Re: [RFC PATCH v2 2/6] kcov: add build system support for dataflow instrumentation

Nathan Chancellor <[email protected]> Thu, 4 Jun 2026 14:48:40 -0700

Newsgroups	org.kernel.vger.workflows,dev.linux.lists.llvm,org.kernel.vger.linux-doc,org.kernel.vger.linux-kbuild,org.kernel.vger.linux-kernel,org.kernel.vger.rust-for-linux
Message-ID	<20260604214840.GA3915915@ax162>

On Thu, Jun 04, 2026 at 10:45:19AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 03, 2026 at 07:43:29PM +0200, Yunseong Kim wrote:
> > Add CFLAGS_KCOV_DATAFLOW and RUSTFLAGS_KCOV_DATAFLOW exports to
> > scripts/Makefile.kcov, containing:
> >   -fsanitize-coverage=dataflow-args,dataflow-ret -g
> >   (with optional -fno-inline via CONFIG_KCOV_DATAFLOW_NO_INLINE)
> > 
> > scripts/Makefile.lib applies these flags when a module's Makefile sets:
> >   KCOV_DATAFLOW_file.o := y   (per-file)
> >   KCOV_DATAFLOW := y          (per-directory)
> > 
> > Also supports CONFIG_KCOV_DATAFLOW_INSTRUMENT_ALL for global enablement.
> > The flags are only applied to kernel objects (same guard as basic KCOV).
> > 
> > Signed-off-by: Yunseong Kim <[email protected]>
> > ---
> >  scripts/Makefile.kcov | 6 ++++++
> >  scripts/Makefile.lib  | 7 +++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
> > index 78305a84ba9d..101173fe194b 100644
> > --- a/scripts/Makefile.kcov
> > +++ b/scripts/Makefile.kcov
> > @@ -2,10 +2,16 @@
> >  kcov-flags-y					+= -fsanitize-coverage=trace-pc
> >  kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)	+= -fsanitize-coverage=trace-cmp
> >  
> > +# KCOV dataflow: trace function args and return values
> > +kcov-dataflow-flags-y := -fsanitize-coverage=dataflow-args,dataflow-ret -g
> > +kcov-dataflow-flags-$(CONFIG_KCOV_DATAFLOW_NO_INLINE) += -fno-inline
> 
> https://clang.llvm.org/docs/ClangCommandLineReference.html
> 
> Has no mention of -fno-inline, furthermore, what are the exact
> semantics? Does it inhibit __always_inline?
> 

Based on clang/test/CodeGen/always-inline.c [1], I believe the semantics
are the same as GCC's '-fno-inline' [2], which avoids inlining except
for always_inline functions.

[1]: https://github.com/llvm/llvm-project/blob/1d13b74cf086629d5cdae5f44ef4a62cebcaf3ff/clang/test/CodeGen/always-inline.c
[2]: https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-fno-inline

-- 
Cheers,
Nathan