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

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Message-ID: <202101201226.A03F16DF24@keescook>
Date:   Wed, 20 Jan 2021 12:28:50 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Timur Tabi <timur@...nel.org>,
        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 Tue, Jan 19, 2021 at 12:18:17PM -0800, Randy Dunlap wrote:
> On 1/19/21 11:45 AM, 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:
> 
> Yes, good patch. Should have been like this to begin with IMO.
> 
> > 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);
> 
> Is 'addr' always set here?
> It is only conditionally set above.

It should be, yes. Though I agree, it's not obvious. ptr_to_hashval()
will write to it when returning 0. So if that fails, addr will have 0
written. Both only happen under DUMP_PREFIX_ADDRESS.


-- 
Kees Cook