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@xxxxxxxxxx> --- 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
