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From:   Mauricio Faria de Oliveira <mfo@canonical.com>
Date:   Wed, 27 Jul 2022 14:11:39 -0300
Message-ID: <CAO9xwp2Ud-60Eb0cdpsBpq_ixOBaiOannxZ7SsbfHKrjj8mGDQ@mail.gmail.com>
Subject: Re: [RFC PATCH 3/6] sysctl, mod_devicetable: shadow struct
 ctl_table.procname for file2alias
To:     Masahiro Yamada <masahiroy@kernel.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-modules <linux-modules@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>
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On Tue, Jul 26, 2022 at 6:27 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> On Fri, Jul 22, 2022 at 11:24 AM Mauricio Faria de Oliveira
> <mfo@canonical.com> wrote:
> >
> > In order to expose a sysctl entry to modpost (file2alias.c, precisely)
> > we have to shadow 'struct ctl_table' in mod_devicetable.h, as scripts
> > should not access kernel headers or its types (see file2alias.c).
> >
> > The required field is '.procname' (basename of '/proc/sys/.../entry').
> >
> > Since 'struct ctl_table' is annotated for structure randomization and
> > we need a known offset for '.procname' (remember, no kernel headers),
> > take it out of the randomized portion (as in, eg, 'struct task_struct').
> >
> > Of course, add build-time checks for struct size and .procname offset
> > between both structs. (This has to be done on kernel side; for headers.)
> >
> > With that in place, use the regular macros in devicetable-offsets.c to
> > define SIZE_... and OFF_... macros for the shadow struct and the field
> > of interest.
> >
> > Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
> > ---
> >  fs/proc/proc_sysctl.c             | 19 +++++++++++++++++++
> >  include/linux/mod_devicetable.h   | 25 +++++++++++++++++++++++++
> >  include/linux/sysctl.h            | 11 ++++++++++-
> >  kernel/sysctl.c                   |  1 +
> >  scripts/mod/devicetable-offsets.c |  3 +++
> >  5 files changed, 58 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > index 021e83fe831f..ebbf8702387e 100644
> > --- a/fs/proc/proc_sysctl.c
> > +++ b/fs/proc/proc_sysctl.c
> > @@ -19,6 +19,24 @@
> >  #include <linux/kmemleak.h>
> >  #include "internal.h"
> >
> > +#ifdef CONFIG_MODULES
> > +#include <linux/mod_devicetable.h>
> > +
> > +static void check_struct_sysctl_device_id(void)
> > +{
> > +       /*
> > +        * The shadow struct sysctl_device_id for file2alias.c needs
> > +        * the same size of struct ctl_table and offset for procname.
> > +        */
> > +       BUILD_BUG_ON(sizeof(struct sysctl_device_id)
> > +                       != sizeof(struct ctl_table));
> > +       BUILD_BUG_ON(offsetof(struct sysctl_device_id, procname)
> > +                       != offsetof(struct ctl_table, procname));
>
>
> Nit:
>
> If you use static_assert(), you can remove
>  check_struct_sysctl_device_id().
>
>
> You can write static_assert() out of a function.

That's a nice cleanup; thanks!

>
>
>
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 223376959d29..15073621cfa8 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -2487,6 +2487,7 @@ int __init sysctl_init_bases(void)
> >
> >         return 0;
> >  }
> > +
>
>
> Noise.

Fixed.


>
>
>
>
> >  #endif /* CONFIG_SYSCTL */
> >  /*
> >   * No sense putting this after each symbol definition, twice,
> > diff --git a/scripts/mod/devicetable-offsets.c b/scripts/mod/devicetable-offsets.c
> > index c0d3bcb99138..43b2549940d2 100644
> > --- a/scripts/mod/devicetable-offsets.c
> > +++ b/scripts/mod/devicetable-offsets.c
> > @@ -262,5 +262,8 @@ int main(void)
> >         DEVID(ishtp_device_id);
> >         DEVID_FIELD(ishtp_device_id, guid);
> >
> > +       DEVID(sysctl_device_id);
> > +       DEVID_FIELD(sysctl_device_id, procname);
> > +
> >         return 0;
> >  }
> > --
> > 2.25.1
> >
>
>
> --
> Best Regards
> Masahiro Yamada


--
Mauricio Faria de Oliveira