malloc_usable_size of malloc_large returns incorrect value.

[Pusnow]

OSv uses malloc_large for huge memory request, and it stores alloced size (page alloced) in header (page_range).

osv/core/mempool.cc

Lines 844 to 850 in 69ea100

	if (ret_header) {
	on_alloc(size);
	void* obj = ret_header;
	obj += offset;
	trace_memory_malloc_large(obj, requested_size, size, alignment);
	return obj;
	}

However, in large_object_size it only returns size in header.

osv/core/mempool.cc

Lines 1070 to 1075 in 69ea100

	static unsigned large_object_size(void *obj)
	{
	obj = align_down(obj - 1, page_size);
	auto header = static_cast<page_range*>(obj);
	return header->size;
	}

I think malloc_usable_size should subtract offset or header size.

Example code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
	
	void* ptr = malloc(0x4010);
	int size = malloc_usable_size(ptr);

	printf("ptr: %p size: %lx\n", ptr, size); 

	return 0;
}

Output:

Cmdline: /hello
ptr: 0xffff800001b64040 size: 5000 

I think the size should be 0x4FC0.

[wkozaczuk]

You might be right. What I am more worried that std_realloc that depends on it might be wrong:

static inline void* std_realloc(void* object, size_t size)
{
    if (!object)
        return malloc(size);
    if (!size) {
        free(object);
        return nullptr;
    }

    size_t old_size = object_size(object);
    size_t copy_size = size > old_size ? old_size : size;
    void* ptr = malloc(size);
    if (ptr) {
        memcpy(ptr, object, copy_size);
        free(object);
    }

    return ptr;
}

[wkozaczuk]

We clearly have bigger issue - here is the related issue #784.