From: Al Viro <viro@ftp.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>,
John Reiser <jreiser@BitWagon.com>,
Andrew Morton <akpm@linux-foundation.org>,
security@kernel.org, tytso@mit.edu,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
mpm@selenic.com, linux-sparse@vger.kernel.org
Subject: Re: Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data)
Date: Mon, 17 Dec 2007 17:48:06 +0000 [thread overview]
Message-ID: <20071217174806.GC8181@ftp.linux.org.uk> (raw)
In-Reply-To: <alpine.LFD.0.9999.0712170856050.21557@woody.linux-foundation.org>
On Mon, Dec 17, 2007 at 09:28:57AM -0800, Linus Torvalds wrote:
>
>
> On Sat, 15 Dec 2007, Herbert Xu wrote:
> >
> > There ought to be a warning about this sort of thing.
>
> We could add it to sparse. The appended (untested) patch seems to say
> there's a lot of those signed divides-by-power-of-twos.
I'm not sure that you are warning about the right things. If you want
a real nightmare scenario in that area, consider this:
int x[20];
int *p = x + n;
int *q = x + m;
p - q
((char *)p - (char *)q)/4
((char *)p - (char *)q)/sizeof(int)
The first two are equivalent on all targets we care about. However, an
attempt to make the second one "more portable" silently creates the
code that'll do something entirely different as soon as we get m > n...
next prev parent reply other threads:[~2007-12-17 17:48 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <E1J3RD5-0006yU-00@gondolin.me.apana.org.au>
2007-12-17 17:28 ` Signed divides vs shifts (Re: [Security] /dev/urandom uses uninit bytes, leaks user data) Linus Torvalds
2007-12-17 17:48 ` Al Viro [this message]
2007-12-17 17:55 ` Eric Dumazet
2007-12-17 18:05 ` Ray Lee
2007-12-17 18:10 ` Eric Dumazet
2007-12-17 18:12 ` Ray Lee
2007-12-17 18:23 ` Al Viro
2007-12-17 18:28 ` [Security] Signed divides vs shifts (Re: " Linus Torvalds
2007-12-17 19:08 ` Al Viro
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20071217174806.GC8181@ftp.linux.org.uk \
--to=viro@ftp.linux.org.uk \
--cc=akpm@linux-foundation.org \
--cc=herbert@gondor.apana.org.au \
--cc=jreiser@BitWagon.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-sparse@vger.kernel.org \
--cc=mpm@selenic.com \
--cc=security@kernel.org \
--cc=torvalds@linux-foundation.org \
--cc=tytso@mit.edu \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).