![Federico Vaga](/assets/img/avatar_default.png)
Translated documents: - stable-kernel-rules.rst - deprecated.rst - kernel-enforcement-statement.rst - license-rules.rst Added document to have valid links - netdev-FAQ.rst Modifications to main documentation - add label in deprecated.rst Signed-off-by: Federico Vaga <federico.vaga@vaga.pv.it> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
122 lines
5.6 KiB
ReStructuredText
122 lines
5.6 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
.. _deprecated:
|
|
|
|
=====================================================================
|
|
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`)
|