Re: cp(1) fails to copy file from /proc

[Mike Frysinger]

On Saturday 18 April 2009 16:58:52 Jim Meyering wrote:
> Mike Frysinger wrote:
> > On Friday 17 April 2009 18:28:07 James Youngman wrote:
> >> The patch itself looks good, but it might be worth leaving in a
> >> comment indicating why the optimisation should not be reintroduced...
> >
> > and/or a new test (i prefer the "and"):
> >  if [ -e /proc/cpuinfo ] ; then
> >    cp /proc/cpuinfo cpuinfo.cp
> >    cat /proc/cpuinfo > cpuinfo.cat
> >    cmp cpuinfo.cp cpuinfo.cat
> >  fi
>
> Of course ;-)
> As promised, I've added a test for this below.
>
> We can't use /proc/cpuinfo, at least not precisely like that,
> because its cpu speed line can change due to frequency scaling.
> Also, that file is usually too small to trigger the failure.

hmm, i seem to remember cpuinfo being a pita when doing remote `ssh cp ...`, 
but must have been some other file.  you got the idea at any rate :).

> Here's a more complete patch, with a title and NEWS reflecting
> that I now think it's a linux kernel bug.

i think that characterization is incorrect.  POSIX certainly states that a 
read value smaller than what was requested is not an error in and of itself.  
i havent looked at the kernel internals as to why this occurs, but if i had to 
make a guess, it's related to the seq_file stuff as a common helper in place 
of many common buggy procfs implementations.  and in that case, the current 
behavior is to be expected.
-mike

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