CPP Pointer Issue

[aagontuk]

• map definition:

struct map_data {
    X val[N];
};

• map is mmaped with MAP_SHARED and using map fd.

map_mmaped = mmap(NULL, map_sz, PROT_READ | PROT_WRITE, MAP_SHARED, data_map_fd, 0);

• Userspace address of the map: 0x7f694c096000 (Address of the first element in the mmap()-able array)

• Userspace address of heap allocated int: 0x562826c616b0

• kernel address of the map: 00000000f1d1465d (Address of the first element in the mmap()-able array)

• kernel address of the heap allocated int: 000000001706ee08 (Fetched from the map)

• In the kernel mmap()-able array maps are allocated here

data = bpf_map_area_mmapable_alloc(array_size, numa_node);

Adding printk("Allocated map addr: %p\n", data) is printing:

[ 1050.709008] Allocated map addr: `0000000036391975`
[ 1050.709125] Allocated map addr: `00000000721a997f`
[ 1050.709225] Allocated map addr: `000000007d021192`
[ 1050.709245] Allocated map addr: `000000007262e39a`

[aagontuk]

mmap()-able array map allocation

• Memory allocation is done in this line:

data = bpf_map_area_mmapable_alloc(array_size, numa_node);

• Which eventually calls __vmalloc_node_range() with VM_USERMAP flag.
• This allocation can be seen cat /proc/vmallocinfo by searching user for VM_USERMAP flag.
• vmalloc address range is defined here: https://elixir.bootlin.com/linux/v5.19-rc2/source/arch/x86/include/asm/pgtable_64_types.h#L132
• There is no size limit for allocated map. Maximum the kernel can provide.

TODO

• How the allocated memory is linked to the map fd.
• For vmalloc() high addresses in older kernels.

[aagontuk]

References

• vmalloc and Friends
• https://lore.kernel.org/linux-mm/20190103145954.16942-4-rpenyaev@suse.de/