GNU bug report logs - #21942
Files with incorrect file sizes

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From: Stephan Müller <fruktopus <at> gmail.com>
To: Jim Meyering <jim <at> meyering.net>
Cc: 21942 <at> debbugs.gnu.org
Subject: bug#21942: [bug-diffutils] bug#21942: Files with incorrect file sizes
Date: Fri, 20 Nov 2015 23:33:34 +0100

Am Fri, 20 Nov 2015 23:12:40 +0100
schrieb Jim Meyering <jim <at> meyering.net>:

> On Fri, Nov 20, 2015 at 9:06 PM, Stephan Müller <fruktopus <at> gmail.com>
> wrote:
> > Am Fri, 20 Nov 2015 18:28:37 +0100
> > schrieb Jim Meyering <jim <at> meyering.net>:
> >
> >> On Tue, Nov 17, 2015 at 12:44 PM, Stephan Müller
> >> <fruktopus <at> gmail.com> wrote:
> >> > recently I had to debug weird problem. Finally I figured it out.
> >> >
> >> > Virtual file systems like /sys or /proc usually don't care about
> >> > file sizes. All files have a size of 0. This leads to
> >> > difficulties as diff sometimes looks for file sizes.
> >> >
> >> > Say you do:
> >> >
> >> >> $ cp /proc/cmdline my_cmdline
> >> >> $ diff /proc/cmdline my_cmdline ; echo $?
> >> >> 0      // ok, files don't differ
> >> >> $ diff --brief /proc/cmdline my_cmdline
> >> >> Files /proc/cmdline and mycmdline differ
> >> >
> >> > The --brief option triggers a binary compare, as we aren't
> >> > interested in the actual differences this makes sense. As a first
> >> > step, file sizes are compared (0 vs ~150) and the files are
> >> > reported as different.
> >>
> >> thanks for the report.
> >> What version of diffutils are you using?
> >> I think this has been fixed for some time.
> >> I was unable to reproduce with 2.8.1 nor with the latest built from
> >> git. I.e., I created an empty file and used diff-2.8.1 to compare
> >> it with the nominally-
> >> zero-length /proc/cmdline file, and diff did the right thing.
> >> Also, I ran stat to show st_size of each file is indeed 0:
> >>
> >>   $ :
> >> > /tmp/k; /p/p/diffutils-2.8.1/bin/diff /proc/cmdline /tmp/k; \
> >> > stat --format %s /proc/cmdline /tmp/k
> >>   1d0
> >>   < ro root=LABEL=...
> >>   0
> >>   0
> >>
> >> In fact, I went ahead and built all available versions and tested
> >> them like this:
> >>
> >>   $ for i in /p/p/*/bin/diff; do p=diffutils-$i; echo $i; $i
> >> /proc/cmdline /tmp/k > /dev/null && echo bad; done
> >>   /p/p/diffutils-2.7/bin/diff
> >>   /p/p/diffutils-2.8.1/bin/diff
> >>   /p/p/diffutils-2.8/bin/diff
> >>   /p/p/diffutils-2.9/bin/diff
> >>   /p/p/diffutils-3.0/bin/diff
> >>   /p/p/diffutils-3.1/bin/diff
> >>   /p/p/diffutils-3.2/bin/diff
> >>   /p/p/diffutils-3.3/bin/diff
> >
> > Hi,
> >
> > I am using v.3.3 of diffutils
> >
> > $ diff -v
> > diff (GNU diffutils) 3.3
> >
> > but I think you misunderstood the problem. Sorry for being
> > ambiguous. I am not diffing against an empty file. That works well.
> > The point is procfs doesn't care about size, but 'normal' file
> > systems do. So for example on my system I have (after
> > cp /proc/cmdline mycmdline)
> >
> > $ stat --format %s /proc/cmdline mycmdline
> > 0
> > 140
> >
> > The result of diffing /proc/cmdline against mycmdline depends on the
> > --brief flag.
> >
> > STEPS TO REPRODUCE:
> >
> > cp /proc/cmdline mycmdline
> > diff --brief /proc/cmdline mycmdline > /dev/null ; echo ?$
> > 1
> > diff /proc/cmdline mycmdline ; echo $?
> > 0
> >
> > EXPECTED RESULT:
> >
> > cp /proc/cmdline mycmdline
> > diff --brief /proc/cmdline mycmdline > /dev/null ; echo ?$
> > 0
> > diff /proc/cmdline mycmdline ; echo $?
> > 0
> 
> Oh, indeed. Thank you for clarifying. That feels like a bug.
> Here's a knee-jerk patch that refrains from using the
> st_size-comparing heuristic when either of the sizes is zero. This
> may well be wrong. I have only barely tested the diff.c code path.

Thanks, that makes the problem at least (even) less unlikely. But if we
cant trust file sizes we're doomed. What do you think about a flag
controlling comparison by size and a notice if files differ by size.

I can craft a patch for this.

  ~s

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