Date: Wed, 14 Feb 2018 13:12:03 -0800 From: Matthew Wilcox <willy@...radead.org> To: Joe Perches <joe@...ches.com> Cc: Andrew Morton <akpm@...ux-foundation.org>, Matthew Wilcox <mawilcox@...rosoft.com>, linux-mm@...ck.org, Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com Subject: Re: [PATCH v2 2/8] mm: Add kvmalloc_ab_c and kvzalloc_struct 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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