Re: [PATCH v2 2/8] mm: Add kvmalloc_ab_c and kvzalloc_struct

[Matthew Wilcox <willy@infradead.org>]

On Wed, Feb 14, 2018 at 12:45:52PM -0800, Joe Perches wrote:
> On Wed, 2018-02-14 at 12:11 -0800, Matthew Wilcox wrote:
> > We have kvmalloc_array in order to safely allocate an array with a
> > number of elements specified by userspace (avoiding arithmetic overflow
> > leading to a buffer overrun).  But it's fairly common to have a header
> > in front of that array (eg specifying the length of the array), so we
> > need a helper function for that situation.
> > 
> > kvmalloc_ab_c() is the workhorse that does the calculation, but in spite
> > of our best efforts to name the arguments, it's really hard to remember
> > which order to put the arguments in.  kvzalloc_struct() eliminates that
> > effort; you tell it about the struct you're allocating, and it puts the
> > arguments in the right order for you (and checks that the arguments
> > you've given are at least plausible).
> > 
> > For comparison between the three schemes:
> > 
> > 	sev = kvzalloc(sizeof(*sev) + sizeof(struct v4l2_kevent) * elems,
> > 			GFP_KERNEL);
> > 	sev = kvzalloc_ab_c(elems, sizeof(struct v4l2_kevent), sizeof(*sev),
> > 			GFP_KERNEL);
> > 	sev = kvzalloc_struct(sev, events, elems, GFP_KERNEL);
> 
> Perhaps kv[zm]alloc_buf_and_array is better naming.

I think that's actively misleading.  The programmer isn't allocating a
buf, they're allocating a struct.  kvzalloc_hdr_arr was the earlier name,
and that made some sense; they're allocating an array with a header.
But nobody thinks about it like that; they're allocating a structure
with a variably sized array at the end of it.

If C macros had decent introspection, I'd like it to be:

	sev = kvzalloc_struct(elems, GFP_KERNEL);

and have the macro examine the structure pointed to by 'sev', check
the last element was an array, calculate the size of the array element,
and call kvzalloc_ab_c.  But we don't live in that world, so I have to
get the programmer to tell me the structure and the name of the last
element in it.

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