Re: [PATCH] bugfix for drivers/char/mem.c

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> There is a bug in the function read_kmem() in drivers/char/mem.c  -
> the file position is incorrectly updated after a read() from /dev/kmem.
> This means that successive reads from /dev/kmem without an intervening
> seek will return incorrect data. 
> 
> The bug has been around for quite a while, and is present in all the
> 2.2.x kernels.  The following patch (against v.2.3.12) fixes it.
> 
> 
> S.
> 
> --- v2.3.12/linux/drivers/char/mem.c	Sun Aug  1 11:09:11 1999
> +++ linux/drivers/char/mem.c	Sun Aug  1 11:25:07 1999
> @@ -245,11 +245,14 @@
>  		count -= read;
>  	}
>  
> -	virtr = vread(buf, (char *)p, count);
> -	if (virtr < 0)
> -		return virtr;
> -	*ppos += p + virtr;
> -	return virtr + read;
> +	if(count) {
> +		if((virtr = vread(buf, (char *)p, count)) < 0)
> +			return virtr;
> +		read += virtr;
> +	}
> +
> +	*ppos += read;
> +	return read;
>  }
>  
>  /*
>

I see the problem. Can I suggest a simpler fix, based on the fact that
vread() returns 0 when count == 0?

-	*ppos += p + virtr;
+	*ppos += read + virtr;

Kanoj
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