Re: [PATCH v5 02/10] x86: Compile-time asm code validation

Josh Poimboeuf
2015-06-12T14:10:37Z

On Fri, Jun 12, 2015 at 12:18:16PM +0100, Pedro Alves wrote:
> On 06/11/2015 03:10 PM, Josh Poimboeuf wrote:
>
> > C would definitely make more sense when analyzing object code. In fact,
> > asmvalidate is written in C. But then I guess we'd have to re-implement
> > the .cfi stuff and populate the DWARF sections manually instead of
> > letting the assembler do it.
>
> Was doing all this directly in the assembler considered? That is,
> e.g., add some knob that makes it error/warn in the same conditions
> you're making the validator catch. For tail calls, you'd e.g., add
> some new ".nonlocal" directive that you'd use to whitelist the
> following jump. And then if it's possible run a CFI generator
> as a separate step over the source, it sounds like it should also
> be possible to have the assembler do it instead too (again with
> some new high level directive to trigger/help it).

In general I think doing these types of things in the assembler would be
a good idea. Missing or inaccurate debug data for asm code seems to be
a common problem for other projects as well. As Andy pointed out,
they're doing similar things in musl [1]. So it might be useful to add
an option to the assembler which validates that the code conforms to
certain structural rules, and then inserts frame pointer and/or .cfi
directives.

That said, the kernel has much more custom features than other projects.
There are some sneaky macros, like _ASM_EXTABLE and ALTERNATIVE, which
hide code in various sections. Unless we're able to somehow either stop
using these macros or isolate them to a few places, I doubt that such a
general purpose assembler option would work.

[1] http://www.openwall.com/lists/musl/2015/05/31/5

--
Josh