linux-kernel - Re: [PATCH v2] mm/swap: Use offsetof instead of custom _

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Message-ID: <20190312082612.GH5721@dhcp22.suse.cz>
Date:   Tue, 12 Mar 2019 09:26:12 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Pi-Hsun Shih <pihsun@...omium.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Minchan Kim <minchan@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Aaron Lu <aaron.lu@...el.com>, Tejun Heo <tj@...nel.org>,
        Omar Sandoval <osandov@...com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] mm/swap: Use offsetof instead of custom __swapoffset
 macro.

On Tue 12-03-19 16:18:59, Pi-Hsun Shih wrote:
> Use offsetof to calculate offset of a field to take advantage of
> compiler built-in version when possible, and avoid UBSAN warning when
> compiling with Clang:
> 
> 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
> 
> Signed-off-by: Pi-Hsun Shih <pihsun@...omium.org>

Yes this makes more sense. I still think the warning is bogus but
whatever. A cleanup is worth it.

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  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? */
> -- 
> 2.21.0.360.g471c308f928-goog
> 

-- 
Michal Hocko
SUSE Labs