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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

  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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