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SubjectRe: [GIT] Networking
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On Wed, Sep 2, 2015 at 10:35 PM, David Miller <davem@davemloft.net> wrote:
>
> Another merge window, another set of networking changes. I've heard
> rumblings that the lightweight tunnels infrastructure has been voted
> networking change of the year.

.. and others say that the most notable feature is the idiotic bugs
that it introduces, and the compiler even complains about.

Christ, people. Learn C, instead of just stringing random characters
together until it compiles (with warnings).

This:

static bool rate_control_cap_mask(struct ieee80211_sub_if_data *sdata,
struct ieee80211_supported_band *sband,
struct ieee80211_sta *sta, u32 *mask,
u8 mcs_mask[IEEE80211_HT_MCS_MASK_LEN])

is horribly broken to begin with, because array arguments in C don't
actually exist. Sadly, compilers accept it for various bad historical
reasons, and silently turn it into just a pointer argument. There are
arguments for them, but they are from weak minds.

But happily gcc has a really really valid warning (kudos - I often end
up ragging on the bad warnings gcc has, but this one is a keeper),
because a few lines down the mistake then turns into pure and utter
garbage.

It's garbage that was basically encouraged by the first mistake
(thinking that C allows array arguments), namely:

for (i = 0; i < sizeof(mcs_mask); i++)

the "sizeof(mcs_mask)" is _shit_. Since array arguments don't actually
exist in C, it is the size of the pointer, not the array. The first
mistake makes the bug look like reasonable code. Although I'd argue
that the code would actually be bad regardless, since "sizeof" is the
size in bytes, and the code actually wants the number of entries (and
we do have ARRAY_SIZE() for that).

Sure, in this case the entries are just one byte each, so it would
have *worked* had it not been for the array argument issue, but it's
misleading and the code is just fundamentally buggy and nonsensical in
two entirely different ways that fed on each other.

That line should read

for (i = 0; i < IEEE80211_HT_MCS_MASK_LEN; i++)

and the argument should just have been declared as the pointer it actually is.

A later patch then added onto the pile of manure by adding *another*
broken array argument, but at least that one then used the proper loop
for traversal of that array.

The fact that I notice this bug from a very basic "let's just compile
each pull request and make sure it isn't complete crap" is sad.

Now, it *looks* like the code was just moved, and the "sizeof()" was
initially correct (because it was a size of an actual array). Well, it
was "correct" in the sense that it generated the right code, even if
the whole confusion between "number of entries" and "size in bytes"
was still there. Then it got moved and turned from "confused but
happens to generate correct code" into "buggy pile of bovine manure".
See commit 90c66bd2232a ("mac80211: remove ieee80211_tx_rate
dependency in rate mask code").

So I can see how this bug happened, and I am only slightly upset with
Lorenzo who is the author of that commit.

What I can't see is why the code has existed in at least two
maintainer trees (Johannes' and David's) for a couple of weeks, and
nobody cared about the new compiler warnings? And it was sent to me
despite that new warning?

I realy want people to take a really hard look at functions that use
arrays as arguments. It really is very misleading, even if it can look
"prettier", and some people will argue that it's "documentation" about
how the pointer is a particular size. But it's neither. It's basically
just lying about what is going on, and the only thing it documents is
"I don't know how to C". Misleading documentation isn't documentation,
it's a mistake.

I see it in that file for at least the functions rate_idx_match_mask()
and rate_control_cap_mask(). I tried - and failed - to come up with a
reasonable grep pattern to try to see how common it is, and I'm too
lazy to add some sparse check for it.

Please people. When I see these kinds of obviously bogus code
problems, that just makes me very upset. Because it makes me worry
about all the non-obvious stuff that I miss. Sadly, this time I had
pushed out the merge early (because I wanted to test the wireless
changes on my laptop), so now the bug is out there.

I'm not sure what the practical *impact* of the bug is. Yes, it only
traverses four or eight rate entries (depending on 32-bit or
64-bitness of the kernel) out of the ten that it should. But maybe in
practice one of the first entries are always good enough matches. So
maybe _testing_ doesn't actually show this bug, but I sure wish people
just took compiler warnings more seriously (and were a lot more
careful about moving things to functions, and never ever used the
"function argument is an array" model).

Linus


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