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SubjectRe: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
On Wed, Oct 13, 2021 at 04:20:55PM +0200, Borislav Petkov wrote:
> On Wed, Oct 13, 2021 at 04:07:23PM +0200, Willy Tarreau wrote:
> > Yes I agree with the "potentially" here. If it can potentially be (i.e.
> > the kernel is allowed by contract to later change the way it's currently
> > done) then we have to save them even if it means lower code efficiency.
> >
> > If, however, the kernel performs such savings on purpose because it is
> > willing to observe a stricter saving than the AMD64 ABI, we can follow
> > it but only once it's written down somewhere that it is by contract and
> > will not change.
>
> Right, and Micha noted that such a change to the document can be done.

great.

> And we're basically doing that registers restoring anyway, in POP_REGS.

That's what I based my analysis on when I wanted to verify Ammar's
finding. I would tend to think that if we're burning cycles popping
plenty of registers it's probably for a reason, maybe at least a good
one, which is that it's the only way to make sure we're not leaking
internal kernel data! This is not a concern for kernel->kernel nor
user->user calls but for user->kernel calls it definitely is one, and
I don't think we could relax that series of pop without causing leaks
anyway.

Willy

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