Messages in this thread | | | Date | Thu, 29 Jul 2021 23:00:48 -0700 | From | Kees Cook <> | Subject | Re: [PATCH 01/64] media: omap3isp: Extract struct group for memcpy() region |
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On Thu, Jul 29, 2021 at 11:20:39AM +0300, Dan Carpenter wrote: > On Thu, Jul 29, 2021 at 07:56:27AM +0200, Greg Kroah-Hartman wrote: > > On Wed, Jul 28, 2021 at 11:37:30PM +0200, David Sterba wrote: > > > On Wed, Jul 28, 2021 at 02:37:20PM -0700, Bart Van Assche wrote: > > > > On 7/28/21 2:14 AM, Dan Carpenter wrote: > > > > > On Wed, Jul 28, 2021 at 10:59:22AM +0200, David Sterba wrote: > > > > >>> drivers/media/platform/omap3isp/ispstat.c | 5 +-- > > > > >>> include/uapi/linux/omap3isp.h | 44 +++++++++++++++++------ > > > > >>> 2 files changed, 36 insertions(+), 13 deletions(-) > > > > >>> > > > > >>> diff --git a/drivers/media/platform/omap3isp/ispstat.c b/drivers/media/platform/omap3isp/ispstat.c > > > > >>> index 5b9b57f4d9bf..ea8222fed38e 100644 > > > > >>> --- a/drivers/media/platform/omap3isp/ispstat.c > > > > >>> +++ b/drivers/media/platform/omap3isp/ispstat.c > > > > >>> @@ -512,7 +512,7 @@ int omap3isp_stat_request_statistics(struct ispstat *stat, > > > > >>> int omap3isp_stat_request_statistics_time32(struct ispstat *stat, > > > > >>> struct omap3isp_stat_data_time32 *data) > > > > >>> { > > > > >>> - struct omap3isp_stat_data data64; > > > > >>> + struct omap3isp_stat_data data64 = { }; > > > > >> > > > > >> Should this be { 0 } ? > > > > >> > > > > >> We've seen patches trying to switch from { 0 } to { } but the answer > > > > >> was that { 0 } is supposed to be used, > > > > >> http://www.ex-parrot.com/~chris/random/initialise.html > > > > >> > > > > >> (from https://lore.kernel.org/lkml/fbddb15a-6e46-3f21-23ba-b18f66e3448a@suse.com/ ) > > > > > > > > > > In the kernel we don't care about portability so much. Use the = { } > > > > > GCC extension. If the first member of the struct is a pointer then > > > > > Sparse will complain about = { 0 }. > > > > > > > > +1 for { }. > > > > > > Oh, I thought the tendency is is to use { 0 } because that can also > > > intialize the compound members, by a "scalar 0" as it appears in the > > > code. > > > > > > > Holes in the structure might not be initialized to anything if you do > > either one of these as well. > > > > Or did we finally prove that is not the case? I can not remember > > anymore... > > Yep. The C11 spec says that struct holes are initialized. > > https://lore.kernel.org/netdev/20200731140452.GE24045@ziepe.ca/
This is, unfortunately, misleading. The frustrating key word is "partial" in "updated in C11 to require zero'ing padding when doing partial initialization of aggregates". If one initializes _all_ the struct members ... the padding doesn't get initialized. :( (And until recently, _trailing_ padding wasn't getting initialized even when other paddings were.)
I've tried to collect all the different ways the compiler might initialize a variable in this test: https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/tree/lib/test_stackinit.c?h=for-next/kspp
FWIW, there's no difference between -std=gnu99 and -std=c11, and the test shows that padding is _not_ universally initialized (unless your compiler supports -ftrivial-auto-var-init=zero, which Clang does, and GCC will shortly[1]). Running this with GCC 10.3.0, I see this...
As expected, having no initializer leaves padding (as well as members) uninitialized:
stackinit: small_hole_none FAIL (uninit bytes: 24) stackinit: big_hole_none FAIL (uninit bytes: 128) stackinit: trailing_hole_none FAIL (uninit bytes: 32)
Here, "zero" means "= { };" and they get padding initialized:
stackinit: small_hole_zero ok stackinit: big_hole_zero ok stackinit: trailing_hole_zero ok
Here, "static_partial" means "= { .one_member = 0 };", and "dynamic_partial" means "= { .one_member = some_variable };". These are similarly initialized:
stackinit: small_hole_static_partial ok stackinit: big_hole_static_partial ok stackinit: trailing_hole_static_partial ok
stackinit: small_hole_dynamic_partial ok stackinit: big_hole_dynamic_partial ok stackinit: trailing_hole_dynamic_partial ok
But when _all_ members are initialized, the padding is _not_:
stackinit: small_hole_static_all FAIL (uninit bytes: 3) stackinit: big_hole_static_all FAIL (uninit bytes: 124) stackinit: trailing_hole_static_all FAIL (uninit bytes: 7)
stackinit: small_hole_dynamic_all FAIL (uninit bytes: 3) stackinit: big_hole_dynamic_all FAIL (uninit bytes: 124) stackinit: trailing_hole_dynamic_all FAIL (uninit bytes: 7)
As expected, assigning to members outside of initialization leaves padding uninitialized:
stackinit: small_hole_runtime_partial FAIL (uninit bytes: 23) stackinit: big_hole_runtime_partial FAIL (uninit bytes: 127) stackinit: trailing_hole_runtime_partial FAIL (uninit bytes: 24)
stackinit: small_hole_runtime_all FAIL (uninit bytes: 3) stackinit: big_hole_runtime_all FAIL (uninit bytes: 124) stackinit: trailing_hole_runtime_all FAIL (uninit bytes: 7)
> What doesn't initialize struct holes is assignments: > > struct foo foo = *bar;
Right. Object to object assignments do not clear padding:
stackinit: small_hole_assigned_copy XFAIL (uninit bytes: 3) stackinit: big_hole_assigned_copy XFAIL (uninit bytes: 124) stackinit: trailing_hole_assigned_copy XFAIL (uninit bytes: 7)
And whole-object assignments of cast initializers follow the pattern of basic initializers, which makes sense given the behavior of initializers and direct assignment tests above. e.g.: obj = (type){ .member = ... };
stackinit: small_hole_assigned_static_partial ok stackinit: small_hole_assigned_dynamic_partial ok stackinit: big_hole_assigned_dynamic_partial ok stackinit: big_hole_assigned_static_partial ok stackinit: trailing_hole_assigned_dynamic_partial ok stackinit: trailing_hole_assigned_static_partial ok
stackinit: small_hole_assigned_static_all FAIL (uninit bytes: 3) stackinit: small_hole_assigned_dynamic_all FAIL (uninit bytes: 3) stackinit: big_hole_assigned_static_all FAIL (uninit bytes: 124) stackinit: big_hole_assigned_dynamic_all FAIL (uninit bytes: 124) stackinit: trailing_hole_assigned_dynamic_all FAIL (uninit bytes: 7) stackinit: trailing_hole_assigned_static_all FAIL (uninit bytes: 7)
So, yeah, it's not very stable.
-Kees
[1] https://gcc.gnu.org/pipermail/gcc-patches/2021-July/576341.html
-- Kees Cook
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