Merge tag 'docs-4.20' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "This is a fairly typical cycle for documentation. There's some welcome
  readability improvements for the formatted output, some LICENSES
  updates including the addition of the ISC license, the removal of the
  unloved and unmaintained 00-INDEX files, the deprecated APIs document
  from Kees, more MM docs from Mike Rapoport, and the usual pile of typo
  fixes and corrections"

* tag 'docs-4.20' of git://git.lwn.net/linux: (41 commits)
  docs: Fix typos in histogram.rst
  docs: Introduce deprecated APIs list
  kernel-doc: fix declaration type determination
  doc: fix a typo in adding-syscalls.rst
  docs/admin-guide: memory-hotplug: remove table of contents
  doc: printk-formats: Remove bogus kobject references for device nodes
  Documentation: preempt-locking: Use better example
  dm flakey: Document "error_writes" feature
  docs/completion.txt: Fix a couple of punctuation nits
  LICENSES: Add ISC license text
  LICENSES: Add note to CDDL-1.0 license that it should not be used
  docs/core-api: memory-hotplug: add some details about locking internals
  docs/core-api: rename memory-hotplug-notifier to memory-hotplug
  docs: improve readability for people with poorer eyesight
  yama: clarify ptrace_scope=2 in Yama documentation
  docs/vm: split memory hotplug notifier description to Documentation/core-api
  docs: move memory hotplug description into admin-guide/mm
  doc: Fix acronym "FEKEK" in ecryptfs
  docs: fix some broken documentation references
  iommu: Fix passthrough option documentation
  ...

1 files changed, 119 insertions, 0 deletions

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
new file mode 100644
index 000000000000..0ef5a63c06ba
--- /dev/null
+++ b/Documentation/process/deprecated.rst
@@ -0,0 +1,119 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================================================================
+Deprecated Interfaces, Language Features, Attributes, and Conventions
+=====================================================================
+
+In a perfect world, it would be possible to convert all instances of
+some deprecated API into the new API and entirely remove the old API in
+a single development cycle. However, due to the size of the kernel, the
+maintainership hierarchy, and timing, it's not always feasible to do these
+kinds of conversions at once. This means that new instances may sneak into
+the kernel while old ones are being removed, only making the amount of
+work to remove the API grow. In order to educate developers about what
+has been deprecated and why, this list has been created as a place to
+point when uses of deprecated things are proposed for inclusion in the
+kernel.
+
+__deprecated
+------------
+While this attribute does visually mark an interface as deprecated,
+it `does not produce warnings during builds any more
+<https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
+because one of the standing goals of the kernel is to build without
+warnings and no one was actually doing anything to remove these deprecated
+interfaces. While using `__deprecated` is nice to note an old API in
+a header file, it isn't the full solution. Such interfaces must either
+be fully removed from the kernel, or added to this file to discourage
+others from using them in the future.
+
+open-coded arithmetic in allocator arguments
+--------------------------------------------
+Dynamic size calculations (especially multiplication) should not be
+performed in memory allocator (or similar) function arguments due to the
+risk of them overflowing. This could lead to values wrapping around and a
+smaller allocation being made than the caller was expecting. Using those
+allocations could lead to linear overflows of heap memory and other
+misbehaviors. (One exception to this is literal values where the compiler
+can warn if they might overflow. Though using literals for arguments as
+suggested below is also harmless.)
+
+For example, do not use ``count * size`` as an argument, as in::
+
+	foo = kmalloc(count * size, GFP_KERNEL);
+
+Instead, the 2-factor form of the allocator should be used::
+
+	foo = kmalloc_array(count, size, GFP_KERNEL);
+
+If no 2-factor form is available, the saturate-on-overflow helpers should
+be used::
+
+	bar = vmalloc(array_size(count, size));
+
+Another common case to avoid is calculating the size of a structure with
+a trailing array of others structures, as in::
+
+	header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
+			 GFP_KERNEL);
+
+Instead, use the helper::
+
+	header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
+
+See :c:func:`array_size`, :c:func:`array3_size`, and :c:func:`struct_size`,
+for more details as well as the related :c:func:`check_add_overflow` and
+:c:func:`check_mul_overflow` family of functions.
+
+simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
+----------------------------------------------------------------------
+The :c:func:`simple_strtol`, :c:func:`simple_strtoll`,
+:c:func:`simple_strtoul`, and :c:func:`simple_strtoull` functions
+explicitly ignore overflows, which may lead to unexpected results
+in callers. The respective :c:func:`kstrtol`, :c:func:`kstrtoll`,
+:c:func:`kstrtoul`, and :c:func:`kstrtoull` functions tend to be the
+correct replacements, though note that those require the string to be
+NUL or newline terminated.
+
+strcpy()
+--------
+:c:func:`strcpy` performs no bounds checking on the destination
+buffer. This could result in linear overflows beyond the
+end of the buffer, leading to all kinds of misbehaviors. While
+`CONFIG_FORTIFY_SOURCE=y` and various compiler flags help reduce the
+risk of using this function, there is no good reason to add new uses of
+this function. The safe replacement is :c:func:`strscpy`.
+
+strncpy() on NUL-terminated strings
+-----------------------------------
+Use of :c:func:`strncpy` does not guarantee that the destination buffer
+will be NUL terminated. This can lead to various linear read overflows
+and other misbehavior due to the missing termination. It also NUL-pads the
+destination buffer if the source contents are shorter than the destination
+buffer size, which may be a needless performance penalty for callers using
+only NUL-terminated strings. The safe replacement is :c:func:`strscpy`.
+(Users of :c:func:`strscpy` still needing NUL-padding will need an
+explicit :c:func:`memset` added.)
+
+If a caller is using non-NUL-terminated strings, :c:func:`strncpy()` can
+still be used, but destinations should be marked with the `__nonstring
+<https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
+attribute to avoid future compiler warnings.
+
+strlcpy()
+---------
+:c:func:`strlcpy` reads the entire source buffer first, possibly exceeding
+the given limit of bytes to copy. This is inefficient and can lead to
+linear read overflows if a source string is not NUL-terminated. The
+safe replacement is :c:func:`strscpy`.
+
+Variable Length Arrays (VLAs)
+-----------------------------
+Using stack VLAs produces much worse machine code than statically
+sized stack arrays. While these non-trivial `performance issues
+<https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
+eliminate VLAs, they are also a security risk. Dynamic growth of a stack
+array may exceed the remaining memory in the stack segment. This could
+lead to a crash, possible overwriting sensitive contents at the end of the
+stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
+memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)