Re: [PATCH 3/3] kbuild: Implement Clang's stack initialization

[Kees Cook]

On Thu, Apr 11, 2019 at 1:06 AM Masahiro Yamada
<yamada.masahiro@xxxxxxxxxxxxx> wrote:
>
> On Thu, Apr 11, 2019 at 1:16 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > CONFIG_INIT_STACK_ALL turns on stack initialization based on
> > -ftrivial-auto-var-init in Clang builds and on
> > -fplugin-arg-structleak_plugin-byref-all in GCC builds.
>
> Is CONFIG_INIT_STACK_ALL wired up to GCC plugin in any way?
> I could not understand it from the code.

No, it's only available under Clang. Clang is all-or-nothing, and the
GCC plugin has a degrees up to "all passed by reference" which isn't
truly "all" (i.e. Clang will initialize variables that aren't passed
by reference and trigger a compiler warning about being
uninitialized.)

> >  choice
> >         prompt "Initialize kernel stack variables at function entry"
> >         depends on CC_HAS_AUTO_VAR_INIT || GCC_PLUGINS
> > +       default INIT_STACK_ALL if CC_HAS_AUTO_VAR_INIT
>
> Why should this be enabled by default?
> Ins't it a performance regression
> since it inserts instructions in function prologue?

There are very few users of Clang right now (mainly Android), so I
figured it'd be nice to start Clang builds from a "protected by
default" here, especially given Linus's thoughts on making this always
happen[1]. I don't want to do it for GCC yet, since that would likely
come as a huge surprise to everyone else. :) But I'm happy to change
this, of course.

-Kees

[1] https://lkml.kernel.org/r/CA+55aFykZL+cSBJjBBts7ebEFfyGPdMzTmLSxKnT_29=j942dA@xxxxxxxxxxxxxx

--
Kees Cook