149 lines
6.6 KiB
ReStructuredText
149 lines
6.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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.. _deprecated:
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=====================================================================
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Deprecated Interfaces, Language Features, Attributes, and Conventions
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=====================================================================
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In a perfect world, it would be possible to convert all instances of
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some deprecated API into the new API and entirely remove the old API in
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a single development cycle. However, due to the size of the kernel, the
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maintainership hierarchy, and timing, it's not always feasible to do these
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kinds of conversions at once. This means that new instances may sneak into
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the kernel while old ones are being removed, only making the amount of
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work to remove the API grow. In order to educate developers about what
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has been deprecated and why, this list has been created as a place to
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point when uses of deprecated things are proposed for inclusion in the
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kernel.
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__deprecated
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------------
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While this attribute does visually mark an interface as deprecated,
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it `does not produce warnings during builds any more
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<https://git.kernel.org/linus/771c035372a036f83353eef46dbb829780330234>`_
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because one of the standing goals of the kernel is to build without
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warnings and no one was actually doing anything to remove these deprecated
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interfaces. While using `__deprecated` is nice to note an old API in
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a header file, it isn't the full solution. Such interfaces must either
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be fully removed from the kernel, or added to this file to discourage
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others from using them in the future.
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open-coded arithmetic in allocator arguments
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--------------------------------------------
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Dynamic size calculations (especially multiplication) should not be
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performed in memory allocator (or similar) function arguments due to the
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risk of them overflowing. This could lead to values wrapping around and a
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smaller allocation being made than the caller was expecting. Using those
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allocations could lead to linear overflows of heap memory and other
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misbehaviors. (One exception to this is literal values where the compiler
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can warn if they might overflow. Though using literals for arguments as
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suggested below is also harmless.)
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For example, do not use ``count * size`` as an argument, as in::
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foo = kmalloc(count * size, GFP_KERNEL);
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Instead, the 2-factor form of the allocator should be used::
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foo = kmalloc_array(count, size, GFP_KERNEL);
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If no 2-factor form is available, the saturate-on-overflow helpers should
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be used::
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bar = vmalloc(array_size(count, size));
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Another common case to avoid is calculating the size of a structure with
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a trailing array of others structures, as in::
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header = kzalloc(sizeof(*header) + count * sizeof(*header->item),
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GFP_KERNEL);
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Instead, use the helper::
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header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
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See :c:func:`array_size`, :c:func:`array3_size`, and :c:func:`struct_size`,
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for more details as well as the related :c:func:`check_add_overflow` and
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:c:func:`check_mul_overflow` family of functions.
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simple_strtol(), simple_strtoll(), simple_strtoul(), simple_strtoull()
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----------------------------------------------------------------------
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The :c:func:`simple_strtol`, :c:func:`simple_strtoll`,
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:c:func:`simple_strtoul`, and :c:func:`simple_strtoull` functions
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explicitly ignore overflows, which may lead to unexpected results
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in callers. The respective :c:func:`kstrtol`, :c:func:`kstrtoll`,
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:c:func:`kstrtoul`, and :c:func:`kstrtoull` functions tend to be the
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correct replacements, though note that those require the string to be
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NUL or newline terminated.
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strcpy()
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--------
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:c:func:`strcpy` performs no bounds checking on the destination
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buffer. This could result in linear overflows beyond the
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end of the buffer, leading to all kinds of misbehaviors. While
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`CONFIG_FORTIFY_SOURCE=y` and various compiler flags help reduce the
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risk of using this function, there is no good reason to add new uses of
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this function. The safe replacement is :c:func:`strscpy`.
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strncpy() on NUL-terminated strings
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-----------------------------------
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Use of :c:func:`strncpy` does not guarantee that the destination buffer
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will be NUL terminated. This can lead to various linear read overflows
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and other misbehavior due to the missing termination. It also NUL-pads the
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destination buffer if the source contents are shorter than the destination
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buffer size, which may be a needless performance penalty for callers using
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only NUL-terminated strings. The safe replacement is :c:func:`strscpy`.
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(Users of :c:func:`strscpy` still needing NUL-padding will need an
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explicit :c:func:`memset` added.)
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If a caller is using non-NUL-terminated strings, :c:func:`strncpy()` can
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still be used, but destinations should be marked with the `__nonstring
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<https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html>`_
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attribute to avoid future compiler warnings.
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strlcpy()
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---------
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:c:func:`strlcpy` reads the entire source buffer first, possibly exceeding
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the given limit of bytes to copy. This is inefficient and can lead to
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linear read overflows if a source string is not NUL-terminated. The
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safe replacement is :c:func:`strscpy`.
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Variable Length Arrays (VLAs)
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-----------------------------
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Using stack VLAs produces much worse machine code than statically
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sized stack arrays. While these non-trivial `performance issues
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<https://git.kernel.org/linus/02361bc77888>`_ are reason enough to
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eliminate VLAs, they are also a security risk. Dynamic growth of a stack
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array may exceed the remaining memory in the stack segment. This could
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lead to a crash, possible overwriting sensitive contents at the end of the
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stack (when built without `CONFIG_THREAD_INFO_IN_TASK=y`), or overwriting
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memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
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Implicit switch case fall-through
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---------------------------------
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The C language allows switch cases to "fall-through" when a "break" statement
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is missing at the end of a case. This, however, introduces ambiguity in the
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code, as it's not always clear if the missing break is intentional or a bug.
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As there have been a long list of flaws `due to missing "break" statements
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<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
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"implicit fall-through".
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In order to identify intentional fall-through cases, we have adopted a
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pseudo-keyword macro 'fallthrough' which expands to gcc's extension
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__attribute__((__fallthrough__)). `Statement Attributes
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<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
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When the C17/C18 [[fallthrough]] syntax is more commonly supported by
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C compilers, static analyzers, and IDEs, we can switch to using that syntax
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for the macro pseudo-keyword.
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All switch/case blocks must end in one of:
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break;
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fallthrough;
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continue;
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goto <label>;
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return [expression];
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