On Thu, Jun 04, 2020 at 10:49:19AM -0700, Kees Cook wrote: > 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 Yeah. My reasoning for is that it will take a while --at least one development cycle more-- to completely get rid of all the 0/1-arrays. Also, this was motivated by the following comments from Christian König: "May I suggest that we add a section how to correctly do this to Documentation/process/coding-style.rst or similar document? I've seen a bunch of different approaches and some even doesn't work with some gcc versions and result in a broken binary."[1] > 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. > I agree with this. I can add the comments in quotes you suggest to this patch. But I think we can add this note while I continue working on the flexible-array conversions. Once that work is complete, I can go back and update the documentation. :) What do you think? Thanks -- Gustavo [1] https://lore.kernel.org/lkml/1065d63e-7959-e4b4-af4e-70607ba92296@xxxxxxx/ > > > > 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
