docs: Introduce deprecated APIs list

As discussed in the "API replacement/deprecation" thread[1], this makes
an effort to document what things shouldn't get (re)added to the kernel,
by introducing Documentation/process/deprecated.rst.

[1] https://lists.linuxfoundation.org/pipermail/ksummit-discuss/2018-September/005282.html

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 119 insertions, 0 deletions

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
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+.. 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`)