linux-kernel - Re: [PATCH 1/4] stringbuf: A string buffer implementation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <200710272009.31430.rusty@rustcorp.com.au>
Date:	Sat, 27 Oct 2007 20:09:30 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...l.org>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 1/4] stringbuf: A string buffer implementation

On Saturday 27 October 2007 06:57:14 Matt Mackall wrote:
> Well I expect once you start letting people easily build strings by
> concatenation, you'll very shortly afterwards have people using them
> in loops.  And having hidden O(n^2) behavior in there is a little sad, 
> even though n will tend to be small and well-bounded. If we can do
> something simple to avoid it, we should.

Hi Matt,

        I avoid typing even a single character of optimization until it's 
justified.  This is partially a reaction against the machoptimization 
tendencies of many kernel programmers, but it's mainly a concern at the 
kernel's complexity creep.

Meanwhile, of course, I've now spent far too long analyzing this :)

Building a 1000 byte string 1 byte at a time involves 6 reallocs (SLAB) or 10 
reallocs (SLUB).  Frankly, that's good enough without an explicit alloc 
length field (better in some ways).

As to keeping an explicit length vs strlen(): those 1000 calls on my test 
machine take 1491ns per call with an explicit length vs 1496ns per call with 
strlen().  That's not worth 4 bytes, let alone a single line of code, O(n^2) 
or no.

As the nail in the coffin, callers only use ->buf, so are insulated from any 
such optimizations if we decided to do them in future.

Hope that helps,
Rusty.
PS.  I don't think we should switch this to a simple char ** tho, as 
the "struct stringbuf" gives us some type safety and reminds people not to 
simply kfree it.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/