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Date: Mon, 20 Jan 2020 12:14:11 +0100
From: Michal Hocko <>
To: Daniel Axtens <>
Subject: Re: [PATCH 4/5] [VERY RFC] mm: kmalloc(_node): return NULL
 immediately for SIZE_MAX

On Mon 20-01-20 18:43:43, Daniel Axtens wrote:
> kmalloc is sometimes compiled with an size that at compile time may be
> equal to SIZE_MAX.
> For example, struct_size(struct, array member, array elements) returns the
> size of a structure that has an array as the last element, containing a
> given number of elements, or SIZE_MAX on overflow.
> However, struct_size operates in (arguably) unintuitive ways at compile time.
> Consider the following snippet:
> struct foo {
> 	int a;
> 	int b[0];
> };
> struct foo *alloc_foo(int elems)
> {
> 	struct foo *result;
> 	size_t size = struct_size(result, b, elems);
> 	if (__builtin_constant_p(size)) {
> 		BUILD_BUG_ON(size == SIZE_MAX);
> 	}
> 	result = kmalloc(size, GFP_KERNEL);
> 	return result;
> }
> I expected that size would only be constant if alloc_foo() was called
> within that translation unit with a constant number of elements, and the
> compiler had decided to inline it. I'd therefore expect that 'size' is only
> SIZE_MAX if the constant provided was a huge number.
> However, instead, this function hits the BUILD_BUG_ON, even if never
> called.
> include/linux/compiler.h:394:38: error: call to ‘__compiletime_assert_32’ declared with attribute error: BUILD_BUG_ON failed: size == SIZE_MAX

This sounds more like a bug to me. Have you tried to talk to compiler

> This is with gcc 9.2.1, and I've also observed it with an gcc 8 series
> compiler.
> My best explanation of this is:
>  - elems is a signed int, so a small negative number will become a very
>    large unsigned number when cast to a size_t, leading to overflow.
>  - Then, the only way in which size can be a constant is if we hit the
>    overflow case, in which 'size' will be 'SIZE_MAX'.
>  - So the compiler takes that value into the body of the if statement and
>    blows up.
> But I could be totally wrong.
> Anyway, this is relevant to slab.h because kmalloc() and kmalloc_node()
> check if the supplied size is a constant and take a faster path if so. A
> number of callers of those functions use struct_size to determine the size
> of a memory allocation. Therefore, at compile time, those functions will go
> down the constant path, specialising for the overflow case.
> When my next patch is applied, gcc will then throw a warning any time
> kmalloc_large could be called with a SIZE_MAX size, as gcc deems SIZE_MAX
> to be too big an allocation.
> So, make functions that check __builtin_constant_p check also against
> SIZE_MAX in the constant path, and immediately return NULL if we hit it.

I am not sure I am happy about an additional conditional path in the hot
path of the allocator. Especially when we already have a check for
Michal Hocko

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