linux-kernel - Re: rfc: treewide scripted patch mechanism? (was: Re: [PATCH] Makefile: Convert -Wimplicit-fallthrough=3 to just -Wimplicit-fallthrough for clang)QUILT

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Date:   Mon, 26 Aug 2019 10:58:39 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joe Perches <joe@...ches.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Julia Lawall <julia.lawall@...6.fr>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux@...glegroups.com,
        Linux Next Mailing List <linux-next@...r.kernel.org>
Subject: Re: rfc: treewide scripted patch mechanism? (was: Re: [PATCH]
 Makefile: Convert -Wimplicit-fallthrough=3 to just -Wimplicit-fallthrough for clang)QUILT

Hi Linus,

On Wed, Aug 21, 2019 at 2:41 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Tue, Aug 20, 2019 at 4:37 PM Joe Perches <joe@...ches.com> wrote:
> > > So I'm putting my foot down on yet another broken string copy
> > > interface from people who do not understand this fundamental issue.
> >
> > I think you are mistaken about the stracpy limits as
> > the only limit is not the source size but the dest.
> >
> > Why should the source be size limited?
>
> You just proved my point. You don't understand that sources can also
> be limited, and the limit on a source can be *smaller* than the limit
> of a destination.
>
> Did we learn *NOTHING* from the complete and utter disaster that was strlcpy()?
>
> Do you not understand why strlcpy() was unacceptably bad, and why the
> people who converted strncpy() to it introduced real bugs?
>
> The fact is, it's not just the destination that has a size limit. The
> source often has one too.
>
> And no, the source is not always guaranteed to be NUL-terminated, nor
> is the source buffer guaranteed to be larger than the destination
> buffer.
>
> Now, if you *know* that the source is smaller than the destination
> size, you can do:
>
>     len = strnlen(src, srclen);
>     memcpy(dst, len);
>     dst[len] = 0;
>
> and that's not wrong, but that works only when
>
>  (a) you actually do the above
>
>  (b) you have no data races on src (or you at least only require that
> 'dst' is NUL-terminated, not that 'len' is necessarily the correct
> length of the result
>
>  (c) you actually know as the programmer that yes, the source is
> definitely smaller than the destination.
>
> and honestly, people don't get _any_ of that right.

(d) you know the untouched trailing end of dst[] does not leak data.

Anything else we're missing?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds