linux-kernel - Re: [PATCH v2] init: fix -Wmissing-variable-declarations clang warning

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Date:   Mon, 11 Sep 2023 13:49:22 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, Tom Rix <trix@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Bill Wendling <morbo@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] init: fix -Wmissing-variable-declarations clang warning

On Mon, Sep 11, 2023 at 1:46 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> On Tue, Sep 5, 2023 at 3:43 PM Justin Stitt <justinstitt@...gle.com> wrote:
> >
> > When building x86/defconfig with Clang-18 I encounter the following warning:
> > | init/main.c:189:13: warning: no previous extern declaration for non-static variable 'envp_init' [-Wmissing-variable-declarations]
> > |   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
> > | init/main.c:189:7: note: declare 'static' if the variable is not intended to be used outside of this translation unit
> > |   189 | const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
> >
> > Make `envp_init` static and provide `handle_initrd` its own copy.
> >
> > This silences the warning and makes the code more readable as you no
> > longer have to track down extern definitions inside of `handle_initrd`.
> > It is now more self-contained.
>
> But this is now creating duplicate string arrays in the kernel image.
>
> Why can't this be:
> 1. declared in include/linux/initrd.h
> 2. defined in init/main.c
> 3. referenced in init/do_mounts.c
>
> The extern declaration in init/do_mounts_initrd.c isn't even correct!
> It's missing a `const`! That's the kind of monkey business this
> warning is trying to help avoid. C has headers for this reason, let's
> use them.
>
> Perhaps users of call_usermodehelper_setup need an update, or
> envp_init actually should not be const.

I prefer v1 as the basis; though as kbuild test robot noted to you,
there are other references to envp_init that now need to be cleaned up
in addition.

When you say you build tested v1, I assume you only built init/main.o
and not vmlinux. Make sure to build test ALL files which refer to
variables you've modified.

call_usermodehelper_setup and its callers either need to be cleaned
up, or we should consider dropping const.

>
> >
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1920
> > Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> > ---
> > The kernel test robot didn't like v1 of this patch and I prefer Kees'
> > approach to simplifying this code anyways so I'm sending this v2.
> > Hopefully the CI builds clean (as I am locally using their repro).
> >
> > Changes in v2:
> > - Make envp_init static and provide handle_initrd() with a copy (thanks Kees)
> > - Rebase onto mainline (2dde18cd1d8fa)
> > - Link to v1: https://lore.kernel.org/r/20230830-missingvardecl2-init-main-c-v1-1-59007a637259@google.com
> > ---
> > Note: build-tested only.
> >
> > It should be noted that `handle_initrd` has been marked as deprecated
> > and perhaps the entire thing can be removed as per it's own message:
> > | using deprecated initrd support, will be removed in 2021.
> > ---
> >  init/do_mounts_initrd.c | 4 ++--
> >  init/main.c             | 2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/init/do_mounts_initrd.c b/init/do_mounts_initrd.c
> > index 425f4bcf4b77..154bd0de85a6 100644
> > --- a/init/do_mounts_initrd.c
> > +++ b/init/do_mounts_initrd.c
> > @@ -87,7 +87,7 @@ static void __init handle_initrd(char *root_device_name)
> >  {
> >         struct subprocess_info *info;
> >         static char *argv[] = { "linuxrc", NULL, };
> > -       extern char *envp_init[];
> > +       static char *envp[] = { "HOME=/", "TERM=linux", NULL, };
> >         int error;
> >
> >         pr_warn("using deprecated initrd support, will be removed in 2021.\n");
> > @@ -100,7 +100,7 @@ static void __init handle_initrd(char *root_device_name)
> >         init_mkdir("/old", 0700);
> >         init_chdir("/old");
> >
> > -       info = call_usermodehelper_setup("/linuxrc", argv, envp_init,
> > +       info = call_usermodehelper_setup("/linuxrc", argv, envp,
> >                                          GFP_KERNEL, init_linuxrc, NULL, NULL);
> >         if (!info)
> >                 return;
> > diff --git a/init/main.c b/init/main.c
> > index ad920fac325c..9a473107bb8f 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -186,7 +186,7 @@ static int __init set_reset_devices(char *str)
> >  __setup("reset_devices", set_reset_devices);
> >
> >  static const char *argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
> > -const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
> > +static const char *envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
> >  static const char *panic_later, *panic_param;
> >
> >  static bool __init obsolete_checksetup(char *line)
> >
> > ---
> > base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
> > change-id: 20230830-missingvardecl2-init-main-c-93dc1013ff8a
> >
> > Best regards,
> > --
> > Justin Stitt <justinstitt@...gle.com>
> >
>
>
> --
> Thanks,
> ~Nick Desaulniers



-- 
Thanks,
~Nick Desaulniers