whose system he was bitterly opposed. And, finally, the extension of ordinary327 experience had not the absolute generality which was needed in order to bring Matter into relation with that ultimate abstraction whence, like everything else, it has now to be derived. That the millionaire was genuine, ¡°in person and not a caricature,¡± as Dick put it, was evident. Both the nurse, his relative, and his wife, were chatting with him as Jeff delivered the heavy packed ball made up of the gum. 233 "I guess not," said Landor, tolerantly, as he turned[Pg 106] his horse over to his orderly; "but, anyway," he added to Ellton, "we had a picnic¡ªof a sort." Si, unable to think of anything better, went with him. The train had stopped on a switch, and seemed likely to rust fast to the rails, from the way other trains were going by in both directions. The bridge gang, under charge of a burly, red-faced young Englishman, was in the rear car, with their tools, equipments, bedding and cooking utensils. THE DEACON HAS SOME EXPERIENCES WITH THE QUADRUPED. "You are not within a mile of the truth. I know it. Look here: I believe that is Gen. Rosecrans's own cow. She's gone, and I got an order to look around for her. I've never seen her, but from the description given me I believe that's she. Who brought her here?" "Deacon, these brothers and sisters who have come here with me to-night are, like myself, deeply interested in the moral condition of the army, where we all have sons or kinsmen. Now, can't you sit right there and tell us of your observations and experiences, as a Christian man and father, from day to day, of every day that you were down there? Tell us everything, just as it happened each day, that we may be able to judge for ourselves." HAS AN ENCOUNTER WITH THE PROVOST-MARSHAL. "Wonder which one o' them is the 200th Injianny's?" said Si to Shorty. "And your mother, and Harry?" The daughter must be the girl who was talking to him now. She sat on a little stool by the fire, and had brought out some sewing. "Over at Grandturzel¡ªcan't see wot's burning from here. Git buckets and come!" These things, however, gave little concern to the worthy who commanded the Kentish division. Tyler, though an excellent blacksmith, possessed few of the qualities requisite for forming a good general. Provided there was no very sensible diminution in the number of his followers, he cared not a straw for the score or two who, after quarrelling, or perhaps fighting, withdrew in such disgust that they vowed rather to pay the full tax for ever than submit to the insolence of the rebels. One man could fight as well as another, reasoned he; and, provided he was obeyed, what mattered it by whom. Dick went and Tom came¡ªit was sure to be all one in the end. But this burst of indignation soon passed away, and upon the suggestion of the prudent Sir Robert Hailes, he sent an evasive answer, with a command that the Commons should attend him at Windsor on the Sunday following. That it was a stratagem to gain entrance to the Tower, was the opinion of several, but, after much discussion, it was decided that the man should be admitted, and that the monk should be exhibited merely to intimidate the rebels, until the result of this promised communication should be known. HoMEŮͬÐÔÁµcbcb
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