.. _phk_cheri_5:
How Varnish met CHERI 5/N
=========================
Varnish Workspaces
------------------
To process a HTTP request or response, varnish must allocate bits
of memory which will only be used for the duration of that processing,
and all of it can be released back at the same time.
To avoid calling ``malloc(3)`` a lot, which comes with a locking
overhead in a heavily multithreaded process, but even more to
avoid having to keep track of all these allocations in order to be able
to ``free(3)`` them all, varnish has "workspaces":
.. code-block:: none
struct ws {
[¡]
char *s; /* (S)tart of buffer */
char *f; /* (F)ree/front pointer */
char *r; /* (R)eserved length */
char *e; /* (E)nd of buffer */
};
The ``s`` pointer points at the start of a slab of memory, owned
exclusively by the current thread and ``e`` points to the end.
Initially ``f`` is the same as ``s``, but as allocations are made
from the workspace, it moves towards ``e``. The ``r`` pointer is
used to make "reservations", we will ignore that for now.
Workspaces look easy to create:
.. code-block:: none
ws->s = space;
ws->e = ws->s + len;
ws->f = ws->s;
ws->r = NULL;
¡ only, given the foot-shooting-abetting nature of the C language,
we have bolted on a lot of seat-belts:
.. code-block:: none
#define WS_ID_SIZE 4
struct ws {
unsigned magic;
#define WS_MAGIC 0x35fac554
char id[WS_ID_SIZE]; /* identity */
char *s; /* (S)tart of buffer */
char *f; /* (F)ree/front pointer */
char *r; /* (R)eserved length */
char *e; /* (E)nd of buffer */
};
void
WS_Init(struct ws *ws, const char *id, void *space, unsigned len)
{
unsigned l;
DSLb(DBG_WORKSPACE,
"WS_Init(%s, %p, %p, %u)", id, ws, space, len);
assert(space != NULL);
assert(PAOK(space));
INIT_OBJ(ws, WS_MAGIC);
ws->s = space;
l = PRNDDN(len - 1);
ws->e = ws->s + l;
memset(ws->e, WS_REDZONE_END, len - l);
ws->f = ws->s;
assert(id[0] & 0x20); // cheesy islower()
bstrcpy(ws->id, id);
WS_Assert(ws);
}
Let me walk you through that:
The ``DSLb()`` call can be used to trace all operations on the
workspace, so we can see what actually goes on.
(Hint: Your ``malloc(3)`` may have something similar,
look for ``utrace`` in the manual page.)
Next we check the provided space pointer is not NULL, and
that it is properly aligned, these are both following
a varnish style-pattern, to sprinkle asserts liberally,
both as code documentation, but also because it allows
the compiler to optimize things better.
The ``INIT_OBJ() and ``magic`` field is a style-pattern
we use throughout varnish: Each structure is tagged with
a unique magic, which can be used to ensure that pointers
are what we are told, when they get passed through a ``void*``.
We set the ``s`` pointer.
We calculate a length at least one byte shorter than what
we were provided, align it, and point ``e`` at that.
We fill that extraspace at and past ``e``, with a "canary" to
stochastically detect overruns. It catches most but not
all overruns.
We set the name of the workspace, ensuring it is not already
marked as overflowed.
And finally check that the resulting workspace complies with
the defined invariants, as captured in the ``WS_Assert()``
function.
With CHERI, it looks like this:
.. code-block:: none
void
WS_Init(struct ws *ws, const char *id, void *space, unsigned len)
{
unsigned l;
DSLb(DBG_WORKSPACE,
"WS_Init(%s, %p, %p, %u)", id, ws, space, len);
assert(space != NULL);
INIT_OBJ(ws, WS_MAGIC);
assert(PAOK(space));
ws->s = cheri_bounds_set(space, len);
ws->e = ws->s + len
ws->f = ws->s;
assert(id[0] & 0x20); // cheesy islower()
bstrcpy(ws->id, id);
WS_Assert(ws);
}
All the gunk to implement a canary to detect overruns went
away, because with CHERI we can restrict the ``s`` pointer so writing
outside the workspace is *by definition* impossible, as long as your
pointer is derived from ``s``.
Less memory wasted, much stronger check and more readable source-code,
what's not to like ?
When an allocation is made from the workspace, CHERI makes it possible
to restrict the returned pointer to just the allocated space:
.. code-block:: none
void *
WS_Alloc(struct ws *ws, unsigned bytes)
{
char *r;
[¡]
r = ws->f;
ws->f += bytes;
return(cheri_bounds_set(r, bytes));
}
Varnish String Buffers
----------------------
Back in the mists of time, Dag-Erling Sm?rgrav and I designed a
safe string API called ``sbuf`` for the FreeBSD kernel.
The basic idea is you set up your buffer, you call functions to stuff
text into it, and those functions do all the hard work to ensure
you do not overrun the buffer. When the string is complete, you
call a function to "finish" the buffer, and if returns a flag which
tells you if overrun (or other problems) happened, and then you can
get a pointer to the resulting string from another function.
Varnish has adopted sbuf's under the name ``vsb``. This should
really not surprise anybody: Dag-Erling was also involved
in the birth of varnish.
It should be obvious that internally ``vsb`` almost always operate
on a bigger buffer than the result, so this is another obvious
place to have CHERI cut a pointer down to size:
.. code-block:: none
char *
VSB_data(const struct vsb *s)
{
assert_VSB_integrity(s);
assert_VSB_state(s, VSB_FINISHED);
return (cheri_bounds_set(s->s_buf, s->s_len + 1));
}
Still no bugs though.
*/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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