Re: [PATCH v6 12/20] arm64:ilp32: add sys_ilp32.c and a separate table (in entry.S) to use it

Arnd Bergmann
2015-12-17T20:11:23Z

On Thursday 17 December 2015 18:27:53 Catalin Marinas wrote:
> On Wed, Dec 16, 2015 at 12:42:38AM +0300, Yury Norov wrote:

> > +#define compat_sys_lookup_dcookie sys_lookup_dcookie
> > +#define compat_sys_pread64 sys_pread64
> > +#define compat_sys_pwrite64 sys_pwrite64
> > +#define compat_sys_readahead sys_readahead
> > +#define compat_sys_shmat sys_shmat
>
> I wonder whether we need wrappers (actually, not only for these but
> sys_read etc.). These functions take either a pointer or a size_t
> argument which are 32-bit with ILP32 but treated as 64-bit by an LP64
> kernel. Can we guarantee that user space zeros the top 32-bit of the
> arguments passed here?

I'm pretty sure that is safe. I haven't read the calling conventions
specification for arm64 ilp32, but usually all function arguments are
passed as 64-bit registers with proper sign-extend or zero-extend.

Most other syscalls rely on this behavior too, not just the ones that
are being modified here.

> With compat/AArch32, this is guaranteed by the kernel since EL0 won't be
> able to touch the top part but here I'm not entirely sure. As long as
> user space used Wn registers for 32-bit types, we are probably fine (the
> architecture guarantees the top 32-bit zeroing following a MOV, LDR etc.
> instruction into a Wn register). We just need to mention this in the ABI
> document (ilp32.txt).

I think the aarch32 case is actually the hard one, because it has to
worry about explicitly sign-extending 32-bit arguments (signed int or
signed long) that might be negative, e.g. user space passes -1
as 0xffffffff, which the kernel entry turns into 0x00000000ffffffff
when it should use 0xffffffffffffffff. The COMPAT_SYSCALL_DEFINEx
macros take care of this.

> > +
> > +#define compat_sys_open_by_handle_at sys_open_by_handle_at
> > +#define compat_sys_openat sys_openat
>
> So using sys_openat() forces O_LARGEFILE and we don't have a problem
> with (f)truncate. We may have an issue with AArch32 compat though.

aarch32 uses the correct compat functions in asm/unistd32.h

Arnd