Re: [PATCH] arm64: increase stack size for KASAN_EXTRA

Qian Cai
2018-12-11T21:52:44Z

On Tue, 2018-12-11 at 22:43 +0100, Arnd Bergmann wrote:
> On Tue, Dec 11, 2018 at 6:18 PM Qian Cai <cai@lca.pw> wrote:
> >
> > On Tue, 2018-12-11 at 13:42 +0100, Arnd Bergmann wrote:
> > > On Fri, Dec 7, 2018 at 11:35 PM Qian Cai <cai@lca.pw> wrote:
> > > >
> > > > If the kernel is configured with KASAN_EXTRA, the stack size is
> > > > increasted significantly due to enable this option will set
> > > > -fstack-reuse to "none" in GCC [1]. As the results, it could trigger
> > > > stack overrun quite often with 32k stack size compiled using GCC 8. For
> > > > example, this reproducer
> > > >
> > > > size
> > > > 7536 shrink_inactive_list
> > > > 7440 shrink_page_list
> > > > 6560 fscache_stats_show
> > > > 3920 jbd2_journal_commit_transaction
> > > > 3216 try_to_unmap_one
> > > > 3072 migrate_page_move_mapping
> > > > 3584 migrate_misplaced_transhuge_page
> > > > 3920 ip_vs_lblcr_schedule
> > > > 4304 lpfc_nvme_info_show
> > > > 3888 lpfc_debugfs_nvmestat_data.constprop
> > > >
> > > > There are other 49 functions are over 2k in size while compiling kernel
> > > > with "-Wframe-larger-than=" on this machine. Hence, it is too much work
> > > > to change Makefiles for each object to compile without
> > > > -fsanitize-address-use-after-scope individually.
> > > >
> > > > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715#c23
> > >
> > > Could you clarify: are the numbers you see with or without the bugfix
> > > from that bugzilla?
> > >
> >
> > The numbers were from GCC8 which does NOT contain this patch [1].
> >
> > GCC9 is awesome which reduced the numbers in half even for KASAN_EXTRA. Only
> > thing is that GCC9 has not been officially released yet, so it is a bit
> > inconvenient for users need to compile the compiler by themselves first.
> >
> > I am fine either way to drop this patch or keep it until GCC9 is GA.
> >
> > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81715#c35
>
> Maybe we can make the constant depend on the compiler version?

I am not too keen to do the version-check considering some LTS versions could
just back-port those patches and the render the version-check incorrectly.

> It may also be possible to reduce the KASAN_THREAD_SHIFT
> constant for the normal case with gcc-9 and go back to the
> default frame size then.