Re: [PATCH][next] wifi: mac80211: ieee80211_i: Avoid dozens of -Wflex-array-member-not-at-end warnings

[Johannes Berg <johannes@sipsolutions.net>]

On Fri, 2024-10-25 at 15:10 -0600, Gustavo A. R. Silva wrote:
> 
> On 25/10/24 14:48, Johannes Berg wrote:
> > On Fri, 2024-10-25 at 14:36 -0600, Gustavo A. R. Silva wrote:
> > > > > 
> > > > > Yeah, I was actually going to mention this commit, as it's the one that introduced
> > > > > that `bool radar_detected` to the flex struct. However, it wasn't obvious to me
> > > > > how `struct ieee80211_chanctx_conf conf` could overwrite `radar_detected` as I didn't
> > > > > see `conf->drv_priv` being accessed through `struct struct ieee80211_chanctx_conf`.
> > > > 
> > > > You have to look at the drivers, see hwsim_clear_chanctx_magic() for
> > > > example; I wonder why hwsim_check_chanctx_magic() never caught this.
> > > 
> > > Sorry, I actually meant through `struct ieee80211_chanctx`. Something like:
> > > 
> > > struct ieee80211_chanctx *foo;
> > > ...
> > > 
> > > foo->conf.drv_priv[i] = something;
> > > 
> > > or
> > > 
> > > struct bar *ptr = (void *)foo->conf->drv_priv;
> > > then write something into *ptr...
> > > 
> > > In the above cases the code will indeed overwrite `radar_detected`.
> > 
> > Right, that's what it does though, no? Except it doesn't have, in the
> > driver, "foo->conf." because mac80211 only gives it a pointer to conf,
> > so it's only "conf->drv_priv" (and it's the "struct bar" example.)
> 
> OK, so do you mean that pointer to `conf` is actually coming from
> `foo->conf`?

Well depends what code you're looking at? I guess we should get more
concrete now. Let's say hwsim:

struct hwsim_chanctx_priv {
        u32 magic;
};

...

static inline void hwsim_set_chanctx_magic(struct ieee80211_chanctx_conf *c)
{
        struct hwsim_chanctx_priv *cp = (void *)c->drv_priv;
        cp->magic = HWSIM_CHANCTX_MAGIC;
}

probably shouldn't be marked 'inline' now that I look at it :-)

This is being called in hwsim itself, of course:

static int mac80211_hwsim_add_chanctx(struct ieee80211_hw *hw,
                                      struct ieee80211_chanctx_conf *ctx)
{
        hwsim_set_chanctx_magic(ctx);
...

which is only referenced as a function pointer in the ops:

static const struct ieee80211_ops mac80211_hwsim_mchan_ops = {
...
	.add_chanctx = mac80211_hwsim_add_chanctx,

(via some macros)


And that's called by mac80211:

static inline int drv_add_chanctx(struct ieee80211_local *local,
                                  struct ieee80211_chanctx *ctx)
{
        int ret = -EOPNOTSUPP;

        might_sleep();
        lockdep_assert_wiphy(local->hw.wiphy);

        trace_drv_add_chanctx(local, ctx);
        if (local->ops->add_chanctx)
                ret = local->ops->add_chanctx(&local->hw, &ctx->conf);


so you see that the struct ieee80211_chanctx is never passed to the
driver, but instead &ctx->conf, which is the struct with the flex array
for driver priv.

So in this example, struct hwsim_chanctx_priv::magic overlaps the
radar_detected value.


(The allocation happens via chanctx_data_size.)

johannes