[PATCH 4/5] [VERY RFC] mm: kmalloc(_node): return NULL immediately for SIZE_MAX

[Daniel Axtens]

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 â__compiletime_assert_32â declared with attribute error: BUILD_BUG_ON failed: size == SIZE_MAX

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.

This brings kmalloc() and kmalloc_node() into line with the array functions
kmalloc_array() and kmalloc_array_node() for the overflow case. The overall
compiled size change per bloat-o-meter is in the noise (a reduction of
<0.01%).

Signed-off-by: Daniel Axtens <dja@xxxxxxxxxx>
---
 include/linux/slab.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 03a389358562..8141c6b1882a 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -544,6 +544,9 @@ static __always_inline void *kmalloc(size_t size, gfp_t flags)
 #ifndef CONFIG_SLOB
 		unsigned int index;
 #endif
+		if (unlikely(size == SIZE_MAX))
+			return NULL;
+
 		if (size > KMALLOC_MAX_CACHE_SIZE)
 			return kmalloc_large(size, flags);
 #ifndef CONFIG_SLOB
@@ -562,6 +565,9 @@ static __always_inline void *kmalloc(size_t size, gfp_t flags)
 
 static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 {
+	if (__builtin_constant_p(size) && size == SIZE_MAX)
+		return NULL;
+
 #ifndef CONFIG_SLOB
 	if (__builtin_constant_p(size) &&
 		size <= KMALLOC_MAX_CACHE_SIZE) {
-- 
2.20.1