linux-kernel - Re: [PATCH 0/2] introduce DUMP_PREFIX

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Message-ID: <29122c86-bfea-2f25-d111-00641cc660ba@kernel.org>
Date:   Tue, 19 Jan 2021 13:55:29 -0600
From:   Timur Tabi <timur@...nel.org>
To:     Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
        Petr Mladek <pmladek@...e.com>, roman.fietze@...na.com,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>, linux-mm@...ck.org,
        Akinobu Mita <akinobu.mita@...il.com>
Subject: Re: [PATCH 0/2] introduce DUMP_PREFIX_UNHASHED for hex dumps

On 1/19/21 1:45 PM, Kees Cook wrote:
> How about this so the base address is hashed once, with the offset added
> to it for each line instead of each line having a "new" hash that makes
> no sense:
> 
> diff --git a/lib/hexdump.c b/lib/hexdump.c
> index 9301578f98e8..20264828752d 100644
> --- a/lib/hexdump.c
> +++ b/lib/hexdump.c
> @@ -242,12 +242,17 @@ void print_hex_dump(const char *level, const char *prefix_str, int prefix_type,
>   		    const void *buf, size_t len, bool ascii)
>   {
>   	const u8 *ptr = buf;
> +	const u8 *addr;
>   	int i, linelen, remaining = len;
>   	unsigned char linebuf[32 * 3 + 2 + 32 + 1];
>   
>   	if (rowsize != 16 && rowsize != 32)
>   		rowsize = 16;
>   
> +	if (prefix_type == DUMP_PREFIX_ADDRESS &&
> +	    ptr_to_hashval(ptr, &addr))
> +		addr = 0;
> +
>   	for (i = 0; i < len; i += rowsize) {
>   		linelen = min(remaining, rowsize);
>   		remaining -= rowsize;
> @@ -258,7 +263,7 @@ void print_hex_dump(const char *level, const char *prefix_str, int prefix_type,
>   		switch (prefix_type) {
>   		case DUMP_PREFIX_ADDRESS:
>   			printk("%s%s%p: %s\n",
> -			       level, prefix_str, ptr + i, linebuf);
> +			       level, prefix_str, addr + i, linebuf);

Well, this is better than nothing, but not by much.  Again, as long as 
%px exists for printk(), I just cannot understand any resistance to 
allowing it in print_hex_dump().

Frankly, I think this patch and my patch should both be added.  During 
debugging, it's very difficult if not impossible to work with hashed 
addresses.  I use print_hex_dump() with an unhashed address all the 
time, either by applying my patch to my own kernel or just replacing the 
%p with %px.