On Tue, Jul 25, 2017 at 04:48:13PM -0700, Ricardo Neri wrote:
> I meant to say the 4 most significant bytes. In this case, the
> 64-address 0xffffffffffff1234 would lie in the kernel memory while
> 0xffff1234 would correctly be in the user space memory.

That explanation is better.

> Yes, perhaps the check above is not needed. I included that check as
> part of my argument validation. In a 64-bit kernel, this function could
> be called with val with non-zero most significant bytes.

So say that in the comment so that it is obvious *why*.

> I have looked into this closely and as far as I can see, the 4 least
> significant bytes will wrap around when using 64-bit signed numbers as
> they would when using 32-bit signed numbers. For instance, for two
> positive numbers we have:
> 
> 7fff:ffff + 7000:0000 = efff:ffff.
> 
> The addition above overflows.

Yes, MSB changes.

> When sign-extended to 64-bit numbers we would have:
> 
> 0000:0000:7fff:ffff + 0000:0000:7000:0000 = 0000:0000:efff:ffff.
> 
> The addition above does not overflow. However, the 4 least significant
> bytes overflow as we expect.

No they don't - you are simply using 64-bit regs:

   0x00005555555546b8 <+8>:     movq   $0x7fffffff,-0x8(%rbp)
   0x00005555555546c0 <+16>:    movq   $0x70000000,-0x10(%rbp)
   0x00005555555546c8 <+24>:    mov    -0x8(%rbp),%rdx
   0x00005555555546cc <+28>:    mov    -0x10(%rbp),%rax
=> 0x00005555555546d0 <+32>:    add    %rdx,%rax

rax            0xefffffff       4026531839
rbx            0x0      0
rcx            0x0      0
rdx            0x7fffffff       2147483647

...

eflags         0x206    [ PF IF ]

(OF flag is not set).

> We can clamp the 4 most significant bytes.
> 
> For a two's complement negative numbers we can have:
> 
> ffff:ffff + 8000:0000 = 7fff:ffff with a carry flag.
> 
> The addition above overflows.

Yes.

> When sign-extending to 64-bit numbers we would have:
> 
> ffff:ffff:ffff:ffff + ffff:ffff:8000:0000 = ffff:ffff:7fff:ffff with a
> carry flag.
> 
> The addition above does not overflow. However, the 4 least significant
> bytes overflew and wrapped around as they would when using 32-bit signed
> numbers.

Right. Ok.

And come to think of it now, I'm wondering, whether it would be
better/easier/simpler/more straight-forward, to do the 32-bit operations
with 32-bit types and separate 32-bit functions and have the hardware do
that for you.

This way you can save yourself all that ugly and possibly error-prone
casting back and forth and have the code much more readable too.

Hmmm.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imend├Ârffer, Jane Smithard, Graham Norton, HRB 21284 
(AG N├╝rnberg)
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