Re: [PATCH v5 1/2] drivers/misc: sysgenid: add system generation id driver

[Catangiu, Adrian Costin]

On 09/02/2021, 16:53, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:

    On Mon, Feb 01, 2021 at 07:24:53PM +0200, Adrian Catangiu wrote:
    > - Background and problem
    >
    > The System Generation ID feature is required in virtualized or
    > containerized environments by applications that work with local copies
    > or caches of world-unique data such as random values, uuids,
    > monotonically increasing counters, etc.
    > Such applications can be negatively affected by VM or container
    > snapshotting when the VM or container is either cloned or returned to
    > an earlier point in time.
    >
    > Furthermore, simply finding out about a system generation change is
    > only the starting point of a process to renew internal states of
    > possibly multiple applications across the system. This process could
    > benefit from a driver that provides an interface through which
    > orchestration can be easily done.
    >
    > - Solution
    >
    > The System Generation ID is a simple concept meant to alleviate the
    > issue by providing a monotonically increasing u32 counter that changes
    > each time the VM or container is restored from a snapshot.
    >
    > The `sysgenid` driver exposes a monotonic incremental System Generation
    > u32 counter via a char-dev FS interface that provides sync and async
    > SysGen counter updates notifications. It also provides SysGen counter
    > retrieval and confirmation mechanisms.
    >
    > The counter starts from zero when the driver is initialized and
    > monotonically increments every time the system generation changes.
    >
    > The `sysgenid` driver exports the `void sysgenid_bump_generation()`
    > symbol which can be used by backend drivers to drive system generation
    > changes based on hardware events.
    > System generation changes can also be driven by userspace software
    > through a dedicated driver ioctl.
    >
    > Userspace applications or libraries can then (a)synchronously consume
    > the system generation counter through the provided FS interface to make
    > any necessary internal adjustments following a system generation
    > update.
    >
    > Signed-off-by: Adrian Catangiu <acatan@xxxxxxxxxx>
    > ---
    >  Documentation/misc-devices/sysgenid.rst            | 236 ++++++++++++++++
    >  Documentation/userspace-api/ioctl/ioctl-number.rst |   1 +
    >  MAINTAINERS                                        |   8 +
    >  drivers/misc/Kconfig                               |  16 ++
    >  drivers/misc/Makefile                              |   1 +
    >  drivers/misc/sysgenid.c                            | 307 +++++++++++++++++++++
    >  include/uapi/linux/sysgenid.h                      |  17 ++
    >  7 files changed, 586 insertions(+)
    >  create mode 100644 Documentation/misc-devices/sysgenid.rst
    >  create mode 100644 drivers/misc/sysgenid.c
    >  create mode 100644 include/uapi/linux/sysgenid.h
    >
    > diff --git a/Documentation/misc-devices/sysgenid.rst b/Documentation/misc-devices/sysgenid.rst
    > new file mode 100644
    > index 0000000..4337ca0
    > --- /dev/null
    > +++ b/Documentation/misc-devices/sysgenid.rst
    > @@ -0,0 +1,236 @@
    > +.. SPDX-License-Identifier: GPL-2.0
    > +
    > +========
    > +SYSGENID
    > +========
    > +
    > +The System Generation ID feature is required in virtualized or
    > +containerized environments by applications that work with local copies
    > +or caches of world-unique data such as random values, UUIDs,
    > +monotonically increasing counters, etc.
    > +Such applications can be negatively affected by VM or container
    > +snapshotting when the VM or container is either cloned or returned to
    > +an earlier point in time.
    > +
    > +The System Generation ID is a simple concept meant to alleviate the
    > +issue by providing a monotonically increasing counter that changes
    > +each time the VM or container is restored from a snapshot.
    > +The driver for it lives at ``drivers/misc/sysgenid.c``.
    > +
    > +The ``sysgenid`` driver exposes a monotonic incremental System
    > +Generation u32 counter via a char-dev FS interface accessible through
    > +``/dev/sysgenid`` that provides sync and async SysGen counter update
    > +notifications. It also provides SysGen counter retrieval and
    > +confirmation mechanisms.
    > +
    > +The counter starts from zero when the driver is initialized and
    > +monotonically increments every time the system generation changes.
    > +
    > +The ``sysgenid`` driver exports the ``void sysgenid_bump_generation()``
    > +symbol which can be used by backend drivers to drive system generation
    > +changes based on hardware events.
    > +System generation changes can also be driven by userspace software
    > +through a dedicated driver ioctl.
    > +
    > +Userspace applications or libraries can (a)synchronously consume the
    > +system generation counter through the provided FS interface, to make
    > +any necessary internal adjustments following a system generation update.
    > +
    > +Driver FS interface:
    > +
    > +``open()``:
    > +  When the device is opened, a copy of the current Sys-Gen-Id (counter)
    > +  is associated with the open file descriptor. The driver now tracks
    > +  this file as an independent *watcher*. The driver tracks how many
    > +  watchers are aware of the latest Sys-Gen-Id counter and how many of
    > +  them are *outdated*; outdated being those that have lived through
    > +  a Sys-Gen-Id change but not yet confirmed the new generation counter.
    > +
    > +``read()``:
    > +  Read is meant to provide the *new* system generation counter when a
    > +  generation change takes place. The read operation blocks until the
    > +  associated counter is no longer up to date, at which point the new
    > +  counter is provided/returned.
    > +  Nonblocking ``read()`` uses ``EAGAIN`` to signal that there is no
    > +  *new* counter value available. The generation counter is considered
    > +  *new* for each open file descriptor that hasn't confirmed the new
    > +  value following a generation change. Therefore, once a generation
    > +  change takes place, all ``read()`` calls will immediately return the
    > +  new generation counter and will continue to do so until the
    > +  new value is confirmed back to the driver through ``write()``.
    > +  Partial reads are not allowed - read buffer needs to be at least
    > +  32 bits in size.
    > +
    > +``write()``:
    > +  Write is used to confirm the up-to-date Sys Gen counter back to the
    > +  driver.
    > +  Following a VM generation change, all existing watchers are marked
    > +  as *outdated*. Each file descriptor will maintain the *outdated*
    > +  status until a ``write()`` confirms the up-to-date counter back to
    > +  the driver.
    > +  Partial writes are not allowed - write buffer should be exactly
    > +  32 bits in size.
    > +
    > +``poll()``:
    > +  Poll is implemented to allow polling for generation counter updates.
    > +  Such updates result in ``EPOLLIN`` polling status until the new
    > +  up-to-date counter is confirmed back to the driver through a
    > +  ``write()``.
    > +
    > +``ioctl()``:
    > +  The driver also adds support for waiting on open file descriptors
    > +  that haven't acknowledged a generation counter update, as well as a
    > +  mechanism for userspace to *force* a generation update:
    > +
    > +  - SYSGENID_WAIT_WATCHERS: blocks until there are no more *outdated*
    > +    watchers, or if a ``timeout`` argument is provided, until the
    > +    timeout expires.
    > +    If the current caller is *outdated* or a generation change happens
    > +    while waiting (thus making current caller *outdated*), the ioctl
    > +    returns ``-EINTR`` to signal the user to handle event and retry.
    > +  - SYSGENID_FORCE_GEN_UPDATE: forces a generation counter increment.
    > +    It takes a ``minimum-generation`` argument which represents the
    > +    minimum value the generation counter will be incremented to. For
    > +    example if current generation is ``5`` and ``SYSGENID_FORCE_GEN_UPDATE(8)``
    > +    is called, the generation counter will increment to ``8``.
    > +    This IOCTL can only be used by processes with CAP_CHECKPOINT_RESTORE
    > +    or CAP_SYS_ADMIN capabilities.
    > +
    > +``mmap()``:
    > +  The driver supports ``PROT_READ, MAP_SHARED`` mmaps of a single page
    > +  in size. The first 4 bytes of the mapped page will contain an
    > +  up-to-date u32 copy of the system generation counter.
    > +  The mapped memory can be used as a low-latency generation counter
    > +  probe mechanism in critical sections - see examples.
    >

    From what I have seen this mmap interface is still fundamentally racy.
    How about splitting it to a separate patch?

Hi all!
Michael, you're right that this is fundamentally racy on its own. Even if we eliminate the
IRQ race by mmaping HW directly, it is still fundamentally racy when looking one or more
layers above.
The only way to eliminate races is having a system-level overlord entity that quiesces the
system before a snapshot then, on snapshot-load/resume, makes sure all software adjusts
before unquiescing and allowing safe workload continuation.
Without this overlord synchronization there will be races in the callers of the sysgenid API
even if the API syscall itself is race-free.
Let's take an OpenSSL session token for example. OpenSSL is a prime target for the mmap()
interface so it can be fast (checks against memory - read/write syscalls in the hotpath will
likely not work). Even if the library code is made 100% race-free, the session token still lives
in the caller of the library and/or in the OS network stack, etc.

We even have the same problem with the transactional read/write API. It too, is racy if
snapshots can happen at any arbitrary time during an active workload. A library or application
can go through the read/write transaction successfully (race-free) and generate a
"safe/unique" secret, then get snapshotted while that secret is in-flight with no way to catch
it unless we'd check generation at every layer, every step of the way (which looks impossible
to me).

Bottom line is we don't have a way to track all of these in-flight secrets and dynamically scrub
them from existence with snapshot events happening arbitrarily.

But, we can do the next best thing. Simplify the problem space, drop the arbitrary part and
take some control over the snapshot flow. Use an overlord entity that quiesces the system
before snapshot, and un-quiesces post-snapshot-resume _only after_ system components
have readjusted to new environment/generation.
Add sysgenid framework that standardizes an API for system software to find out about
needing to readjust and at the same time provide a mechanism for the overlord entity to wait
for everyone to be done, the system to have readjusted, so it can un-quiesce.

In this context, this is how I see the utility and safe-usage of the sysgenid API:

- The poll(), read(), write() operations are used in event loops so software is notified (while
  in quiesced state) that generation has changed and they need to do their specific internal
  adjustments. If done transactionally, they can ACK that they are ready to thrive in the new
  generation.
- The WAIT_WATCHERS ioctl() is used by the overlord to know when system is ready for
  un-quiesce.
- SysGenID needs a (currently missing) IOCTL or some other type of knob for users to
  enable/disable tracking, where tracking means sysgenid driver keeping track of that FD as
  a *watcher* - keeping track if it is up-to-date or outdated.
- There is a class of software (like PRNG libraries or OpenSSL since we talked about it earlier)
  that doesn't fit the event-loop model and also has strict latency requirements. These can
  take advantage of the mmap() interface and lazily adjust on-demand whenever they are
  called _after un-quiesce_.
  For a well-designed service stack, these libraries should not be called while system is
  quiesced. When workload is resumed by the overlord, on the first call into these libs, they
  will JIT readjust race-free.
  Users of mmap() interface would disable watcher tracking since they lazily on-demand
  readjust and waiting on them doesn't make sense.

Now I know that there is a lot of subtlety involved here, but this is a hard problem without
an easy simple solution. If you and people here agree on this approach I will document all of
this in great detail with big fat warnings on how to safely use it (in driver documentation).

I'm sorry for the massive reply, but the proposed mechanism and its details are a direct
result of the valuable feedback coming from this upstream review, and I want to make sure
we align on all details so we get it right!

Let me know what you all think!

    > +``close()``:
    > +  Removes the file descriptor as a system generation counter *watcher*.
    > +
    > +Example application workflows
    > +-----------------------------
    > +
    > +1) Watchdog thread simplified example::
    > +
    > +     void watchdog_thread_handler(int *thread_active)
    > +     {
    > +             unsigned int genid;
    > +             int fd = open("/dev/sysgenid", O_RDWR | O_CLOEXEC, S_IRUSR | S_IWUSR);
    > +
    > +             do {
    > +                     // read new gen ID - blocks until VM generation changes
    > +                     read(fd, &genid, sizeof(genid));
    > +
    > +                     // because of VM generation change, we need to rebuild world
    > +                     reseed_app_env();
    > +
    > +                     // confirm we're done handling gen ID update
    > +                     write(fd, &genid, sizeof(genid));
    > +             } while (atomic_read(thread_active));
    > +
    > +             close(fd);
    > +     }
    > +
    > +2) ASYNC simplified example::
    > +
    > +     void handle_io_on_sysgenfd(int sysgenfd)
    > +     {
    > +             unsigned int genid;
    > +
    > +             // read new gen ID - we need it to confirm we've handled update
    > +             read(fd, &genid, sizeof(genid));
    > +
    > +             // because of VM generation change, we need to rebuild world
    > +             reseed_app_env();
    > +
    > +             // confirm we're done handling the gen ID update
    > +             write(fd, &genid, sizeof(genid));
    > +     }
    > +
    > +     int main() {
    > +             int epfd, sysgenfd;
    > +             struct epoll_event ev;
    > +
    > +             epfd = epoll_create(EPOLL_QUEUE_LEN);
    > +
    > +             sysgenfd = open("/dev/sysgenid",
    > +                            O_RDWR | O_CLOEXEC | O_NONBLOCK,
    > +                            S_IRUSR | S_IWUSR);
    > +
    > +             // register sysgenid for polling
    > +             ev.events = EPOLLIN;
    > +             ev.data.fd = sysgenfd;
    > +             epoll_ctl(epfd, EPOLL_CTL_ADD, sysgenfd, &ev);
    > +
    > +             // register other parts of your app for polling
    > +             // ...
    > +
    > +             while (1) {
    > +                     // wait for something to do...
    > +                     int nfds = epoll_wait(epfd, events,
    > +                             MAX_EPOLL_EVENTS_PER_RUN,
    > +                             EPOLL_RUN_TIMEOUT);
    > +                     if (nfds < 0) die("Error in epoll_wait!");
    > +
    > +                     // for each ready fd
    > +                     for(int i = 0; i < nfds; i++) {
    > +                             int fd = events[i].data.fd;
    > +
    > +                             if (fd == sysgenfd)
    > +                                     handle_io_on_sysgenfd(sysgenfd);
    > +                             else
    > +                                     handle_some_other_part_of_the_app(fd);
    > +                     }
    > +             }
    > +
    > +             return 0;
    > +     }
    > +
    > +3) Mapped memory polling simplified example::
    > +
    > +     /*
    > +      * app/library function that provides cached secrets
    > +      */
    > +     char * safe_cached_secret(app_data_t *app)
    > +     {
    > +             char *secret;
    > +             volatile unsigned int *const genid_ptr = get_sysgenid_mapping(app);
    > +     again:
    > +             secret = __cached_secret(app);
    > +
    > +             if (unlikely(*genid_ptr != app->cached_genid)) {
    > +                     app->cached_genid = *genid_ptr;

    And this is racy, isn't it? There is not system call to synchronize
    with interrupts, so the value this reads could be out of date.

Yes, without system quiescing and controlled snapshot moments, all of these are racy.
I will rewrite documentation to capture all details and pitfalls and how to use sysgenid
safely.

    > +                     barrier();
    > +
    > +                     // rebuild world then confirm the genid update (thru write)
    > +                     rebuild_caches(app);
    > +
    > +                     ack_sysgenid_update(app);
    > +
    > +                     goto again;
    > +             }
    > +
    > +             return secret;
    > +     }
    > +
    > +4) Orchestrator simplified example::
    > +
    > +     /*
    > +      * orchestrator - manages multiple applications and libraries used by
    > +      * a service and tries to make sure all sensitive components gracefully
    > +      * handle VM generation changes.
    > +      * Following function is called on detection of a VM generation change.
    > +      */
    > +     int handle_sysgen_update(int sysgen_fd, unsigned int new_gen_id)
    > +     {
    > +             // pause until all components have handled event
    > +             pause_service();
    > +
    > +             // confirm *this* watcher as up-to-date
    > +             write(sysgen_fd, &new_gen_id, sizeof(unsigned int));
    > +
    > +             // wait for all *others* for at most 5 seconds.
    > +             ioctl(sysgen_fd, VMGENID_WAIT_WATCHERS, 5000);
    > +
    > +             // all applications on the system have rebuilt worlds
    > +             resume_service();
    > +     }
    > diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
    > index a4c75a2..d74fed4 100644
    > --- a/Documentation/userspace-api/ioctl/ioctl-number.rst
    > +++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
    > @@ -356,6 +356,7 @@ Code  Seq#    Include File                                           Comments
    >  0xDB  00-0F  drivers/char/mwave/mwavepub.h
    >  0xDD  00-3F                                                          ZFCP device driver see drivers/s390/scsi/
    >                                                                       <mailto:aherrman@xxxxxxxxxx>
    > +0xE4  01-02  uapi/linux/sysgenid.h                                   SysGenID misc driver
    >  0xE5  00-3F  linux/fuse.h
    >  0xEC  00-01  drivers/platform/chrome/cros_ec_dev.h                   ChromeOS EC driver
    >  0xF3  00-3F  drivers/usb/misc/sisusbvga/sisusb.h                     sisfb (in development)
    > diff --git a/MAINTAINERS b/MAINTAINERS
    > index d3e847f..f1158b0 100644
    > --- a/MAINTAINERS
    > +++ b/MAINTAINERS
    > @@ -17203,6 +17203,14 @@ L:   linux-mmc@xxxxxxxxxxxxxxx
    >  S:   Maintained
    >  F:   drivers/mmc/host/sdhci-pci-dwc-mshc.c
    >
    > +SYSGENID
    > +M:   Adrian Catangiu <acatan@xxxxxxxxxx>
    > +L:   linux-kernel@xxxxxxxxxxxxxxx
    > +S:   Supported
    > +F:   Documentation/misc-devices/sysgenid.rst
    > +F:   drivers/misc/sysgenid.c
    > +F:   include/uapi/linux/sysgenid.h
    > +
    >  SYSTEM CONFIGURATION (SYSCON)
    >  M:   Lee Jones <lee.jones@xxxxxxxxxx>
    >  M:   Arnd Bergmann <arnd@xxxxxxxx>
    > diff --git a/drivers/misc/Kconfig b/drivers/misc/Kconfig
    > index fafa8b0..931d716 100644
    > --- a/drivers/misc/Kconfig
    > +++ b/drivers/misc/Kconfig
    > @@ -456,6 +456,22 @@ config PVPANIC
    >         a paravirtualized device provided by QEMU; it lets a virtual machine
    >         (guest) communicate panic events to the host.
    >
    > +config SYSGENID
    > +     tristate "System Generation ID driver"
    > +     default N
    > +     help
    > +       This is a System Generation ID driver which provides a system
    > +       generation counter. The driver exposes FS ops on /dev/sysgenid
    > +       through which it can provide information and notifications on system
    > +       generation changes that happen because of VM or container snapshots
    > +       or cloning.
    > +       This enables applications and libraries that store or cache
    > +       sensitive information, to know that they need to regenerate it
    > +       after process memory has been exposed to potential copying.
    > +
    > +       To compile this driver as a module, choose M here: the
    > +       module will be called sysgenid.
    > +
    >  config HISI_HIKEY_USB
    >       tristate "USB GPIO Hub on HiSilicon Hikey 960/970 Platform"
    >       depends on (OF && GPIOLIB) || COMPILE_TEST
    > diff --git a/drivers/misc/Makefile b/drivers/misc/Makefile
    > index d23231e..4b4933d 100644
    > --- a/drivers/misc/Makefile
    > +++ b/drivers/misc/Makefile
    > @@ -57,3 +57,4 @@ obj-$(CONFIG_HABANA_AI)             += habanalabs/
    >  obj-$(CONFIG_UACCE)          += uacce/
    >  obj-$(CONFIG_XILINX_SDFEC)   += xilinx_sdfec.o
    >  obj-$(CONFIG_HISI_HIKEY_USB) += hisi_hikey_usb.o
    > +obj-$(CONFIG_SYSGENID)               += sysgenid.o
    > diff --git a/drivers/misc/sysgenid.c b/drivers/misc/sysgenid.c
    > new file mode 100644
    > index 0000000..95b3421
    > --- /dev/null
    > +++ b/drivers/misc/sysgenid.c
    > @@ -0,0 +1,307 @@
    > +// SPDX-License-Identifier: GPL-2.0
    > +/*
    > + * System Generation ID driver
    > + *
    > + * Copyright (C) 2020 Amazon. All rights reserved.
    > + *
    > + *   Authors:
    > + *     Adrian Catangiu <acatan@xxxxxxxxxx>
    > + *
    > + */
    > +#include <linux/acpi.h>
    > +#include <linux/kernel.h>
    > +#include <linux/minmax.h>
    > +#include <linux/miscdevice.h>
    > +#include <linux/mm.h>
    > +#include <linux/module.h>
    > +#include <linux/poll.h>
    > +#include <linux/random.h>
    > +#include <linux/uuid.h>
    > +#include <linux/sysgenid.h>
    > +
    > +struct sysgenid_data {
    > +     unsigned long           map_buf;
    > +     wait_queue_head_t       read_waitq;
    > +     atomic_t                generation_counter;
    > +
    > +     unsigned int            watchers;
    > +     atomic_t                outdated_watchers;
    > +     wait_queue_head_t       outdated_waitq;
    > +     spinlock_t              lock;
    > +};
    > +static struct sysgenid_data sysgenid_data;
    > +
    > +struct file_data {
    > +     unsigned int acked_gen_counter;
    > +};
    > +
    > +static void sysgenid_uevent(unsigned int gen_counter);
    > +
    > +static int equals_gen_counter(unsigned int counter)
    > +{
    > +     return counter == atomic_read(&sysgenid_data.generation_counter);
    > +}
    > +
    > +static void _bump_generation(int min_gen)
    > +{
    > +     unsigned long flags;
    > +     int counter;
    > +
    > +     spin_lock_irqsave(&sysgenid_data.lock, flags);
    > +     counter = max(min_gen, 1 + atomic_read(&sysgenid_data.generation_counter));
    > +     atomic_set(&sysgenid_data.generation_counter, counter);
    > +     *((int *) sysgenid_data.map_buf) = counter;
    > +     atomic_set(&sysgenid_data.outdated_watchers, sysgenid_data.watchers);
    > +
    > +     wake_up_interruptible(&sysgenid_data.read_waitq);
    > +     wake_up_interruptible(&sysgenid_data.outdated_waitq);
    > +     spin_unlock_irqrestore(&sysgenid_data.lock, flags);
    > +
    > +     sysgenid_uevent(counter);
    > +}
    > +
    > +void sysgenid_bump_generation(void)
    > +{
    > +     _bump_generation(0);
    > +}
    > +EXPORT_SYMBOL(sysgenid_bump_generation);
    > +
    > +static void put_outdated_watchers(void)
    > +{
    > +     if (atomic_dec_and_test(&sysgenid_data.outdated_watchers))
    > +             wake_up_interruptible(&sysgenid_data.outdated_waitq);
    > +}
    > +
    > +static int sysgenid_open(struct inode *inode, struct file *file)
    > +{
    > +     struct file_data *fdata = kzalloc(sizeof(struct file_data), GFP_KERNEL);
    > +     unsigned long flags;
    > +
    > +     if (!fdata)
    > +             return -ENOMEM;
    > +
    > +     spin_lock_irqsave(&sysgenid_data.lock, flags);
    > +     fdata->acked_gen_counter = atomic_read(&sysgenid_data.generation_counter);
    > +     ++sysgenid_data.watchers;
    > +     spin_unlock_irqrestore(&sysgenid_data.lock, flags);
    > +
    > +     file->private_data = fdata;
    > +
    > +     return 0;
    > +}
    > +
    > +static int sysgenid_close(struct inode *inode, struct file *file)
    > +{
    > +     struct file_data *fdata = file->private_data;
    > +     unsigned long flags;
    > +
    > +     spin_lock_irqsave(&sysgenid_data.lock, flags);
    > +     if (!equals_gen_counter(fdata->acked_gen_counter))
    > +             put_outdated_watchers();
    > +     --sysgenid_data.watchers;
    > +     spin_unlock_irqrestore(&sysgenid_data.lock, flags);
    > +
    > +     kfree(fdata);
    > +
    > +     return 0;
    > +}
    > +
    > +static ssize_t sysgenid_read(struct file *file, char __user *ubuf,
    > +                                                      size_t nbytes, loff_t *ppos)
    > +{
    > +     struct file_data *fdata = file->private_data;
    > +     ssize_t ret;
    > +     int gen_counter;
    > +
    > +     if (nbytes == 0)
    > +             return 0;
    > +     /* disallow partial reads */
    > +     if (nbytes < sizeof(gen_counter))
    > +             return -EINVAL;
    > +
    > +     if (equals_gen_counter(fdata->acked_gen_counter)) {
    > +             if (file->f_flags & O_NONBLOCK)
    > +                     return -EAGAIN;
    > +             ret = wait_event_interruptible(
    > +                     sysgenid_data.read_waitq,
    > +                     !equals_gen_counter(fdata->acked_gen_counter)
    > +             );
    > +             if (ret)
    > +                     return ret;
    > +     }
    > +
    > +     gen_counter = atomic_read(&sysgenid_data.generation_counter);
    > +     ret = copy_to_user(ubuf, &gen_counter, sizeof(gen_counter));
    > +     if (ret)
    > +             return -EFAULT;
    > +
    > +     return sizeof(gen_counter);

    interrupt could be in flight at this point. I think you need
    to synch with that IRQ otherwise your read will return stale data.

Since the user also needs to ACK the right gen counter, the race is not here
(stale generation will fail ack). But you're on-point, this race does exist but
during write().
Interrupt could be in flight when user ACKs generation.

Alas I don't think we can safely support arbitrarily timed snapshots.
Safely used snapshots should each follow a full
quiesce -> take-snapshot -> load-snapshot -> full-system-adjust -> un-quiesce
cycle before repeating.
In such a scenario, there is no race here.

    > +}
    > +
    > +static ssize_t sysgenid_write(struct file *file, const char __user *ubuf,
    > +                                                       size_t count, loff_t *ppos)
    > +{
    > +     struct file_data *fdata = file->private_data;
    > +     unsigned int new_acked_gen;
    > +     unsigned long flags;
    > +
    > +     /* disallow partial writes */
    > +     if (count != sizeof(new_acked_gen))
    > +             return -EINVAL;
    > +     if (copy_from_user(&new_acked_gen, ubuf, count))
    > +             return -EFAULT;
    > +
    > +     spin_lock_irqsave(&sysgenid_data.lock, flags);
    > +     /* wrong gen-counter acknowledged */
    > +     if (!equals_gen_counter(new_acked_gen)) {
    > +             spin_unlock_irqrestore(&sysgenid_data.lock, flags);
    > +             return -EINVAL;
    > +     }
    > +     if (!equals_gen_counter(fdata->acked_gen_counter)) {
    > +             fdata->acked_gen_counter = new_acked_gen;
    > +             put_outdated_watchers();
    > +     }
    > +     spin_unlock_irqrestore(&sysgenid_data.lock, flags);
    > +
    > +     return (ssize_t)count;
    > +}
    > +
    > +static __poll_t sysgenid_poll(struct file *file, poll_table *wait)
    > +{
    > +     __poll_t mask = 0;
    > +     struct file_data *fdata = file->private_data;
    > +
    > +     if (!equals_gen_counter(fdata->acked_gen_counter))
    > +             return EPOLLIN | EPOLLRDNORM;
    > +
    > +     poll_wait(file, &sysgenid_data.read_waitq, wait);
    > +
    > +     if (!equals_gen_counter(fdata->acked_gen_counter))
    > +             mask = EPOLLIN | EPOLLRDNORM;
    > +
    > +     return mask;
    > +}
    > +
    > +static long sysgenid_ioctl(struct file *file,
    > +                                                unsigned int cmd, unsigned long arg)
    > +{
    > +     struct file_data *fdata = file->private_data;
    > +     unsigned long timeout_ns, min_gen;
    > +     ktime_t until;
    > +     int ret = 0;
    > +
    > +     switch (cmd) {
    > +     case SYSGENID_WAIT_WATCHERS:
    > +             timeout_ns = arg * NSEC_PER_MSEC;
    > +             until = timeout_ns ? ktime_set(0, timeout_ns) : KTIME_MAX;
    > +
    > +             ret = wait_event_interruptible_hrtimeout(
    > +                     sysgenid_data.outdated_waitq,
    > +                     (!atomic_read(&sysgenid_data.outdated_watchers) ||
    > +                                     !equals_gen_counter(fdata->acked_gen_counter)),
    > +                     until
    > +             );
    > +             break;
    > +     case SYSGENID_FORCE_GEN_UPDATE:
    > +             if (!checkpoint_restore_ns_capable(current_user_ns()))
    > +                     return -EACCES;
    > +             min_gen = arg;
    > +             _bump_generation(min_gen);
    > +             break;
    > +     default:
    > +             ret = -EINVAL;
    > +             break;
    > +     }
    > +     return ret;
    > +}
    > +
    > +static int sysgenid_mmap(struct file *file, struct vm_area_struct *vma)
    > +{
    > +     struct file_data *fdata = file->private_data;
    > +
    > +     if (vma->vm_pgoff != 0 || vma_pages(vma) > 1)
    > +             return -EINVAL;
    > +
    > +     if ((vma->vm_flags & VM_WRITE) != 0)
    > +             return -EPERM;
    > +
    > +     vma->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
    > +     vma->vm_flags &= ~VM_MAYWRITE;
    > +     vma->vm_private_data = fdata;
    > +
    > +     return vm_insert_page(vma, vma->vm_start,
    > +                                               virt_to_page(sysgenid_data.map_buf));
    > +}
    > +
    > +static const struct file_operations fops = {
    > +     .owner          = THIS_MODULE,
    > +     .mmap           = sysgenid_mmap,
    > +     .open           = sysgenid_open,
    > +     .release        = sysgenid_close,
    > +     .read           = sysgenid_read,
    > +     .write          = sysgenid_write,
    > +     .poll           = sysgenid_poll,
    > +     .unlocked_ioctl = sysgenid_ioctl,
    > +};
    > +
    > +static struct miscdevice sysgenid_misc = {
    > +     .minor = MISC_DYNAMIC_MINOR,
    > +     .name = "sysgenid",
    > +     .fops = &fops,
    > +};
    > +
    > +static void sysgenid_uevent(unsigned int gen_counter)
    > +{
    > +     char event_string[22];
    > +     char *envp[] = { event_string, NULL };
    > +     struct device *dev = sysgenid_misc.this_device;
    > +
    > +     if (dev) {
    > +             sprintf(event_string, "SYSGENID=%u", gen_counter);
    > +             kobject_uevent_env(&dev->kobj, KOBJ_CHANGE, envp);
    > +     }
    > +}
    > +
    > +static int __init sysgenid_init(void)
    > +{
    > +     int ret;
    > +
    > +     sysgenid_data.map_buf = get_zeroed_page(GFP_KERNEL);
    > +     if (!sysgenid_data.map_buf)
    > +             return -ENOMEM;
    > +
    > +     atomic_set(&sysgenid_data.generation_counter, 0);
    > +     atomic_set(&sysgenid_data.outdated_watchers, 0);
    > +     init_waitqueue_head(&sysgenid_data.read_waitq);
    > +     init_waitqueue_head(&sysgenid_data.outdated_waitq);
    > +     spin_lock_init(&sysgenid_data.lock);
    > +
    > +     ret = misc_register(&sysgenid_misc);
    > +     if (ret < 0) {
    > +             pr_err("misc_register() failed for sysgenid\n");
    > +             goto err;
    > +     }
    > +
    > +     return 0;
    > +
    > +err:
    > +     free_pages(sysgenid_data.map_buf, 0);
    > +     sysgenid_data.map_buf = 0;
    > +
    > +     return ret;
    > +}
    > +
    > +static void __exit sysgenid_exit(void)
    > +{
    > +     misc_deregister(&sysgenid_misc);
    > +     free_pages(sysgenid_data.map_buf, 0);
    > +     sysgenid_data.map_buf = 0;
    > +}
    > +
    > +module_init(sysgenid_init);
    > +module_exit(sysgenid_exit);
    > +
    > +MODULE_AUTHOR("Adrian Catangiu");
    > +MODULE_DESCRIPTION("System Generation ID");
    > +MODULE_LICENSE("GPL");
    > +MODULE_VERSION("0.1");
    > diff --git a/include/uapi/linux/sysgenid.h b/include/uapi/linux/sysgenid.h
    > new file mode 100644
    > index 0000000..ba370c8
    > --- /dev/null
    > +++ b/include/uapi/linux/sysgenid.h
    > @@ -0,0 +1,17 @@
    > +/* SPDX-License-Identifier: GPL-2.0+ WITH Linux-syscall-note */
    > +
    > +#ifndef _UAPI_LINUX_SYSGENID_H
    > +#define _UAPI_LINUX_SYSGENID_H
    > +
    > +#include <linux/ioctl.h>
    > +
    > +#define SYSGENID_IOCTL                       0xE4
    > +#define SYSGENID_WAIT_WATCHERS               _IO(SYSGENID_IOCTL, 1)
    > +#define SYSGENID_FORCE_GEN_UPDATE    _IO(SYSGENID_IOCTL, 2)
    > +
    > +#ifdef __KERNEL__
    > +void sysgenid_bump_generation(void);
    > +#endif /* __KERNEL__ */
    > +
    > +#endif /* _UAPI_LINUX_SYSGENID_H */
    > +
    > --
    > 2.7.4
    >
    >
    >
    >
    > Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.





Amazon Development Center (Romania) S.R.L. registered office: 27A Sf. Lazar Street, UBC5, floor 2, Iasi, Iasi County, 700045, Romania. Registered in Romania. Registration number J22/2621/2005.