Re: [PATCH] net: ethtool: avoid allocation failure for dump_regs

[David Arcari <darcari@redhat.com>]

On 01/18/2017 11:45 AM, David Miller wrote:
> From: David Arcari <darcari@redhat.com>
> Date: Wed, 18 Jan 2017 08:34:05 -0500
>
>> If the user executes 'ethtool -d' for an interface and the associated
>> get_regs_len() function returns 0, the user will see a call trace from
>> the vmalloc() call in ethtool_get_regs().  This patch modifies
>> ethtool_get_regs() to avoid the call to vmalloc when the size is zero.
>>
>> Signed-off-by: David Arcari <darcari@redhat.com>
> I think when the driver indicates this, it is equivalent to saying that
> the operation isn't supported.
>
> Also, this guards us against ->get_regs() methods that don't handle
> zero length requests properly.  I see many which are going to do
> really terrible things in that situation.
>
> Therefore, if get_regs_len() returns zero, treat it the safe as if the
> ethtool operations were NULL.
>
> Thanks.

That was actually the fix that I was originally considering, but it
turns out
there is a problem with it.

I found that the vmalloc error was occurring because
ieee80211_get_regs_len() in
net/mac80211/ethtool.c was returning zero.  The ieee80211_get_regs in
the same
file returns the hw version. It turns out that this information is used
by the
at76c50x-usb driver in the user space ethtool to report which HW variant
is in
use.  Returning an error when regs_len() returns zero would break this
functionality.

-Dave