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@xxxxxxxxxx>
>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>
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
