Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list

[H. Peter Anvin]

On 10/13/21 05:43, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 05:23:11AM +0700, Ammar Faizi wrote:
> > "Figure 3.4: Register Usage" is not the answer, if it were, nolibc.h
> > would be broken as it is missing "rdi", "rsi", "rdx" in the clobber list.
>
> It is not about what happens in practice but what the contract is:
> syscall argument registers can potentially get clobbered and userspace
> should treat them as such. Because if the kernel decides to actually
> clobber them for whatever reason and some userspace thing thinks
> otherwise, then it is the userspace thing's problem as it doesn't adhere
> to the well known ABI.

Currently the kernel doesn't, but some past kernels have zeroed some of 
these registers rather than preserving them.

	-hpa