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Date:   Mon, 15 Jun 2020 12:00:09 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH] efi: Replace zero-length array and use struct_size() helper

On Fri, 29 May 2020 at 19:32, Gustavo A. R. Silva <gustavoars@...nel.org> wrote:
>
> On Fri, May 29, 2020 at 01:31:54AM -0700, Kees Cook wrote:
> > On Wed, May 27, 2020 at 12:14:25PM -0500, Gustavo A. R. Silva wrote:
> > > The current codebase makes use of the zero-length array language
> > > extension to the C90 standard, but the preferred mechanism to declare
> > > variable-length types such as these ones is a flexible array member[1][2],
> > > introduced in C99:
> > >
> > > struct foo {
> > >         int stuff;
> > >         struct boo array[];
> > > };
> > >
> > > By making use of the mechanism above, we will get a compiler warning
> > > in case the flexible array does not occur last in the structure, which
> > > will help us prevent some kind of undefined behavior bugs from being
> > > inadvertently introduced[3] to the codebase from now on.
> > >
> > > Also, notice that, dynamic memory allocations won't be affected by
> > > this change:
> > >
> > > "Flexible array members have incomplete type, and so the sizeof operator
> > > may not be applied. As a quirk of the original implementation of
> > > zero-length arrays, sizeof evaluates to zero."[1]
> > >
> > > sizeof(flexible-array-member) triggers a warning because flexible array
> > > members have incomplete type[1]. There are some instances of code in
> > > which the sizeof operator is being incorrectly/erroneously applied to
> > > zero-length arrays and the result is zero. Such instances may be hiding
> > > some bugs. So, this work (flexible-array member conversions) will also
> > > help to get completely rid of those sorts of issues.
> > >
> > > Lastly, make use of the sizeof_field() helper instead of an open-coded
> > > version.
> > >
> > > This issue was found with the help of Coccinelle and audited _manually_.
> > >
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > [2] https://github.com/KSPP/linux/issues/21
> > > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > >
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> >
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> >
>
> Thanks :)
>

Queued in efi/urgent, thanks


> Please, see more comments below...
>
> > > ---
> > >  drivers/firmware/efi/efi.c | 3 ++-
> > >  include/linux/efi.h        | 7 ++-----
> > >  2 files changed, 4 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> > > index 7f1657b6c30df..edc5d36caf54e 100644
> > > --- a/drivers/firmware/efi/efi.c
> > > +++ b/drivers/firmware/efi/efi.c
> > > @@ -622,7 +622,8 @@ int __init efi_config_parse_tables(const efi_config_table_t *config_tables,
> > >                     rsv = (void *)(p + prsv % PAGE_SIZE);
> > >
> > >                     /* reserve the entry itself */
> > > -                   memblock_reserve(prsv, EFI_MEMRESERVE_SIZE(rsv->size));
> > > +                   memblock_reserve(prsv,
> > > +                                    struct_size(rsv, entry, rsv->size));
> > >
> > >                     for (i = 0; i < atomic_read(&rsv->count); i++) {
> > >                             memblock_reserve(rsv->entry[i].base,
> > > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > > index c45ac969ea4eb..328cc52a5fd45 100644
> > > --- a/include/linux/efi.h
> > > +++ b/include/linux/efi.h
> > > @@ -1234,14 +1234,11 @@ struct linux_efi_memreserve {
> > >     struct {
> > >             phys_addr_t     base;
> > >             phys_addr_t     size;
> > > -   } entry[0];
> > > +   } entry[];
> > >  };
> > >
> > > -#define EFI_MEMRESERVE_SIZE(count) (sizeof(struct linux_efi_memreserve) + \
> > > -   (count) * sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
> > > -
> > >  #define EFI_MEMRESERVE_COUNT(size) (((size) - sizeof(struct linux_efi_memreserve)) \
> > > -   / sizeof(((struct linux_efi_memreserve *)0)->entry[0]))
> > > +   / sizeof_field(struct linux_efi_memreserve, entry[0]))
> >
> > Whoa. This is kind of a "reverse struct_size()". I wonder if any other
> > places in the kernel do a similar calculation?
> >
>
> So far this is the only intance of this I've run into.
>
> What I've found is that there are many instances of the open-coded
> version of sizeof_field() and offsetof(). I'm addressing them on the
> way.
>
> Thanks
> --
> Gustavo

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