[PATCH 00/14] Make the user mode driver code a better citizen

[Tetsuo Handa]

On 2020/06/30 5:19, Eric W. Biederman wrote:
> Tetsuo Handa <penguin-kernel at i-love.sakura.ne.jp> writes:
> 
>> On 2020/06/29 4:44, Alexei Starovoitov wrote:
>>> But all the defensive programming kinda goes against general kernel style.
>>> I wouldn't do it. Especially pr_info() ?!
>>> Though I don't feel strongly about it.
>>
>> Honestly speaking, caller should check for errors and print appropriate
>> messages. info->wd.mnt->mnt_root != info->wd.dentry indicates that something
>> went wrong (maybe memory corruption). But other conditions are not fatal.
>> That is, I consider even pr_info() here should be unnecessary.
> 
> They were all should never happen cases.  Which is why my patches do:
> if (WARN_ON_ONCE(...))

No. Fuzz testing (which uses panic_on_warn=1) will trivially hit them.
This bug was unfortunately not found by syzkaller because this path is
not easily reachable via syscall interface.

> 
> That let's the caller know the messed up very clearly while still
> providing a change to continue.
> 
> If they were clearly corruption no ones kernel should ever continue
> BUG_ON would be appropriate.

Please use BUG_ON() (to only corruption case) like I suggested in my updated diff.