Date: Mon, 31 Aug 2009 08:25:13 -0400 From: John Baldwin <jhb@freebsd.org> To: Alan Cox <alc@cs.rice.edu> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, Colin Percival <cperciva@freebsd.org> Subject: Re: svn commit: r196558 - head/usr.bin/look Message-ID: <200908310825.14372.jhb@freebsd.org> In-Reply-To: <4A999104.9030809@cs.rice.edu> References: <200908260330.n7Q3U61l047845@svn.freebsd.org> <200908260734.12893.jhb@freebsd.org> <4A999104.9030809@cs.rice.edu>
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On Saturday 29 August 2009 4:35:16 pm Alan Cox wrote: > John Baldwin wrote: > > On Tuesday 25 August 2009 11:30:06 pm Colin Percival wrote: > > > >> Author: cperciva > >> Date: Wed Aug 26 03:30:06 2009 > >> New Revision: 196558 > >> URL: http://svn.freebsd.org/changeset/base/196558 > >> > >> Log: > >> Don't try to mmap the contents of empty files. This behaviour was harmless > >> prior to r195693, since historical behaviour of mmap(2) was to silently > >> ignore length-zero mmap requests; but mmap now returns EINVAL, which caused > >> look(1) to emit an error message and fail. > >> > > > > FWIW, it did not silently ignore the request. Instead it rounded the size up > > to a page and mapped a page of data. However, if you then passed that pointer > > with a length of 0 to munmap() munmap() would fail. > > > > > > I don't believe that your claim is correct; round_page(0) == 0. Thus, > the value of "size" that would be passed to vm_mmap() would be 0, which > would cause it to immediately return "0", indicating success. In this > case, I believe that the virtual address that would be returned by > mmap(2) would always be the "hint address" for where it should start > looking for free space, which would be the end of the heap/data segment, > unless the application specified its own hint. > > In short, and the reason why I'm correcting you here, is to make clear > that we needn't worry about earlier versions of FreeBSD that don't > implement this change "leaking" or wasting memory from misuse of mmap(2) > in this way. Yes, I think you are correct. -- John Baldwin
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