netdev - Re: [PATCH 12/20] maccess: remove strncpy_from

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Date:   Tue, 19 May 2020 18:41:46 +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 12/20] maccess: remove strncpy_from_unsafe

On Tue, May 19, 2020 at 09:25:57AM -0700, Linus Torvalds wrote:
> On Tue, May 19, 2020 at 6:45 AM Christoph Hellwig <hch@....de> wrote:
> >
> > +       if (IS_ENABLED(CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE) &&
> > +           compat && (unsigned long)unsafe_ptr < TASK_SIZE)
> > +               ret = strncpy_from_user_nofault(dst, user_ptr, size);
> > +       else
> > +               ret = strncpy_from_kernel_nofault(dst, unsafe_ptr, size);
> 
> These conditionals are completely illegible.

I had a lot of folks complaining about things like:

#ifdef CONFIG_FOO
	if (foo)
		do_stuff();
	else
#endif
		do_something_else();

which I personally don't mind at all, so I switched to this style.

>   static long bpf_strncpy_from_legacy(void *dest, const void
> *unsafe_ptr, long size, bool legacy)
>   {
>   #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>         if (legacy && addr < TASK_SIZE)
>             return strncpy_from_user_nofault(dst, (const void __user
> *) unsafe_ptr, size);
>   #endif
> 
>         return strncpy_from_kernel_nofault(dst, unsafe_ptr, size);
>   }
> 
> and then you'd just use
> 
>         if (bpf_strncpy_from_unsafe(dst, unsafe_ptr, size, compat) < 0)
>                 memset(dst, 0, size);
> 
> and avoid any complicated conditionals, goto's, and make the code much
> easier to understand thanks to having a big comment about the legacy
> case.

Sure.

> In fact, separately I'd probably want that "compat" naming to be
> scrapped entirely in that file.

Not my choice..  Maybe Daniel has a recommendation of what to change
there, but I'd love to not have to deal with that renaming as well
in this series.