Potential handle leaks in dup_worker
Ken Brown
kbrown@cornell.edu
Tue Feb 9 14:19:12 GMT 2021
On 2/9/2021 4:47 AM, Corinna Vinschen via Cygwin-developers wrote:
> 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?
I've come across one place where I think it's needed. Suppose build_fh_name is
called with PC_KEEP_HANDLE. It calls build_fh_pc, which calls set_name, which
calls path_conv::operator<<. I think we need to duplicate conv_handle here.
If this is the only place where duplication is needed, we could just do it in
build_fh_pc or fhandler_base::set_name. We would probably want to add a
path_conv::dup_conv_handle method:
inline void dup_conv_handle (path_conv& pc)
{
conv_handle.close ();
conv_handle.dup (pc.conv_handle);
}
Ken
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