Re: [PATCH] Syscall arguments are unsigned long (full registers)

On 4 Jul 2016, at 16:59, Arnd Bergmann [off-list ref] wrote:

On Monday, July 4, 2016 2:47:10 PM CEST Tautschnig, Michael wrote:

quoted

Thanks a lot for the immediate feedback.

quoted

On 4 Jul 2016, at 16:28, Andi Kleen [off-list ref] wrote:

On Mon, Jul 04, 2016 at 01:52:58PM +0000, Tautschnig, Michael wrote:

quoted

All syscall arguments are passed in as types of the same byte size as
unsigned long (width of full registers). Using a smaller type without a
cast may result in losing bits of information. In all other instances
apart from the ones fixed by the patch the code explicitly introduces
type casts (using, e.g., SYSCALL_DEFINE1).

While goto-cc reported these problems at build time, it is noteworthy
that the calling conventions specified in the System V AMD64 ABI do
ensure that parameters 1-6 are passed via registers, thus there is no
implied risk of misaligned stack access.

Does this actually fix anything?

It will ensure the behaviour on 32 and 64-bit systems is consistent, i.e.,
no truncation occurs. This is to ensure that future uses of these syscalls

   ^^^ no *hidden*

quoted

do not face surprises.

[...]

This is the same truncation that we do with SYSCALL_DEFINE2(),
clearing the top 32 bits of the 'code' parameter to ensure that
user space doesn't pass data unexpectedly.

That change seems reasonable, but why not just use SYSCALL_DEFINE2()
directly for consistency with the other syscalls?

Happy to provide such an updated patch; Andi seemed less confident this should
be going ahead?

Best,
Michael