Merge tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-tracegrafted

Pull tracing fixes from Steven Rostedt:

 - Fix error output of boot instance creation failure

   Currently if a boot instance creation fails, instead of printing out
   the name of the instance that failed, it prints "(null)". That is
   because it prints "cur_str" that had already been processed by
   strsep(). Print the saved name instead.

   While at it, print the error code of the failure.

 - Fix use-after-free for same named historgrams

   Histograms can be named so that they can be used in multiple events.
   But if the named histogram has a variable attached, the second event
   that uses the named histogram which duplicates it and needs to free
   the original after duplication leaves the old variable in place and
   still visible. If another histogram uses than variable, it will use
   the stale one which will try to reference the freed duplicate
   histogram and crash the kernel.

   Free the duplicate variables along with the duplicated histogram
   data.

 - Check return value of kthread_run() in event self test

   The events self tests uses a kthread for testing but does not check
   if it succeeded in creating a kthread. If the kthread creation were
   to fail, the code will still try to call kthread_stop() on the error
   returned.

 - Fix race between reading trace_pipe and updating subbuffer size

   If a user is reading the trace_pipe file at the same time they update
   the ring buffer sub-buffer size, can cause the trace_pipe read to
   read stale data. Add trace_access_lock() around updating the ring
   buffer sub-buffer size.

 - Fix eventfs_inode on failure path in creation of the events directory

   In the creation of the "events" directory, if after allocating the
   eventfs_inode a failure is detected, it calls cleanup_ei() which
   calls free_ei(). The free_ei() will test if eventfs_inode being freed
   has no children. It is a bug if it does. But on the failure case of
   the creation of the "events" directory, the children lists have not
   yet been initialized and the free will trigger a warning because
   list_empty() on an uninitialized list returns false.

   Move the initialization into init_ei() where it makes more sense and
   makes sure that a created eventfs_inode has its lists initialized
   upon creation.

 - Check return value of kthread_run() in ftrace direct sample code

   The sample code that shows how to use the ftrace direct calls does
   not test the return of kthread_run() to see if it succeeds. Return a
   failure if the kthread_run() doesn't succeed.

 - Clear user events state on fork in case of alloc failure

   On fork, the child gets a pointer to the parent's user events state.
   It makes a copy of it then updates the child's pointer to it. But if
   the allocation fails, the duplication function leaves the child with
   a pointer to its parent's descriptor. When the child cleans up its
   data, it will free the parent's descriptor while the parent is still
   using it.

   In the duplication function, set the child's user_event_mm to NULL
   before testing if the allocation succeeded, and when it exits it will
   not free the parent's descriptor.

 - Fix retry exhaustion in simple ring buffer reader swap

   simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
   post-decrements it only after a failed link replacement. On the final
   attempt, a successful replacement leaves retry at zero, while a
   failed replacement leaves it at -1.

   But the check for success expects the retry value to be non-zero and
   exits with an error on zero. This is the opposite result. Fix it.

 - Fail nicely when the remote swap_reader_page() returns an error

   Currently, if the swap_reader_page() of a remote buffer fails, it
   triggers a WARN_ON_ONCE() and continues normally. Instead, have it
   exit with an error and a pr_warn() print instead of a full WARNING.

* tag 'trace-v7.3-2' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace:
  ring-buffer: Stop remote reader update when page swap fails
  tracing: Fix retry exhaustion in simple ring buffer reader swap
  tracing/user_events: Clear copied tracing state before fork duplication
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-multi-modify
  samples/ftrace: Fix kthread_stop() on ERR_PTR in ftrace-direct-modify
  eventfs: Initialize ei->children and ei->list in init_ei()
  tracing: Fix use-after-free in trace_pipe read on sub-buffer order change
  tracing: Fix crash passing ERR_PTR to kthread_stop()
  tracing: Fix use-after-free with same-name named triggers
  tracing: Fix logged instance name on creation failure

1 files changed, 204 insertions, 0 deletions

diff --git a/Documentation/process/stable-api-nonsense.rst b/Documentation/process/stable-api-nonsense.rst
new file mode 100644
index 000000000..a9625ab1f
--- /dev/null
+++ b/Documentation/process/stable-api-nonsense.rst
@@ -0,0 +1,204 @@
+.. _stable_api_nonsense:
+
+The Linux Kernel Driver Interface
+==================================
+
+(all of your questions answered and then some)
+
+Greg Kroah-Hartman <greg@kroah.com>
+
+This is being written to try to explain why Linux **does not have a binary
+kernel interface, nor does it have a stable kernel interface**.
+
+.. note::
+
+  Please realize that this article describes the **in kernel** interfaces, not
+  the kernel to userspace interfaces.
+
+  The kernel to userspace interface is the one that application programs use,
+  the syscall interface.  That interface is **very** stable over time, and
+  will not break.  I have old programs that were built on a pre 0.9something
+  kernel that still work just fine on the latest 2.6 kernel release.
+  That interface is the one that users and application programmers can count
+  on being stable.
+
+
+Executive Summary
+-----------------
+You think you want a stable kernel interface, but you really do not, and
+you don't even know it.  What you want is a stable running driver, and
+you get that only if your driver is in the main kernel tree.  You also
+get lots of other good benefits if your driver is in the main kernel
+tree, all of which has made Linux into such a strong, stable, and mature
+operating system which is the reason you are using it in the first
+place.
+
+
+Intro
+-----
+
+It's only the odd person who wants to write a kernel driver that needs
+to worry about the in-kernel interfaces changing.  For the majority of
+the world, they neither see this interface, nor do they care about it at
+all.
+
+First off, I'm not going to address **any** legal issues about closed
+source, hidden source, binary blobs, source wrappers, or any other term
+that describes kernel drivers that do not have their source code
+released under the GPL.  Please consult a lawyer if you have any legal
+questions, I'm a programmer and hence, I'm just going to be describing
+the technical issues here (not to make light of the legal issues, they
+are real, and you do need to be aware of them at all times.)
+
+So, there are two main topics here, binary kernel interfaces and stable
+kernel source interfaces.  They both depend on each other, but we will
+discuss the binary stuff first to get it out of the way.
+
+
+Binary Kernel Interface
+-----------------------
+Assuming that we had a stable kernel source interface for the kernel, a
+binary interface would naturally happen too, right?  Wrong.  Please
+consider the following facts about the Linux kernel:
+
+  - Depending on the version of the C compiler you use, different kernel
+    data structures will contain different alignment of structures, and
+    possibly include different functions in different ways (putting
+    functions inline or not.)  The individual function organization
+    isn't that important, but the different data structure padding is
+    very important.
+
+  - Depending on what kernel build options you select, a wide range of
+    different things can be assumed by the kernel:
+
+      - different structures can contain different fields
+      - Some functions may not be implemented at all, (i.e. some locks
+	compile away to nothing for non-SMP builds.)
+      - Memory within the kernel can be aligned in different ways,
+	depending on the build options.
+
+  - Linux runs on a wide range of different processor architectures.
+    There is no way that binary drivers from one architecture will run
+    on another architecture properly.
+
+Now a number of these issues can be addressed by simply compiling your
+module for the exact specific kernel configuration, using the same exact
+C compiler that the kernel was built with.  This is sufficient if you
+want to provide a module for a specific release version of a specific
+Linux distribution.  But multiply that single build by the number of
+different Linux distributions and the number of different supported
+releases of the Linux distribution and you quickly have a nightmare of
+different build options on different releases.  Also realize that each
+Linux distribution release contains a number of different kernels, all
+tuned to different hardware types (different processor types and
+different options), so for even a single release you will need to create
+multiple versions of your module.
+
+Trust me, you will go insane over time if you try to support this kind
+of release, I learned this the hard way a long time ago...
+
+
+Stable Kernel Source Interfaces
+-------------------------------
+
+This is a much more "volatile" topic if you talk to people who try to
+keep a Linux kernel driver that is not in the main kernel tree up to
+date over time.
+
+Linux kernel development is continuous and at a rapid pace, never
+stopping to slow down.  As such, the kernel developers find bugs in
+current interfaces, or figure out a better way to do things.  If they do
+that, they then fix the current interfaces to work better.  When they do
+so, function names may change, structures may grow or shrink, and
+function parameters may be reworked.  If this happens, all of the
+instances of where this interface is used within the kernel are fixed up
+at the same time, ensuring that everything continues to work properly.
+
+As a specific examples of this, the in-kernel USB interfaces have
+undergone at least three different reworks over the lifetime of this
+subsystem.  These reworks were done to address a number of different
+issues:
+
+  - A change from a synchronous model of data streams to an asynchronous
+    one.  This reduced the complexity of a number of drivers and
+    increased the throughput of all USB drivers such that we are now
+    running almost all USB devices at their maximum speed possible.
+  - A change was made in the way data packets were allocated from the
+    USB core by USB drivers so that all drivers now needed to provide
+    more information to the USB core to fix a number of documented
+    deadlocks.
+
+This is in stark contrast to a number of closed source operating systems
+which have had to maintain their older USB interfaces over time.  This
+provides the ability for new developers to accidentally use the old
+interfaces and do things in improper ways, causing the stability of the
+operating system to suffer.
+
+In both of these instances, all developers agreed that these were
+important changes that needed to be made, and they were made, with
+relatively little pain.  If Linux had to ensure that it will preserve a
+stable source interface, a new interface would have been created, and
+the older, broken one would have had to be maintained over time, leading
+to extra work for the USB developers.  Since all Linux USB developers do
+their work on their own time, asking programmers to do extra work for no
+gain, for free, is not a possibility.
+
+Security issues are also very important for Linux.  When a
+security issue is found, it is fixed in a very short amount of time.  A
+number of times this has caused internal kernel interfaces to be
+reworked to prevent the security problem from occurring.  When this
+happens, all drivers that use the interfaces were also fixed at the
+same time, ensuring that the security problem was fixed and could not
+come back at some future time accidentally.  If the internal interfaces
+were not allowed to change, fixing this kind of security problem and
+insuring that it could not happen again would not be possible.
+
+Kernel interfaces are cleaned up over time.  If there is no one using a
+current interface, it is deleted.  This ensures that the kernel remains
+as small as possible, and that all potential interfaces are tested as
+well as they can be (unused interfaces are pretty much impossible to
+test for validity.)
+
+
+What to do
+----------
+
+So, if you have a Linux kernel driver that is not in the main kernel
+tree, what are you, a developer, supposed to do?  Releasing a binary
+driver for every different kernel version for every distribution is a
+nightmare, and trying to keep up with an ever changing kernel interface
+is also a rough job.
+
+Simple, get your kernel driver into the main kernel tree (remember we are
+talking about drivers released under a GPL-compatible license here, if your
+code doesn't fall under this category, good luck, you are on your own here,
+you leech).  If your driver is in the tree, and a kernel interface changes,
+it will be fixed up by the person who did the kernel change in the first
+place.  This ensures that your driver is always buildable, and works over
+time, with very little effort on your part.
+
+The very good side effects of having your driver in the main kernel tree
+are:
+
+  - The quality of the driver will rise as the maintenance costs (to the
+    original developer) will decrease.
+  - Other developers will add features to your driver.
+  - Other people will find and fix bugs in your driver.
+  - Other people will find tuning opportunities in your driver.
+  - Other people will update the driver for you when external interface
+    changes require it.
+  - The driver automatically gets shipped in all Linux distributions
+    without having to ask the distros to add it.
+
+As Linux supports a larger number of different devices "out of the box"
+than any other operating system, and it supports these devices on more
+different processor architectures than any other operating system, this
+proven type of development model must be doing something right :)
+
+
+
+------
+
+Thanks to Randy Dunlap, Andrew Morton, David Brownell, Hanna Linder,
+Robert Love, and Nishanth Aravamudan for their review and comments on
+early drafts of this paper.