linux-kernel - Re: [PATCH 02/31] ntsync: Introduce NTSYNC_IOC_CREATE

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Message-ID: <13452408.uLZWGnKmhe@camazotz>
Date: Thu, 15 Feb 2024 13:22:01 -0600
From: Elizabeth Figura <zfigura@...eweavers.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Arnd Bergmann <arnd@...db.de>, Jonathan Corbet <corbet@....net>,
 Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
 linux-api@...r.kernel.org, wine-devel@...ehq.org,
 André Almeida <andrealmeid@...lia.com>,
 Wolfram Sang <wsa@...nel.org>, Arkadiusz Hiler <ahiler@...eweavers.com>,
 Peter Zijlstra <peterz@...radead.org>, Andy Lutomirski <luto@...nel.org>,
 linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 02/31] ntsync: Introduce NTSYNC_IOC_CREATE_SEM.

On Thursday, 15 February 2024 01:28:32 CST Greg Kroah-Hartman wrote:
> On Wed, Feb 14, 2024 at 05:36:38PM -0600, Elizabeth Figura wrote:
> > This corresponds to the NT syscall NtCreateSemaphore().
> > 
> > Semaphores are one of three types of object to be implemented in this driver,
> > the others being mutexes and events.
> > 
> > An NT semaphore contains a 32-bit counter, and is signaled and can be acquired
> > when the counter is nonzero. The counter has a maximum value which is specified
> > at creation time. The initial value of the semaphore is also specified at
> > creation time. There are no restrictions on the maximum and initial value.
> > 
> > Each object is exposed as an file, to which any number of fds may be opened.
> > When all fds are closed, the object is deleted.
> > 
> > Signed-off-by: Elizabeth Figura <zfigura@...eweavers.com>
> > ---
> >  .../userspace-api/ioctl/ioctl-number.rst      |   2 +
> >  drivers/misc/ntsync.c                         | 120 ++++++++++++++++++
> >  include/uapi/linux/ntsync.h                   |  21 +++
> >  3 files changed, 143 insertions(+)
> >  create mode 100644 include/uapi/linux/ntsync.h
> > 
> > diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > index 457e16f06e04..2f5c6994f042 100644
> > --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
> > +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
> > @@ -173,6 +173,8 @@ Code  Seq#    Include File                                           Comments
> >  'M'   00-0F  drivers/video/fsl-diu-fb.h                              conflict!
> >  'N'   00-1F  drivers/usb/scanner.h
> >  'N'   40-7F  drivers/block/nvme.c
> > +'N'   80-8F  uapi/linux/ntsync.h                                     NT synchronization primitives
> > +                                                                     <mailto:wine-devel@...ehq.org>
> >  'O'   00-06  mtd/ubi-user.h                                          UBI
> >  'P'   all    linux/soundcard.h                                       conflict!
> >  'P'   60-6F  sound/sscape_ioctl.h                                    conflict!
> > diff --git a/drivers/misc/ntsync.c b/drivers/misc/ntsync.c
> > index e4969ef90722..3ad86d98b82d 100644
> > --- a/drivers/misc/ntsync.c
> > +++ b/drivers/misc/ntsync.c
> > @@ -5,26 +5,146 @@
> >   * Copyright (C) 2024 Elizabeth Figura
> >   */
> >  
> > +#include <linux/anon_inodes.h>
> > +#include <linux/file.h>
> >  #include <linux/fs.h>
> >  #include <linux/miscdevice.h>
> >  #include <linux/module.h>
> > +#include <linux/slab.h>
> > +#include <uapi/linux/ntsync.h>
> >  
> >  #define NTSYNC_NAME	"ntsync"
> >  
> > +enum ntsync_type {
> > +	NTSYNC_TYPE_SEM,
> > +};
> > +
> > +struct ntsync_obj {
> > +	enum ntsync_type type;
> > +
> > +	union {
> > +		struct {
> > +			__u32 count;
> > +			__u32 max;
> > +		} sem;
> > +	} u;
> > +
> > +	struct file *file;
> > +	struct ntsync_device *dev;
> > +};
> > +
> > +struct ntsync_device {
> > +	struct file *file;
> > +};
> 
> No reference counting is needed for your ntsync_device?  Or are you
> relying on the reference counting of struct file here?
> 
> You pass around pointers to this structure, and save it off into other
> structures, how do you know it is "safe" to do so?

Yes, this relies on the reference counting of struct file. The sync
objects (semaphore etc.) grab a reference when they're created, via
get_file(), and release it when they're destroyed. This reference is
taken from within ioctls on the ntsync_device, so the file must be
valid when we grab a reference. Maybe I'm missing something, though?