Date: Sat, 31 Oct 2009 03:38:24 +0100 (CET) From: Alexander Best <alexbestms@math.uni-muenster.de> To: John Baldwin <jhb@freebsd.org>, <freebsd-hackers@freebsd.org> Cc: Alexander Best <alexbestms@math.uni-muenster.de> Subject: Re: mmap(2) with MAP_ANON honouring offset although it shouldn't Message-ID: <permail-200910310238241e86ffa800003202-a_best01@message-id.uni-muenster.de> In-Reply-To: <200910211349.10174.jhb@freebsd.org>
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John Baldwin schrieb am 2009-10-21:
> On Wednesday 21 October 2009 11:51:04 am Alexander Best wrote:
> > although the mmap(2) manual states in section MAP_ANON:
> > "The offset argument is ignored."
> > this doesn't seem to be true. running
> > printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_NONE, MAP_ANON, -1,
> > 0x12345678));
> > and
> > printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_NONE, MAP_ANON, -1,
> > 0));
> > produces different outputs. i've attached a patch to solve the
> > problem. the
> > patch is similar to the one proposed in this PR, but should apply
> > cleanly to
> > CURRENT: http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/71258
> A simpler patch would be to simply set pos = 0 below the MAP_STACK
> line if
> MAP_ANON is set.
how about the following patch. problem seems to be that pos = 0 needs to be
set before pageoff is being calculated.
i've tested mmap with MAP_STACK and the offset gets discarded just as
documented in mmap(2). with the patch the offset handling with MAP_ANON and
MAP_STACK (implies MAP_ANON) are the same.
another short question:
why does the second call when doing
printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_READ|PROT_WRITE,
MAP_STACK, -1, 0x0));
printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_READ|PROT_WRITE,
MAP_STACK, -1, 0x0));
fail? doesn't MAP_STACK allow mapping the same region twice?
printf("%p\n", mmap((void*)0x1000, 0x1000, PROT_READ|PROT_WRITE,
MAP_STACK, -1, 0x0));
printf("%p\n", mmap((void*)0x2000, 0x1000, PROT_READ|PROT_WRITE,
MAP_STACK, -1, 0x0));
works just as expected.
cheers.
alex
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