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docs: deprecated.rst: Clean up fall-through details 

Add example of fall-through, list-ify the case ending statements, and
adjust the markup for links and readability. While here, adjust
strscpy() details to mention strscpy_pad().

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/202003041102.47A4E4B62@keescook
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Kees Cook 2020-03-04 11:03:24 -08:00 committed by Jonathan Corbet
parent 6505a18e66
commit 76136e028d
1 changed files with 29 additions and 19 deletions
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48
Documentation/process/deprecated.rst
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@ -94,8 +94,8 @@ 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.)
(Users of :c:func:`strscpy` still needing NUL-padding should instead
use strscpy_pad().)
If a caller is using non-NUL-terminated strings, :c:func:`strncpy()` can
still be used, but destinations should be marked with the `__nonstring
@ -144,27 +144,37 @@ memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
Implicit switch case fall-through
---------------------------------
The C language allows switch cases to "fall-through" when a "break" statement
is missing at the end of a case. This, however, introduces ambiguity in the
code, as it's not always clear if the missing break is intentional or a bug.
The C language allows switch cases to fall through to the next case
when a "break" statement is missing at the end of a case. This, however,
introduces ambiguity in the code, as it's not always clear if the missing
break is intentional or a bug. For example, it's not obvious just from
looking at the code if `STATE_ONE` is intentionally designed to fall
through into `STATE_TWO`::
switch (value) {
case STATE_ONE:
do_something();
case STATE_TWO:
do_other();
break;
default:
WARN("unknown state");
}
As there have been a long list of flaws `due to missing "break" statements
<https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
"implicit fall-through".
In order to identify intentional fall-through cases, we have adopted a
pseudo-keyword macro 'fallthrough' which expands to gcc's extension
__attribute__((__fallthrough__)). `Statement Attributes
<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
When the C17/C18 [[fallthrough]] syntax is more commonly supported by
implicit fall-through. In order to identify intentional fall-through
cases, we have adopted a pseudo-keyword macro "fallthrough" which
expands to gcc's extension `__attribute__((__fallthrough__))
<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_.
(When the C17/C18 `[[fallthrough]]` syntax is more commonly supported by
C compilers, static analyzers, and IDEs, we can switch to using that syntax
for the macro pseudo-keyword.
for the macro pseudo-keyword.)
All switch/case blocks must end in one of:
break;
fallthrough;
continue;
goto <label>;
return [expression];
* break;
* fallthrough;
* continue;
* goto <label>;
* return [expression];