Re: [PATCH v2 06/11] kcov: x86: introduce CONFIG_KCOV_UNIQUE

[Alexander Potapenko]

On Fri, Jun 27, 2025 at 10:11 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 26, 2025 at 03:41:53PM +0200, Alexander Potapenko wrote:
> > The new config switches coverage instrumentation to using
> >   __sanitizer_cov_trace_pc_guard(u32 *guard)
> > instead of
> >   __sanitizer_cov_trace_pc(void)
> >
> > This relies on Clang's -fsanitize-coverage=trace-pc-guard flag [1].
> >
> > Each callback receives a unique 32-bit guard variable residing in the
> > __sancov_guards section. Those guards can be used by kcov to deduplicate
> > the coverage on the fly.
>
> This sounds like a *LOT* of data; how big is this for a typical kernel
> build?

I have a 1.6Gb sized vmlinux, which has a .text section of 176Mb.
There are 1809419 calls to __sanitizer_cov_trace_pc_guard, and the
__sancov_guards section has a size of 6Mb, which are only allocated at
runtime.

If we take a vmlinux image from syzbot (e.g.
https://storage.googleapis.com/syzbot-assets/dadedf20b2e3/vmlinux-67a99386.xz),
its .text section is 166Mb, and there are 1893023 calls to
__sanitizer_cov_trace_pc, which will translate to exactly the same
number of __sanitizer_cov_trace_pc_guard, if we apply the unique
coverage instrumentation.