Linux Ethernet Bridging - Nov 2018 - [Bridge] [PATCH][V2] net: bridge: remove redundant checks for null p->dev and p->br

From: Colin Ian King <colin.king at canonical.com>

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 at canonical.com>

---

V2: remove checks instead of moving them before the dereference of
p->dev.
---
 net/bridge/br_sysfs_if.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/net/bridge/br_sysfs_if.c b/net/bridge/br_sysfs_if.c
index 7c87a2fe5248..88715edb119a 100644
--- a/net/bridge/br_sysfs_if.c
+++ b/net/bridge/br_sysfs_if.c
@@ -320,9 +320,6 @@ static ssize_t brport_store(struct kobject *kobj,
 	if (!rtnl_trylock())
 		return restart_syscall();
 
-	if (!p->dev || !p->br)
-		goto out_unlock;
-
 	if (brport_attr->store_raw) {
 		char *buf_copy;
 
-- 
2.19.1

On 25/11/2018 18:08, Colin King wrote:
> From: Colin Ian King <colin.king at canonical.com>
> 
> 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 at canonical.com>
> 
> ---
> 
> V2: remove checks instead of moving them before the dereference of
> p->dev.
> ---
>  net/bridge/br_sysfs_if.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/net/bridge/br_sysfs_if.c b/net/bridge/br_sysfs_if.c
> index 7c87a2fe5248..88715edb119a 100644
> --- a/net/bridge/br_sysfs_if.c
> +++ b/net/bridge/br_sysfs_if.c
> @@ -320,9 +320,6 @@ static ssize_t brport_store(struct kobject *kobj,
>  	if (!rtnl_trylock())
>  		return restart_syscall();
>  
> -	if (!p->dev || !p->br)
> -		goto out_unlock;
> -
>  	if (brport_attr->store_raw) {
>  		char *buf_copy;
>  
> 
Thanks,
Acked-by: Nikolay Aleksandrov <nikolay at cumulusnetworks.com>

From: Colin King <colin.king at canonical.com>
Date: Sun, 25 Nov 2018 16:08:51 +0000
> From: Colin Ian King <colin.king at canonical.com>
> 
> 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 at canonical.com>
Applied.