Re: [PATCH][V2] net: bridge: remove redundant checks for null p->dev and p->br

From: David Miller
Date: Sun Nov 25 2018 - 13:26:20 EST

From: Colin King <colin.king@xxxxxxxxxxxxx>
Date: Sun, 25 Nov 2018 16:08:51 +0000

> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> A recent change added a null check on p->dev after p->dev was being
> dereferenced by the ns_capable check on p->dev. It turns out that
> neither the p->dev and p->br null checks are necessary, and can be
> removed, which cleans up a static analyis warning.
> As Nikolay Aleksandrov noted, these checks can be removed because:
> "My reasoning of why it shouldn't be possible:
> - On port add new_nbp() sets both p->dev and p->br before creating
> kobj/sysfs
> - On port del (trickier) del_nbp() calls kobject_del() before call_rcu()
> to destroy the port which in turn calls sysfs_remove_dir() which uses
> kernfs_remove() which deactivates (shouldn't be able to open new
> files) and calls kernfs_drain() to drain current open/mmaped files in
> the respective dir before continuing, thus making it impossible to
> open a bridge port sysfs file with p->dev and p->br equal to NULL.
> So I think it's safe to remove those checks altogether. It'd be nice to
> get a second look over my reasoning as I might be missing something in
> sysfs/kernfs call path."
> Thanks to Nikolay Aleksandrov's suggestion to remove the check and
> David Miller for sanity checking this.
> Detected by CoverityScan, CID#751490 ("Dereference before null check")
> Fixes: a5f3ea54f3cc ("net: bridge: add support for raw sysfs port options")
> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>