Re: [PATCH v2 1/2] kcov, objtool: Make runtime functions noinstr-compatible

[Peter Zijlstra]

On Thu, Jun 04, 2020 at 04:56:34PM +0200, Marco Elver wrote:
> While we lack a compiler attribute to add to noinstr that would disable
> KCOV, make the KCOV runtime functions return if the caller is in a
> noinstr section. We then whitelist __sanitizer_cov_*() functions in
> objtool.

> __sanitizer_cov_*() cannot safely become safe noinstr functions
> as-is, as they may fault due to accesses to vmalloc's memory.

I would feel very much better with those actually in noinstr, because
without it, there is nothing stopping us from adding a kprobe/hw-
breakpoint or other funny to the function.

Even if they almost instra-return, having a kprobe on the function entry
or condition check is enough to utterly wreck things.

So something like:

void noinstr __sanitizer_cov_trace_*(...)
{
	if (within_noinstr_section(ip))
		return;

	instrumentation_begin();
	write_comp_data(...);
	instrumentation_end();
}

Would make me feel a whole lot better.

> +static __always_inline bool in_noinstr_section(unsigned long ip)
> +{
> +	return (unsigned long)__noinstr_text_start <= ip &&
> +	       ip < (unsigned long)__noinstr_text_end;
> +}

.entry.text is also considered noinstr, although I suppose that all
being in .S files avoids it having annotations inserted, but perhaps a
comment?