selftests/bpf: Reject graph kptr use after RCU unlockgrafted

Add a sleepable verifier test that loads a graph-node local kptr in an
explicit RCU read-side critical section, then passes its node to
bpf_rbtree_remove() after the section ends.

Before the verifier fix, the stale NON_OWN_REF flag makes the node look like
a live borrowed reference and the program is accepted. After the fix, the
pointer is demoted without NON_OWN_REF and the graph kfunc argument is
rejected.

Also exercise a graph kptr loaded while a spin lock provides implicit RCU
protection. The pointer must be invalidated when the lock is released, which
guards the required ordering between non-owning-reference invalidation and
RCU demotion.

Update the existing fault-protected load test state description. The
post-unlock pointer no longer carries NON_OWN_REF, but remains readable
because the load is rewritten to use BPF_PROBE_MEM.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260904084325.52250-7-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 50 insertions, 0 deletions

diff --git a/Documentation/driver-api/gpio/using-gpio.rst b/Documentation/driver-api/gpio/using-gpio.rst
new file mode 100644
index 000000000..894d88855
--- /dev/null
+++ b/Documentation/driver-api/gpio/using-gpio.rst
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+=========================
+Using GPIO Lines in Linux
+=========================
+
+The Linux kernel exists to abstract and present hardware to users. GPIO lines
+as such are normally not user facing abstractions. The most obvious, natural
+and preferred way to use GPIO lines is to let kernel hardware drivers deal
+with them.
+
+For examples of already existing generic drivers that will also be good
+examples for any other kernel drivers you want to author, refer to
+Documentation/driver-api/gpio/drivers-on-gpio.rst
+
+For any kind of mass produced system you want to support, such as servers,
+laptops, phones, tablets, routers, and any consumer or office or business goods
+using appropriate kernel drivers is paramount. Submit your code for inclusion
+in the upstream Linux kernel when you feel it is mature enough and you will get
+help to refine it, see Documentation/process/submitting-patches.rst.
+
+In Linux GPIO lines also have a userspace ABI.
+
+The userspace ABI is intended for one-off deployments. Examples are prototypes,
+factory lines, maker community projects, workshop specimen, production tools,
+industrial automation, PLC-type use cases, door controllers, in short a piece
+of specialized equipment that is not produced by the numbers, requiring
+operators to have a deep knowledge of the equipment and knows about the
+software-hardware interface to be set up. They should not have a natural fit
+to any existing kernel subsystem and not be a good fit for an operating system,
+because of not being reusable or abstract enough, or involving a lot of non
+computer hardware related policy.
+
+Applications that have a good reason to use the industrial I/O (IIO) subsystem
+from userspace will likely be a good fit for using GPIO lines from userspace as
+well.
+
+Do not under any circumstances abuse the GPIO userspace ABI to cut corners in
+any product development projects. If you use it for prototyping, then do not
+productify the prototype: rewrite it using proper kernel drivers. Do not under
+any circumstances deploy any uniform products using GPIO from userspace.
+
+The userspace ABI is a character device for each GPIO hardware unit (GPIO chip).
+These devices will appear on the system as ``/dev/gpiochip0`` thru
+``/dev/gpiochipN``. Examples of how to directly use the userspace ABI can be
+found in the kernel tree ``tools/gpio`` subdirectory.
+
+For structured and managed applications, we recommend that you make use of the
+libgpiod_ library. This provides helper abstractions, command line utilities
+and arbitration for multiple simultaneous consumers on the same GPIO chip.
+
+.. _libgpiod: https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/