linux-kernel - Re: [PATCH] docs: deprecated.rst: Clean up fall-through details

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Date:   Wed, 4 Mar 2020 13:30:53 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Kees Cook <keescook@...omium.org>, Jonathan Corbet <corbet@....net>
Cc:     Joe Perches <joe@...ches.com>,
        Federico Vaga <federico.vaga@...a.pv.it>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] docs: deprecated.rst: Clean up fall-through details



On 3/4/20 13:03, Kees Cook wrote:
> 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@...omium.org>

Acked-by: Gustavo A. R. Silva <gustavo@...eddedor.com>

> ---
>  Documentation/process/deprecated.rst | 48 +++++++++++++++++-----------
>  1 file changed, 29 insertions(+), 19 deletions(-)
> 
> diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> index 179f2a5625a0..f9f196d3a69b 100644
> --- a/Documentation/process/deprecated.rst
> +++ b/Documentation/process/deprecated.rst
> @@ -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
> @@ -122,27 +122,37 @@ 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];
>