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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