From: Michal Hocko <mhocko@kernel.org>
To: Pi-Hsun Shih <pihsun@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <willy@infradead.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Vlastimil Babka <vbabka@suse.cz>,
Minchan Kim <minchan@kernel.org>, Omar Sandoval <osandov@fb.com>,
Huang Ying <ying.huang@intel.com>, Aaron Lu <aaron.lu@intel.com>,
Tejun Heo <tj@kernel.org>, Wei Yang <richard.weiyang@gmail.com>,
open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm/swap: Avoid undefined behavior in __swapoffset
Date: Thu, 7 Mar 2019 13:23:13 +0100 [thread overview]
Message-ID: <20190307122313.GA30234@dhcp22.suse.cz> (raw)
In-Reply-To: <20190307094653.119425-1-pihsun@chromium.org>
On Thu 07-03-19 17:46:50, Pi-Hsun Shih wrote:
> Use offsetof to calculate offset of a field to avoid UBSAN warning like:
>
> ===================================================================
> UBSAN: Undefined behaviour in mm/swapfile.c:3010:38
> member access within null pointer of type 'union swap_header'
> CPU: 6 PID: 1833 Comm: swapon Tainted: G S 4.19.23 #43
> Call trace:
> dump_backtrace+0x0/0x194
> show_stack+0x20/0x2c
> __dump_stack+0x20/0x28
> dump_stack+0x70/0x94
> ubsan_epilogue+0x14/0x44
> ubsan_type_mismatch_common+0xf4/0xfc
> __ubsan_handle_type_mismatch_v1+0x34/0x54
> __se_sys_swapon+0x654/0x1084
> __arm64_sys_swapon+0x1c/0x24
> el0_svc_common+0xa8/0x150
> el0_svc_compat_handler+0x2c/0x38
> el0_svc_compat+0x8/0x18
> ==================================================================
Could you be more specific about what exactly is undefined here and
why offsetof is any better. AFAIR it uses the same construct unless a
compiler defines a built in.
I do not object the change itself because it is cleaner to use the
existing helper but I am wondering why this is fixing ubsan. Is ubsan
defining the compiler variant and consider it safe?
>
> Signed-off-by: Pi-Hsun Shih <pihsun@chromium.org>
> ---
> include/linux/swap.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index fc50e21b3b88..4bfb5c4ac108 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -157,9 +157,9 @@ struct swap_extent {
> /*
> * Max bad pages in the new format..
> */
> -#define __swapoffset(x) ((unsigned long)&((union swap_header *)0)->x)
> #define MAX_SWAP_BADPAGES \
> - ((__swapoffset(magic.magic) - __swapoffset(info.badpages)) / sizeof(int))
> + ((offsetof(union swap_header, magic.magic) - \
> + offsetof(union swap_header, info.badpages)) / sizeof(int))
>
> enum {
> SWP_USED = (1 << 0), /* is slot in swap_info[] used? */
> --
> 2.21.0.352.gf09ad66450-goog
>
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2019-03-07 12:23 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-07 9:46 [PATCH] mm/swap: Avoid undefined behavior in __swapoffset Pi-Hsun Shih
2019-03-07 12:23 ` Michal Hocko [this message]
2019-03-07 12:47 ` Pi-Hsun Shih
2019-03-07 13:23 ` Michal Hocko
2019-03-12 7:02 ` Pi-Hsun Shih
2019-03-12 8:07 ` Michal Hocko
2019-03-12 8:12 ` Pi-Hsun Shih
2019-03-12 8:18 ` [PATCH v2] mm/swap: Use offsetof instead of custom __swapoffset macro Pi-Hsun Shih
2019-03-12 8:26 ` Michal Hocko
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