Re: [PATCH v5 2/4] kernel hacking: new config NO_AUTO_INLINE to disable compiler auto-inline optimizations

Johan Hovold
2018-06-07T08:06:54Z

On Thu, Jun 07, 2018 at 09:47:18AM +0530, Viresh Kumar wrote:

> On 06-06-18, 14:26, Steven Rostedt wrote:
> > On Wed, 6 Jun 2018 16:26:00 +0200
> > Johan Hovold <johan@kernel.org> wrote:
> >
> > > Looks like the greybus code above is working as intended by checking for
> > > unterminated string after the strncpy, even if this does now triggers
> > > the truncation warning.
>
> So why exactly are we generating a warning here ? Is it because it is possible
> that the first n bytes of src may not have the null terminating byte and the
> dest may not be null terminated eventually ?

Yes, new warning in GCC 8:

https://gcc.gnu.org/onlinedocs/gcc-8.1.0/gcc/Warning-Options.html#index-Wstringop-truncation

> Maybe I should just use memcpy here then ?

No, as you note below, you use strncpy to clear the rest of the buffer.

> But AFAIR, I used strncpy() specifically because it also sets all the remaining
> bytes after the null terminating byte with the null terminating byte. And so it
> is pretty easy for me to check if the final string is null terminated by
> checking [max - 1] byte against '\0', which the code is doing right now.
>
> I am not sure what would the best way to get around this incorrect-warning.

It seems gcc just isn't smart enough in this case (where you check for
overflow and never use a non-terminated string), but it is supposed to
detect when the string is unconditionally terminated. So perhaps just
adding a redundant buf[size-1] = '\0' before returning in the error path
or after the error path would shut it up. But that's a bit of a long
shot, I admit.

Probably best to leave things as they are, and let the gcc folks find a
way to handle such false positives.

Thanks,
Johan