From: Markus Wichmann <nullplan@gmx.net>
To: musl@lists.openwall.com
Subject: Re: Wrong info in libc comparison
Date: Sat, 16 Sep 2017 21:34:34 +0200 [thread overview]
Message-ID: <20170916193434.x7oxm3fusmt46srz@voyager> (raw)
In-Reply-To: <20170916171154.GC15263@port70.net>
On Sat, Sep 16, 2017 at 07:11:54PM +0200, Szabolcs Nagy wrote:
> glibc, uclibc, dietlibc, newlib, netbsd, openbsd, freebsd
> qsort are all O(n^2) worst-case, musl qsort is O(n log(n)).
>
Yes, sorry, I mixed that up. Quicksort is loglinear only in the average
case. Same for Shell sort being O(sqrt(n^3)).
> i think this is not a sidetrack, but relevant detail
> for a libc comparision page.
> (the openbsd proof of concept stack clash exploit
> relied on the unbounded stack use in qsort, that
> would not work against musl, but all the other libcs
> are affected.)
No, stack usage is not necessarily linear even with quicksort. dietlibc
and avr-libc have linear stack usage (with avr-libc always recursing
into the first subproblem, and dietlibc always recursing into both
subproblems), that's right, but glibc uses a constant amount of stack
(automatic allocation of an array to hold the maximum possible number of
subproblems, which is equal to the number of bits in a machine word),
and newlib uses recursion into the smaller subproblem, thus using a
logarithmic amount of stack (functionally constant, since you can give
an upper limit).
And uclibc also uses a constant amount of stack. Shell sort doesn't need
recursion.
Ciao,
Markus
next prev parent reply other threads:[~2017-09-16 19:34 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-13 13:51 Markus Wichmann
2017-09-13 18:10 ` Rich Felker
2017-09-13 18:51 ` Markus Wichmann
2017-09-13 19:25 ` Szabolcs Nagy
2017-09-13 19:53 ` Rich Felker
2017-09-15 19:18 ` Markus Wichmann
2017-09-16 9:37 ` Szabolcs Nagy
2017-09-16 14:01 ` Markus Wichmann
2017-09-16 17:11 ` Szabolcs Nagy
2017-09-16 19:34 ` Markus Wichmann [this message]
2017-09-16 16:18 ` Rich Felker
2017-09-16 18:38 ` Markus Wichmann
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