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

Arnd Bergmann
2015-12-18T12:54:13Z

On Friday 18 December 2015 11:42:19 Catalin Marinas wrote:
> On Thu, Dec 17, 2015 at 12:14:20PM -0800, Andrew Pinski wrote:
> > Well (just like LP64 on AARCH64), when passing a 32bit value to a
> > function, the upper 32bits are undefined. I ran into this when I was
> > debugging the GCC go library on ILP32 (though reproduced with pure C
> > code) and the assembly functions inside glibc where pointers are
> > passed with the upper 32bits as undefined.
> > So we have an issue if called with syscall function or using pure
> > assembly to create the syscall functions (which glibc does).
>
> I think the ILP32 syscall ABI should follow the PCS convention where the
> top 32-bit of a register is not guaranteed 0 when the size of the
> argument is 32-bit. So take the read(2) syscall:
>
> ssize_t read(int fd, void *buf, size_t count);
>
> From the ILP32 code perspective, void * and size_t are both 32-bit. It
> would call into the kernel leaving the top 32-bit as undefined (if we
> follow the PCS). Normally, calling a function with the same size
> arguments is not a problem since the compiler generates the callee code
> accordingly. However, we route the syscall directly into the native
> sys_read() where void * and size_t are 64-bit with the top 32-bit left
> undefined.
>
> We have three options here:
>
> 1. Always follow PCS convention across user/kernel call and add wrappers
> in the kernel (preferred)

Yes, I also think this is best.

> 2. Follow the PCS up to glibc and get glibc to zero the top part (not
> always safe with hand-written assembly, though we already do this for
> AArch32 where the PCS only specifies 4 arguments in registers, the
> rest go on the stack)

I assume this needs special handling for syscalls with 64-bit arguments
in both glibc and kernel.

> 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?

> My preference, as stated above, is (1). You can write the wrappers in C
> directly and let the compiler upgrade the types when calling the native
> syscall. But any other option would be fine (take some inspiration from
> other architectures). Unfortunately we don't have COMPAT_SYSCALL_DEFINE
> for all functions that we need to wrap, it would have been easier (so we
> need to add them but probably in the arch/arm64 code).

It would be nice to have that code architecture-independent, so we can
share it with s390 and only need to update one place when new syscalls
get added.

Arnd