Potential handle leaks in dup_worker

Corinna Vinschen corinna-cygwin@cygwin.com
Tue Feb 9 09:47:23 GMT 2021


On Feb  8 12:39, Ken Brown via Cygwin-developers wrote:
> I've had occasion to work through dtable::dup_worker, and I'm seeing the
> potential for leaks of path_conv handles.  I haven't seen any evidence that
> the leaks actually occur, but the code should probably be cleaned up if I'm
> right.
> 
> dup_worker calls clone to create newfh from oldfh.  clone calls copyto,
> which calls operator=, which calls path_conv::operator=, which duplicates
> the path_conv handle from oldfh to newfh.  Then copyto calls reset, which
> calls path_conv::operator<<, which again duplicates the path_conv handle
> from oldfh to newfh without first closing the previous one.  That's the
> first leak.
> 
> Further on, dup_worker calls newfh->pc.reset_conv_handle (), which sets the
> path_conv handle of newfh to NULL without closing the existing handle.  So
> that's a second leak.  This one is easily fixed by calling close_conv_handle
> instead of reset_conv_handle.

Nice detective work, you're right.  For fun, this is easily testable.
Apply this patch to Cygwin:

diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
index 52a020f07d5e..58e993b66c42 100644
--- a/winsup/cygwin/syscalls.cc
+++ b/winsup/cygwin/syscalls.cc
@@ -1475,6 +1475,10 @@ open (const char *unix_path, int flags, ...)
       int opt = PC_OPEN | PC_SYM_NOFOLLOW_PROCFD;
       opt |= (flags & (O_NOFOLLOW | O_EXCL)) ? PC_SYM_NOFOLLOW
 					     : PC_SYM_FOLLOW;
+
+      if (flags & O_NOATIME)
+      	opt |= PC_KEEP_HANDLE;
+
       /* This is a temporary kludge until all utilities can catch up
 	 with a change in behavior that implements linux functionality:
 	 opening a tty should not automatically cause it to become the

And then create an STC like this:

  #define _GNU_SOURCE 1
  #include <stdio.h>
  #include <unistd.h>
  #include <fcntl.h>

  int
  main (int argc, char **argv)
  {
    int fd, fd2;

    fd = open (argv[1], O_RDONLY | O_NOATIME);
    dup (fd);
  }

> As a practical matter, I think the path_conv handle of oldfh is always NULL
> when dup_worker is called, so there's no actual leak.

Right, because conv_handle should only be non-NULL in calls to stat(2)
and friends.

Nevertheless, it's a bad idea to keep this code.  So the question is
this:  Do we actually *need* to duplicate the conv_handle at all?
It doesn't look like this is ever needed.  Perhaps the code should
just never duplicate conv_handle and just always reset it to NULL
instead?


Thanks,
Corinna


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