bpf: mark a NULL kfunc argument precisegrafted

check_kfunc_arg() allows bpf_register_is_null() for nullable arguments
w/o marking the underlying scalar register precise. Hence a checkpoint
created on such a path would prune against arbitrary scalar value.

Fixes: 3bda08b63670 ("bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260904-register-is-null-precise-fixes-v1-5-0f5a360ff15d@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 75 insertions, 0 deletions

diff --git a/Documentation/driver-api/tty/index.rst b/Documentation/driver-api/tty/index.rst
new file mode 100644
index 000000000..c1ffe3d1e
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+.. SPDX-License-Identifier: GPL-2.0
+
+===
+TTY
+===
+
+Teletypewriter (TTY) layer takes care of all those serial devices. Including
+the virtual ones like pseudoterminal (PTY).
+
+TTY structures
+==============
+
+There are several major TTY structures. Every TTY device in a system has a
+corresponding struct tty_port. These devices are maintained by a TTY driver
+which is struct tty_driver. This structure describes the driver but also
+contains a reference to operations which could be performed on the TTYs. It is
+struct tty_operations. Then, upon open, a struct tty_struct is allocated and
+lives until the final close. During this time, several callbacks from struct
+tty_operations are invoked by the TTY layer.
+
+Every character received by the kernel (both from devices and users) is passed
+through a preselected :doc:`tty_ldisc` (in
+short ldisc; in C, struct tty_ldisc_ops). Its task is to transform characters
+as defined by a particular ldisc or by user too. The default one is n_tty,
+implementing echoes, signal handling, jobs control, special characters
+processing, and more. The transformed characters are passed further to
+user/device, depending on the source.
+
+In-detail description of the named TTY structures is in separate documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   tty_driver
+   tty_port
+   tty_struct
+   tty_ldisc
+   tty_buffer
+   tty_ioctl
+   tty_internals
+   console
+
+Writing TTY Driver
+==================
+
+Before one starts writing a TTY driver, they must consider
+:doc:`Serial <../serial/driver>` and :doc:`USB Serial <../../usb/usb-serial>`
+layers first. Drivers for serial devices can often use one of these specific
+layers to implement a serial driver. Only special devices should be handled
+directly by the TTY Layer. If you are about to write such a driver, read on.
+
+A *typical* sequence a TTY driver performs is as follows:
+
+#. Allocate and register a TTY driver (module init)
+#. Create and register TTY devices as they are probed (probe function)
+#. Handle TTY operations and events like interrupts (TTY core invokes the
+   former, the device the latter)
+#. Remove devices as they are going away (remove function)
+#. Unregister and free the TTY driver (module exit)
+
+Steps regarding driver, i.e. 1., 3., and 5. are described in detail in
+:doc:`tty_driver`. For the other two (devices handling), look into
+:doc:`tty_port`.
+
+Other Documentation
+===================
+
+Miscellaneous documentation can be further found in these documents:
+
+.. toctree::
+   :maxdepth: 2
+
+   moxa-smartio
+   n_gsm
+   n_tty