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Date: Tue, 21 Feb 2023 10:25:05 -0800
From: Ralph Little <skelband@...il.com>
To: musl@...ts.openwall.com
Subject: Re: Re: [BUG] ioctl: overflow in implicit constant conversion
Hi,
On Tue, Feb 21, 2023 at 8:58 AM Jeffrey Walton <noloader@...il.com> wrote:
> On Tue, Feb 21, 2023 at 11:05 AM Markus Wichmann <nullplan@....net> wrote:
> > On Mon, Feb 20, 2023 at 09:26:05PM -0800, Ralph Little wrote:
> > > I have been picking up some old pending issues related to the SANE
> project.
> > > One of our CI builds is on Alpine and it is generating warnings for
> ioctl()
> > > calls from the musl library:
> > >
> > > |error: overflow in conversion from 'long unsigned int' to 'int'
> changes
> > > value from '2147577985' to '-2147389311' [-Werror=overflow]
> > > |
> > > ||ioctl (fd, PPRSTATUS, &status);
> > >
> > > ||I see that Olaf Meeuwissen raised this issue a couple of years ago
> and the
> > > discussion petered out somewhat and I don't believe that the issue was
> ever
> > > really resolved:
> > >
> > > https://www.openwall.com/lists/musl/2020/01/20/2
> > >
> > > Is there any possibility that this could be addressed in the near
> future?
> > > I see that Alpine have closed their issue and are not interested in
> patching
> > > their downstream musl:
> > >
> > > https://gitlab.alpinelinux.org/alpine/aports/-/issues/7580#note_287168
> > [...]
> > So, I had a look at it. As far as I can tell, the issue is that musl
> > declares ioctl()'s second argument to be an int. Together with the other
> > defintions, this means that any _IOC_READ constant will overflow and
> > generate those warnings. Also, this is technically undefined behavior,
> > as value bits are shifted into the sign bit of a signed integer.
> >
> > Linux itself defines the ioctl syscall to have a second argument of type
> > unsigned int.
> >
> > So this issue could be resolved by simply making the second argument of
> > the ioctl() function unsigned. Does that create ABI issues? To my
> > knowledge, all ABIs pass ints and unsigned ints the same way. Even if on
> > some 64-bit arch there was a sign extension at the top, only the low
> > 32 bits are defined.
>
> In this case, I think the best course of action is to cast a,b,c to
> unsigned, then perform the shifts, and finally cast back to int. That
> is what the C standard requires. And it should not mess with the ABI.
>
> If the code remains undefined, then it is subject to removal by the
> compiler. The casts, while ugly, keep the code in well defined
> territory. Also, if anyone ever performs testing with
> -fsantize=undefined, then the code will trigger real findings that
> could keep the code from passing through a security gate (for those
> folks who have to work in that kind of environment).
>
> I've had to work bug reports that were a result of the missing casts
> during shifts and rotates. It is not fun. I was able to track all of
> them down with -fsantize=undefined .
>
> Jeff
>
That would be cool by me. I hate making macros complicated but I also
understand the POSIX issue.
Seems a shame that POSIX is so out of step these days with actual
implementations. Perhaps that will change in the future....
Cool to be reminded of the GCC santize settings. I had forgotten about
those.
Cheers,
Ralph
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