Re: Potential static analysis ideas

[Hannes Reinecke <hare@suse.de>]

On 7/26/21 11:28 AM, Julia Lawall wrote:
>>>> Also, over 230 files contain functions that return both NULL and ERR_PTR.
>>>> A random example, chosen for conciseness, is the following from
>>>> fs/overlayfs/inode.c:
>>>>
>>>> struct inode *ovl_lookup_inode(struct super_block *sb, struct dentry *real,
>>>>                                  bool is_upper)
>>>> {
>>>>        struct inode *inode, *key = d_inode(real);
>>>>
>>>>           inode = ilookup5(sb, (unsigned long) key, ovl_inode_test, key);
>>>>        if (!inode)
>>>>                   return NULL;
>>>>
>>>>           if (!ovl_verify_inode(inode, is_upper ? NULL : real,
>>>>                                 is_upper ? real : NULL, false)) {
>>>>                   iput(inode);
>>>>                   return ERR_PTR(-ESTALE);
>>>>           }
>>>>
>>>>           return inode;
>>>> }
>>>>
>>> And that I would consider a coding error.
>>> If a function is able to return an error pointer it should _always_
>>> return an error pointer; here it would be trivial to return -ENXIO
>>> instead of NULL in the first condition.
>>>
>>> Not doing so is just sloppy programming IMO.
>>
>> In this case I agree.
> 
> I looked at the calling context, and it is not so obvious.  There are
> different behaviors in the two cases at both callsites.  It is like what
> Dan describes.  If the thing is not available, we just move on.  If it is
> available some action is needed.  If there is an actual error, some error
> handling is needed.
> 
But isn't 'not available' an error, too?
And isn't that exactly why we have individual errnos?

Why do we have to introduce different classes of errors (errno, NULL 
pointer), when we could have used a simple errno lik -ENXIO?
And, of course, documentation for that function outlining what exactly 
the meaning of the individual error numbers is.
But then we would need that anyway to clarify how the caller should 
interpret a 'NULL' return value.

So: not convinced :-)

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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