To reproduce this using a user-mode build of subr_blist.c:
```
$ clang -fsanitize=address subr_blist.c -lsbuf
$ ./a.out 64
BLIST representing 64 blocks (0 MB of swap), requiring 1K of ram
BLIST raw radix tree contains 2 records
64/64/4096> a 63 63
R=00000000, c=00000063
1/64/4096> a 1 2
=================================================================
==20873==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x606000000060 at pc 0x0000002b979d bp 0x7fffffffdc00 sp 0x7fffffffdbf8
READ of size 8 at 0x606000000060 thread T0
#0 0x2b979c in blst_next_leaf_alloc (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b979c)
#1 0x2b95a9 in blst_leaf_alloc (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b95a9)
#2 0x2b5a2c in blst_meta_alloc (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b5a2c)
#3 0x2b5c2f in blst_meta_alloc (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b5c2f)
#4 0x2b573b in blist_alloc (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b573b)
#5 0x2b8ad6 in main (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b8ad6)
0x606000000060 is located 0 bytes to the right of 64-byte region [0x606000000020,0x606000000060)
allocated by thread T0 here:
#0 0x28c272 in calloc /usr/home/markj/src/freebsd/contrib/llvm-project/compiler-rt/lib/asan/asan_malloc_linux.cpp:154:3
#1 0x2b53ca in blist_create (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b53ca)
#2 0x2b845a in main (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b845a)
#3 0x2359df in _start /usr/home/markj/src/freebsd/lib/csu/amd64/crt1_c.c:75:7
#4 0x8002df007 (<unknown module>)
SUMMARY: AddressSanitizer: heap-buffer-overflow (/usr/home/markj/src/freebsd/sys/kern/a.out+0x2b979c) in blst_next_leaf_alloc
Shadow bytes around the buggy address:
0x4c0bffffffb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x4c0bffffffc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x4c0bffffffd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x4c0bffffffe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x4c0bfffffff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x4c0c00000000: fa fa fa fa 00 00 00 00 00 00 00 00[fa]fa fa fa
0x4c0c00000010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x4c0c00000020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x4c0c00000030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x4c0c00000040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x4c0c00000050: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
==20873==ABORTING
```
This report is basically just saying that blst_next_leaf_alloc() triggered
an out-of-bounds access. Note also that in that example we allocated
two meta nodes even though only one is required.