Re: [GIT PULL] dma-mapping fix for 5.10

[Geert Uytterhoeven]

Hi Linus,

On Sat, Oct 31, 2020 at 8:51 PM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Sat, Oct 31, 2020 at 2:40 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > dma-mapping fix for 5.10:
> >
> >  - fix an integer overflow on 32-bit platforms in the new DMA range code
> >    (Geert Uytterhoeven)
>
> So this is just a stylistic nit, and has no impact on this pull (which
> I've done). But looking at the patch, it triggers one of my "this is
> wrong" patterns.
>
> In particular, this:
>
>         u64 dma_start = 0;
>         ...
>         for (dma_start = ~0ULL; r->size; r++) {
>
> is actually completely bogus in theory, and it's a horribly horribly
> bad pattern to have.
>
> The thing that I hate about that parttern is "~0ULL", which is simply wrong.
>
> The correct pattern for "all bits set" is ~0. NOTHING ELSE. No extra
> letters at the end.
>
> Why? Because using an unsigned type is wrong, and will not extend the
> bits up to a potentially bigger size.
>
> So adding that "ULL" is not just three extra characters to type, it
> actually _detracts_ from the code and makes it more fragile and
> potentially wrong.
>
> It so happens, that yes, in the kernel, "ull" us 64-bit, and you get
> the right results. So the code _works_. But it's wrong, and it now
> requires that the types match exactly (ie it would not be broken if
> somebody ever were to say "I want to use use 128-bit dma addresses and
> u128").

Thanks, you're right, the "ULL" suffix is not needed, and could cause
future issues.

> Another example is using "~0ul", which would give different results on
> a 32-bit kernel and a 64-bit kernel. Again: DON'T DO THAT.

Definitely.

> I repeat: the right thing to do for "all bits set" is just a plain ~0
> or -1. Either of those are fine (technically assumes a 2's complement
> machine, but let's just be honest: that's a perfectly fine assumption,
> and -1 might be preferred by some because it makes that sign extension
> behavior of the integer constant more obvious).

"-1" definitely causes warnings, depending on your compiler (not with
the gcc 9.3.0 I'm currently using, though).

> Don't try to do anything clever or anything else, because it's going
> to be strictly worse.
>
> The old code that that patch removed was "technically correct", but
> just pointless, and actually shows the problem:
>
>         for (dma_start = ~(dma_addr_t)0; r->size; r++) {
>
> the above is indeed a correct way to say "I want all bits set in a
> dma_addr_t", but while correct, it is - once again - strictly inferior
> to just using "~0".

Obviously I was misled by the old code, and instead of changing
the cast, I replaced the cast ("casts are evil") by a suffix. Doh.

Any, I've just sent a patch. Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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