Re: [PATCH v3 3/6] mm: Add build time sanity check for struct page size
From: Baoquan He
Date: Mon Feb 18 2019 - 03:07:33 EST
On 02/17/19 at 08:50am, Kees Cook wrote:
> On Sat, Feb 16, 2019 at 6:02 AM Baoquan He <bhe@xxxxxxxxxx> wrote:
> > Size of struct page might be larger than 64 bytes if debug options
> > enabled, or fields added for debugging intentionally. Yet an upper
> > limit need be added at build time to trigger an alert in case the
> > size is too big to boot up system, warning people to check if it's
> > be done on purpose in advance.
> > Here 1/4 of PAGE_SIZE is chosen since system must have been insane
> > with this value. For those systems with PAGE_SIZE larger than 4KB,
> > 1KB is simply taken.
> > Signed-off-by: Baoquan He <bhe@xxxxxxxxxx>
> > ---
> > mm/page_alloc.c | 2 ++
> > 1 file changed, 2 insertions(+)
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 35fdde041f5c..eb6c8e22333b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -67,6 +67,7 @@
> > #include <linux/lockdep.h>
> > #include <linux/nmi.h>
> > #include <linux/psi.h>
> > +#include <linux/sizes.h>
> > #include <asm/sections.h>
> > #include <asm/tlbflush.h>
> > @@ -7084,6 +7085,7 @@ void __init free_area_init_nodes(unsigned long *max_zone_pfn)
> > unsigned long start_pfn, end_pfn;
> > int i, nid;
> > + BUILD_BUG_ON(sizeof(struct page) > min_t(size_t, SZ_1K, PAGE_SIZE));
> Are there systems with PAGE_SIZE < 1K? Maybe this should just be a
> direct SZ_1K check?
This check was suggested by Kirill, I forgot adding his "Suggested-by",
sorry abou that.
Originally he suggested to add code in generic place like this:
BUILD_BUG_ON(sizeof(struct page) < min(SZ_1K, PAGE_SIZE/4));
In later post, the kbuild test robot reported an build error on i386
>From the report hint, I thought 'PAGE_SIZE/4' also related to the macro
expansion, so change it as min_t(size_t, SZ_1K, PAGE_SIZE).
Just now I tried the build again, changing it back to 'PAGE_SIZE/4' also
BUILD_BUG_ON(sizeof(struct page) > min_t(size_t, SZ_1K, PAGE_SIZE/4));
I guess Kirill wants to make it as self explanatory for this check by
suggesting it as min(SZ_1K, PAGE_SIZE/4), to stress the 'PAGE_SIZE/4'.
As he said in mail thread, "If struct page is more than 1/4 of PAGE_SIZE
something is horribly broken".
Just try to give more details about this adding, not defend. I am
fine to take any of them.
> (Also, perhaps this should use the new static_assert where struct page
> is defined?)
I searched with 'git grep', didn't find static_assert macro or function.
And also no finding in include/linux/mm_types.h. Could you please be
> > /* Record where the zone boundaries are */
> > memset(arch_zone_lowest_possible_pfn, 0,
> > sizeof(arch_zone_lowest_possible_pfn));
> > --
> > 2.17.2
> Kees Cook