linux-kernel - [PATCH v2] docs: deprecated.rst: Add note to the use of struct

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Message-ID: <20200604184456.GA24758@embeddedor>
Date:   Thu, 4 Jun 2020 13:44:56 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Jonathan Corbet <corbet@....net>, Kees Cook <keescook@...omium.org>
Cc:     linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH v2] docs: deprecated.rst: Add note to the use of
 struct_size() helper

Add a note to educate people about the proper use of struct_size() when
the trailing array in the enclosing structure is a one-element array.

Zero-length and one-element arrays will soon be removed from the kernel,
but in the meantime, it's worth letting people know how to correctly
use struct_size() together with such constructs.

Note: this documentation will be updated once all zero-length and
one-element arrays have been completely removed from the kernel.

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
Changes in v2:
 - Suggest stop using one-element arrays and switch to flexible arrays.
 - Update changelog text.

 Documentation/process/deprecated.rst | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
index 652e2aa02a66c..1af3a021f4b33 100644
--- a/Documentation/process/deprecated.rst
+++ b/Documentation/process/deprecated.rst
@@ -85,6 +85,18 @@ Instead, use the helper::
 
 	header = kzalloc(struct_size(header, item, count), GFP_KERNEL);
 
+NOTE: You might want to use the following form in case the trailing array
+is a one-element array, as unlike zero-length arrays and flexible-array
+members, `one-element arrays occupy at least as much space as a single
+object of the type <https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html>`_,
+hence they contribute to the size of the enclosing structure::
+
+	header = kzalloc(struct_size(header, item, count - 1), GFP_KERNEL);
+
+It's also worth noting that one-element arrays --together with zero-length
+arrays-- will soon be completely removed from the codebase and deprecated.
+So, you better STOP using them and switch to flexible arrays instead.
+
 See array_size(), array3_size(), and struct_size(),
 for more details as well as the related check_add_overflow() and
 check_mul_overflow() family of functions.
-- 
2.27.0