On Monday 21 December 2015, Dr. Philipp Tomsich wrote:
>
> > On 18 Dec 2015, at 13:47, Arnd Bergmann <[email protected]> wrote:
> >
> >> 3. Follow the PCS up to glibc but always pass syscall arguments in W
> >> registers, like AArch32 compat support (the least preferred option,
> >> the only advantage is a single wrapper for all syscalls but it would
> >> be doing unnecessary zeroing even for syscalls where it isn't needed)
> >
> > This would mean we cannot pass 64-bit arguments in registers, right?
>
> Note that there’s no 32bit registers (the ‘w’-form always refers to the lower
> 32bits of a 64bit register, with implicit zero-extension)… and load/store
> instructions always use the full base-register (‘x’-form) for address
> calculation.
> I.e. a load/store would inadvertently pickup “random garbage” in the upper
> 32bits, if no explicit zero-extension is applied.
>
> In other words: all zero-extensions for 32bit arguments should be explicit
> on the kernel side.
I think that is what Catalin meant with the single wrapper in the description:
the kernel always zeroes out the upper 32 bits in the initial trap, and then
we only need to do sign-extensions for the few cases that need it, and pass
64-bit
arguments in two lower halves. In contrast, approach 1 adds a separate wrapper
for each syscall that takes care of both sign-extending where necessary and
zero-extending all othe 32-bit arguments but not the 64-bit arguments.
Arnd
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