Re: [RFC][PATCH 00/24] x86/pti: Defer CR3 switch to C code

[Alexandre Chartre]

On 11/9/20 8:35 PM, Dave Hansen wrote:
> On 11/9/20 6:44 AM, Alexandre Chartre wrote:
> >   - map more syscall, interrupt and exception entry code into the user
> >     page-table (map all noinstr code);
>
> This seems like the thing we'd want to tag explicitly rather than make
> it implicit with 'noinstr' code.  Worst-case, shouldn't this be:
>
> #define __entry_func	noinstr
>
> or something?

Yes. I use the easy solution to just use noinstr because noinstr is mostly
use for entry functions. But if we want to use the user page-table beyond
the entry functions then we will definitively need a dedicated tag.

> I'd also like to see a lot more discussion about what the rules are for
> the C code and the compiler.  We can't, for instance, do a normal
> printk() in this entry functions.  Should we stick them in a special
> section and have objtool look for suspect patterns or references?
>
> I'm most worried about things like this:
>
> 	if (something_weird)
> 		pr_warn("this will oops the kernel\n");

That would be similar to noinstr which uses the .noinstr.text section, and if
I remember correctly objtool detects if a noinstr function calls a non-noinst.
Similarly here, an entry function should not call a non-entry function.

alex.