linux-kernel - Re: [PATCH] fs: Avoid signed overflow UBSAN warning

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Message-ID: <20190305042625.GC13380@bombadil.infradead.org>
Date:   Mon, 4 Mar 2019 20:26:25 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Hongbo Yao <yaohongbo@...wei.com>
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...wei.com
Subject: Re: [PATCH] fs: Avoid signed overflow UBSAN warning

On Sat, Feb 23, 2019 at 05:02:10PM +0800, Hongbo Yao wrote:
> I ran into this:
> 	UBSAN: Undefined behaviour in fs/read_write.c:251:11
> 	signed integer overflow:
> 	9223372036854775805 + 9223372036854775805 cannot be represented in
> 	type 'long long int'
> 
> Use unsigned types to do math, it doesn't throw a UBSAN warning
> when it does overflow.This patch doesn't change generated code.

You don't need to be this verbose to fix it.

> -			offset += file->f_pos;
> +			offset = (u64)file->f_pos + (u64)offset;

			offset += (u64)file->f_pos;