Re: [PATCH] lustre: check copy_from_iter/copy_to_iter return code

[Al Viro <viro@ZenIV.linux.org.uk>]

On Thu, Jul 13, 2017 at 10:57:59PM +0200, Arnd Bergmann wrote:

> Thanks for testing it!
> 
> That means we did not copy any data and the kernel continues with
> an uninitialized buffer, right? The problem may be the definition of
> 
> struct kib_immediate_msg {
>         struct lnet_hdr ibim_hdr;        /* portals header */
>         char         ibim_payload[0]; /* piggy-backed payload */
> } WIRE_ATTR;
> 
> The check that Al added will try to ensure that we don't write
> beyond the size of the ibim_payload[] array, which unfortunately
> is defined as a zero-byte array, so I can see why it will now
> fail. However, it's already broken in mainline now, with or without
> my patch.
> 
> Are you able to come up with a fix that avoids the warning in
> 'allmodconfig' and makes the function do something reasonable
> again?

Might make sense to try and use valid C99 for "array of indefinite
size as the last member", i.e.
struct kib_immediate_msg {
         struct lnet_hdr ibim_hdr;        /* portals header */
         char         ibim_payload[]; /* piggy-backed payload */
} WIRE_ATTR;

	Zero-sized array as the last member is gcc hack predating that;
looks like gcc gets confused into deciding that it knows the distance
from the end of object...

	Said that, are we really guaranteed the IBLND_MSG_SIZE bytes
in there?