On Thu, Jun 04, 2020 at 12:21:38PM -0500, Gustavo A. R. Silva wrote: > 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. > > Signed-off-by: Gustavo A. R. Silva <gustavoars@xxxxxxxxxx> > --- > > Kees, > > This is not substitute for the patch I'll write about flexible-arrays > and the deprecation of zero-lenght and one-element arrays. Hm, hm. I think I'd rather just get the 0/1-array docs written, since that will mean this paragraph isn't needed at all. (Or rather, it can be modified to say "if you're using struct_size() on a 1-array, stop using a 1-array, see [link]". If someone is going to switch around their code, they need to switch to flex at the same time, IMO. -Kees > > Thanks > -- > Gustavo > > Documentation/process/deprecated.rst | 11 +++++++++++ > 1 file changed, 11 insertions(+) > > diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst > index 652e2aa02a66c..0b7b37718bf96 100644 > --- a/Documentation/process/deprecated.rst > +++ b/Documentation/process/deprecated.rst > @@ -85,6 +85,17 @@ 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. > + > 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 > -- Kees Cook
