Messages in this thread | | | From | Linus Torvalds <> | Date | Fri, 18 Sep 2020 18:28:30 -0700 | Subject | Re: [GIT PULL] percpu fix for v5.9-rc6 |
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On Fri, Sep 18, 2020 at 3:40 PM Arvind Sankar <nivedita@alum.mit.edu> wrote: > > Ouch, offsetof() and sizeof() will give different results in the > presence of alignment padding.
Indeed. But from an allocation standpoint, the offsetof()+size is I think the correct size. The padding at the end makes very little sense for something like "struct_size()".
Padding at the end is required for sizeof() for a very simple reason: arrays. The "sizeof()" needs to be aligned to the alignment of the entry, because if it isn't, then the standard C array traversal doesn't work.
But you cannot sanely have arrays of these structures of variable size entries either - even if standard C cheerfully allows you to declare them (again: it will not behave like a variable sized array, it will behave like a zero-sized one).
That was in fact one of the test-cases that I submitted to the sparse list - the insanity of allowing arrays of structures that have a flexible array at the end is just the C standard being confused. The C standard may allow it, but I don't think we should allow it in the kernel.
Oh, I can see why somebody would want to have an array of those things - exactly because they want to have some "initializer _without_ the flexible array part", and they actually don't want that variably-sized array at all for that case.
But I'm pretty sure we really really don't want that kind of oddities in the kernel. If we really want a separate "struct head_struct", then I think we should do so explicitly, and have something like
struct real_struct { // Unnamed head struct here struct head_struct { ,,,, }; unsigned int variable_array[]; };
and if you want the part without the flexible array at the end, then you use that "struct head_struct".
Instead of depending on the imho broken model of the C standard that says "in lots of cases, we'll just silently make that flexible array be a zero-sized one".
Linus
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