Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Sun, 31 May 2020 12:43:11 +0200 | Subject | Re: [PATCH] sh: Implement __get_user_u64() required for 64-bit get_user() |
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Hi Adrian,
On Sun, May 31, 2020 at 11:59 AM John Paul Adrian Glaubitz <glaubitz@physik.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@linux-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
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