Re: [PATCH 1/3] leaking_addresses: skip all /proc/PID except /proc/1

[Tobin C. Harding]

On Mon, Feb 26, 2018 at 10:09:31PM -0700, Tycho Andersen wrote:
> Hi Tobin,
> 
> On Tue, Feb 27, 2018 at 03:45:09PM +1100, Tobin C. Harding wrote:
> > When the system is idle it is likely that most files under /proc/PID
> > will be identical for various processes.  Scanning _all_ the PIDs under
> > /proc is unnecessary and implies that we are thoroughly scanning /proc.
> > This is _not_ the case because there may be ways userspace can trigger
> > creation of /proc files that leak addresses but were not present during
> > a scan.  For these two reasons we should exclude all PID directories
> > under /proc except '1/'
> > 
> > Exclude all /proc/PID except /proc/1.
> > 
> > Signed-off-by: Tobin C. Harding <me@xxxxxxxx>
> > ---
> >  scripts/leaking_addresses.pl | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/scripts/leaking_addresses.pl b/scripts/leaking_addresses.pl
> > index 6e5bc57caeaa..fb40e2828f43 100755
> > --- a/scripts/leaking_addresses.pl
> > +++ b/scripts/leaking_addresses.pl
> > @@ -10,6 +10,14 @@
> >  # Use --debug to output path before parsing, this is useful to find files that
> >  # cause the script to choke.
> >  
> > +#
> > +# When the system is idle it is likely that most files under /proc/PID will be
> > +# identical for various processes.  Scanning _all_ the PIDs under /proc is
> > +# unnecessary and implies that we are thoroughly scanning /proc.  This is _not_
> > +# the case because there may be ways userspace can trigger creation of /proc
> > +# files that leak addresses but were not present during a scan.  For these two
> > +# reasons we exclude all PID directories under /proc except '1/'
> > +
> >  use warnings;
> >  use strict;
> >  use POSIX;
> > @@ -472,6 +480,9 @@ sub walk
> >  			my $path = "$pwd/$file";
> >  			next if (-l $path);
> >  
> > +			# skip /proc/PID except /proc/1
> > +			next if ($path =~ /\/proc\/(?:[2-9][0-9]*|1[0-9]+)/);
> 
> Can't we just do,
> 
> substr($path, 0, len("/proc/1/")) eq "/proc/1/" ?
> 
> seems much easier to read than the regex.

This is much better.  I guess it's true what they say, be careful after
reading a book about hammers, everything will look like a nail.


	Tobin