netdev - Re: [PATCH 11/20] bpf: factor out a bpf_trace_copy

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Date:   Tue, 19 May 2020 18:14:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christoph Hellwig <hch@....de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org,
        linux-um <linux-um@...ts.infradead.org>,
        Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/20] bpf: factor out a bpf_trace_copy_string helper

On Tue, May 19, 2020 at 09:07:55AM -0700, Linus Torvalds wrote:
> On Tue, May 19, 2020 at 6:45 AM Christoph Hellwig <hch@....de> wrote:
> >
> > +       switch (fmt_ptype) {
> > +       case 's':
> > +#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
> > +               strncpy_from_unsafe(buf, unsafe_ptr, bufsz);
> > +               break;
> > +#endif
> > +       case 'k':
> > +               strncpy_from_kernel_nofault(buf, unsafe_ptr, bufsz);
> > +               break;
> 
> That 's' case needs a "fallthrough;" for the overlapping case,
> methinks. Otherwise you'll get warnings.

I don't think we need it as the case of

	case 'a':
	case 'b':
		do_stuff();
		break;

has always been fine even with the fallthough warnings.  And the
rest of the stuff gets removed by cpp..