Bug 3501 - File.GetAttributes() returns ReadOnly for writable files
Summary: File.GetAttributes() returns ReadOnly for writable files
Alias: None
Product: Runtime
Classification: Mono
Component: io-layer ()
Version: unspecified
Hardware: PC Mac OS
: --- normal
Target Milestone: ---
Assignee: Bugzilla
Depends on:
Reported: 2012-02-16 13:58 UTC by Jonathan Pryor
Modified: 2014-01-13 00:59 UTC (History)
3 users (show)

Is this bug a regression?: ---
Last known good build:

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Description Jonathan Pryor 2012-02-16 13:58:17 UTC
Context: http://mono-for-android.1047100.n5.nabble.com/System-IO-File-GetAttributes-returns-always-ReadOnly-td5430132.html

The problem is that mono/mono/io-layer/io.c!_wapi_stat_to_file_attributes() ignores st_uid and st_gid (and anything else that would be useful). Result: it _only_ checks S_IWUSR to determine if a file writable:

> if (!(buf->st_mode & S_IWUSR)) {

So, consider a file not owned by the current user, but accessible by the current user because of groups:

> $ whoami
> jon
> $ groups
> staff everyone _appstore localaccounts _appserverusr admin _appserveradm _lpadmin _lpoperator _developer com.apple.access_ssh com.apple.access_screensharing
> $ touch foo
> $ sudo chown root:staff foo
> $ sudo chmod 075 foo
> $  ls -l foo
> ----rwxr-x  1 root  staff  0 Feb 16 13:43 foo

The file is still writable by the current user:

> $ echo foo > foo
> # Note: no error

Mono disagrees:

> $ csharp
> Mono C# Shell, type "help;" for help
> Enter statements below.
> csharp> System.IO.File.GetAttributes("foo");     
> ReadOnly

despite the fact that we can, in fact, write to it:

> csharp> using (var o = System.IO.File.AppendText("foo")) o.WriteLine("bar");
> csharp> ^D
> $ cat foo
> foo
> bar

_wapi_stat_to_file_attributes() should compare st_uid to geteuid(2) (and getuid(2)?), and when they don't match check for group ownership (getgrent(3) & co.), and if group checking fails then use S_IWOTH.
Comment 1 Rodrigo Kumpera 2014-01-13 00:31:17 UTC
This is anything but trivial.

We can check the euid and egid against the supplied struct stat. But, beyond that, we have two options:

1)The getgrgid / getpwuid dance, which does work on OSX/iOS, is very complex and fail flat in face of extended permission systems like SELinux or OSX sandboxes.

2)Use access(2) and accept that it will fail with setuid/setgid programs.
Comment 2 Rodrigo Kumpera 2014-01-13 00:59:54 UTC
Fixed in master using option 2.