Re: [PATCH 1/1] lib/ioremap.c: avoid endless loop under ioremapping page unaligned ranges

From: Michal Hocko <mhocko@kernel.org>
To: zijun_hu <zijun_hu@zoho.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	zijun_hu@htc.com, tj@kernel.org, mingo@kernel.org,
	rientjes@google.com, iamjoonsoo.kim@lge.com,
	mgorman@techsingularity.net
Subject: Re: [PATCH 1/1] lib/ioremap.c: avoid endless loop under ioremapping page unaligned ranges
Date: Thu, 22 Sep 2016 14:47:36 +0200	[thread overview]
Message-ID: <20160922124735.GB11204@dhcp22.suse.cz> (raw)
In-Reply-To: <57E20A69.5010206@zoho.com>

On Wed 21-09-16 12:19:53, zijun_hu wrote:
> From: zijun_hu <zijun_hu@htc.com>
> 
> endless loop maybe happen if either of parameter addr and end is not
> page aligned for kernel API function ioremap_page_range()

Does this happen in practise or this you found it by reading the code?

> in order to fix this issue and alert improper range parameters to user
> WARN_ON() checkup and rounding down range lower boundary are performed
> firstly, loop end condition within ioremap_pte_range() is optimized due
> to lack of relevant macro pte_addr_end()
> 
> Signed-off-by: zijun_hu <zijun_hu@htc.com>
> ---
>  lib/ioremap.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/ioremap.c b/lib/ioremap.c
> index 86c8911..911bdca 100644
> --- a/lib/ioremap.c
> +++ b/lib/ioremap.c
> @@ -64,7 +64,7 @@ static int ioremap_pte_range(pmd_t *pmd, unsigned long addr,
>  		BUG_ON(!pte_none(*pte));
>  		set_pte_at(&init_mm, addr, pte, pfn_pte(pfn, prot));
>  		pfn++;
> -	} while (pte++, addr += PAGE_SIZE, addr != end);
> +	} while (pte++, addr += PAGE_SIZE, addr < end && addr >= PAGE_SIZE);
>  	return 0;
>  }

Ble, this just overcomplicate things. Can we just make sure that the
proper alignment is done in ioremap_page_range which is the only caller
of this (and add VM_BUG_ON in ioremap_pud_range to make sure no new
caller will forget about that).

>  
> @@ -129,7 +129,9 @@ int ioremap_page_range(unsigned long addr,
>  	int err;
>  
>  	BUG_ON(addr >= end);
> +	WARN_ON(!PAGE_ALIGNED(addr) || !PAGE_ALIGNED(end));

maybe WARN_ON_ONCE would be sufficient to prevent from swamping logs if
something just happens to do this too often in some pathological path.

>  
> +	addr = round_down(addr, PAGE_SIZE);

	end = round_up(end, PAGE_SIZE);

wouldn't work?

>  	start = addr;
>  	phys_addr -= addr;
>  	pgd = pgd_offset_k(addr);
> -- 
> 1.9.1
> 
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-- 
Michal Hocko
SUSE Labs