Messages in this thread | | | Date | Wed, 13 Oct 2021 16:24:33 +0200 | From | Willy Tarreau <> | Subject | Re: [PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list |
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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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