From: nobu@...
Date: 2019-04-27T03:45:31+00:00
Subject: [ruby-core:92431] [Ruby trunk Feature#15797] Use realpath(3) instead of custom realpath implementation if available

Issue #15797 has been updated by nobu (Nobuyoshi Nakada).


The tainted-ness issue seems a different story.
Could you make it a separate ticket?

----------------------------------------
Feature #15797: Use realpath(3) instead of custom realpath implementation if available
https://bugs.ruby-lang.org/issues/15797#change-77785

* Author: jeremyevans0 (Jeremy Evans)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
One reason to do this is simplicity, as this approach is ~30 lines of
code instead of ~200.

Performance wise, this performs 25%-115% better, using the following
benchmark on OpenBSD 6.5:

```ruby
require 'benchmark'

f = File
pwd = Dir.pwd
Dir.mkdir('b') unless f.directory?('b')
f.write('b/a', '') unless f.file?('b/a')

args = [
  ["b/a", nil],
  ["#{pwd}/b/a", nil],
  ['a', 'b'],
  ["#{pwd}/b/a", 'b'],
  ["b/a", pwd]
]

args.each do |path, base|
  print "File.realpath(#{path.inspect}, #{base.inspect}): ".ljust(50)
  puts Benchmark.measure{100000.times{f.realpath(path, base)}}
end
```

Before:

```
File.realpath("b/a", nil):                          4.330000   2.990000   7.320000 (  7.316244)
File.realpath("/home/testr/ruby/b/a", nil):         3.560000   2.680000   6.240000 (  6.240951)
File.realpath("a", "b"):                            4.370000   3.080000   7.450000 (  7.452511)
File.realpath("/home/testr/ruby/b/a", "b"):         3.730000   2.640000   6.370000 (  6.371979)
File.realpath("b/a", "/home/testr/ruby"):           3.590000   2.630000   6.220000 (  6.226824)
```

After:

```
File.realpath("b/a", nil):                          1.370000   2.030000   3.400000 (  3.400775)
File.realpath("/home/testr/ruby/b/a", nil):         1.260000   2.770000   4.030000 (  4.024957)
File.realpath("a", "b"):                            2.090000   1.990000   4.080000 (  4.080284)
File.realpath("/home/testr/ruby/b/a", "b"):         1.400000   2.620000   4.020000 (  4.015505)
File.realpath("b/a", "/home/testr/ruby"):           2.150000   2.760000   4.910000 (  4.910634)
```

If someone could benchmark before/after with this patch on Linux and/or MacOS X,
and post the results here, I would appreciate it.

My personal reason for wanting this is that the custom realpath
implementation does not work with OpenBSD's unveil(2) system call,
which limits access to the file system, allowing for security
similar to chroot(2), without most of the downsides.

This change passes all tests except for one assertion related to
taintedness.  Previously, if either argument to `File.realpath` is an
absolute path, then the returned value is considered not tainted.
However, I believe that behavior to be incorrect, because if there is
a symlink anywhere in the path, the returned value can contain a
section that was taken from the file system (unreliable source) that
was not marked as untainted. Example:

```ruby
Dir.mkdir('b') unless File.directory?('b')
File.write('b/a', '') unless File.file?('b/a')
File.symlink('b', 'c') unless File.symlink?('c')
path = File.realpath('c/a'.untaint, Dir.pwd.untaint)
path # "/home/testr/ruby/b/a"
path.tainted? # should be true, as 'b' comes from file system
```

I believe it is safer to always mark the output of realpath as tainted
to prevent this issue, which is what this commit does.

---Files--------------------------------
use-native-realpath.patch (6.31 KB)


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