linux-kernel - Re: [PATCH] sh: Implement __get_user_u64() required for 64-bit get

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Date:   Sun, 31 May 2020 12:43:11 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>
Cc:     Linux-sh list <linux-sh@...r.kernel.org>,
        Rich Felker <dalias@...c.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Michael Karcher <kernel@...rcher.dialup.fu-berlin.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sh: Implement __get_user_u64() required for 64-bit get_user()

Hi Adrian,

On Sun, May 31, 2020 at 11:59 AM John Paul Adrian Glaubitz
<glaubitz@...sik.fu-berlin.de> wrote:
> On 5/31/20 11:54 AM, John Paul Adrian Glaubitz wrote:
> > On 5/31/20 11:52 AM, Geert Uytterhoeven wrote:
> >> As this is the 64-bit variant, I think this single move should be
> >> replaced by a double move:
> >>
> >>        "mov  #0,%R1\n\t" \
> >>        "mov  #0,%S1\n\t" \
> >>
> >> Same for the big endian version below.
> >>
> >> Disclaimer: uncompiled, untested, no SH assembler expert.
> >
> > Right, this makes sense. I'll send a new patch shortly.
>
> Hmm, this change is not the case for __put_user_asm() vs. __put_user_u64().
> But I have to admit, I don't know what the part below "3:\n\t" is for.

It's part of the exception handling, in case the passed (userspace) pointer
points to an inaccessible address, and triggers an exception.

For an invalid store, nothing is done, besides returning -EFAULT.
Hence there's no "mov #0, %1\n\t" in the put_user case.
For an invalid load, the data is replaced by zero, and -EFAULT is returned.

> +__asm__ __volatile__( \
> +       "1:\n\t" \
> +       "mov.l  %2,%R1\n\t" \
> +       "mov.l  %T2,%S1\n\t" \
> +       "2:\n" \

(reordering the two sections for easier explanation)

> +       ".section       __ex_table,\"a\"\n\t" \
> +       ".long  1b, 3b\n\t" \

In case an exception happens for the instruction at 1b, jump to 3b.

Note that the m68k version has two entries here: one for each half of
the 64-bit access[*].
I don't know if that is really needed (and thus SH needs it, too), or if
the exception code handles subsequent instructions automatically.

> +       ".section       .fixup,\"ax\"\n" \
> +       "3:\n\t" \
> +       "mov    #0, %1\n\t" \

Return zero instead of the data at the (invalid) address.

> +       "mov.l  4f, %0\n\t" \
> +       "jmp    @%0\n\t" \

Resume at 2b.
Remember: branch delay slot, so the instruction below is executed first!

> +       " mov   %3, %0\n\t" \

Set err to -EFAULT.

> +       ".balign        4\n" \
> +       "4:     .long   2b\n\t" \
> +       ".previous\n" \

> +       ".previous" \
> +       :"=&r" (err), "=&r" (x) \
> +       :"m" (__m(addr)), "i" (-EFAULT), "0" (err)); })

[*] arch/m68k/include/asm/uaccess_mm.h

                        "1:     "MOVES".l       (%2)+,%1\n"             \
                        "2:     "MOVES".l       (%2),%R1\n"             \

                        "       .section __ex_table,\"a\"\n"            \
                        "       .align  4\n"                            \
                        "       .long   1b,10b\n"                       \
                        "       .long   2b,10b\n"                       \

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