linux-kernel - Re: [PATCH] fs: make d_path-like functions all have unsigned size

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Date:   Tue, 27 Jul 2021 14:50:19 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jordy Zomer <jordy@...ing.systems>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH] fs: make d_path-like functions all have unsigned size

On Tue, Jul 27, 2021 at 12:36:25PM +0200, Greg Kroah-Hartman wrote:
> When running static analysis tools to find where signed values could
> potentially wrap the family of d_path() functions turn out to trigger a
> lot of mess.  In evaluating the code, all of these usages seem safe, but
> pointer math is involved so if a negative number is ever somehow passed
> into these functions, memory can be traversed backwards in ways not
> intended.
> 
> Resolve all of the abuguity by just making "size" an unsigned value,
> which takes the guesswork out of everything involved.

TBH, I'm not sure it's the right approach.  Huge argument passed to d_path()
is a bad idea, no matter what.  Do you really want to have the damn thing
try and fill 3Gb of buffer, all while holding rcu_read_lock() and a global
spinlock or two?  Hell, s/3Gb/1Gb/ and it won't get any better...


How about we do this instead:

d_path(const struct path *path, char *buf, int buflen)
{
	if (unlikely((unsigned)buflen > 0x8000)) {
		buf += (unsigned)buflen - 0x8000;
		buflen = 0x8000;
	}
	as in mainline
}

and take care of both issues?