A long, long time ago (I think it was late 2003), OpenBSD/i386 was vulnerable to a trusted-yet-NULL pointer dereference in the agp code. The attack involved using mmap(2) with MAP_FIXED and a hint of zero.
Shortly afterwards, in addition to fixing the overtrusting code, it was decided never to allow mmap(2) to allow address zero to get mapped, by never making VM_MIN_ADDRESS equal to zero (I actually argued for this change to only be applied to platforms with shared kernel/userland address spaces, but the party's line prevailed), which is why VM_MIN_ADDRESS is nowadays PAGE_SIZE instead of zero. Except on riscv64. The following diff adjusts VM_MIN_ADDRESS to follow the party's line. Completely untested due to lack of hardware. Index: vmparam.h =================================================================== RCS file: /OpenBSD/src/sys/arch/riscv64/include/vmparam.h,v retrieving revision 1.5 diff -u -p -r1.5 vmparam.h --- vmparam.h 2 Jul 2021 10:42:22 -0000 1.5 +++ vmparam.h 21 Mar 2022 19:49:00 -0000 @@ -111,8 +111,7 @@ * VM_MIN_USER_ADDRESS and VM_MAX_USER_ADDRESS define the start and end of the * user address space. */ -// XXX OpenBSD/arm64 starts VM_MIN_ADDRESS from PAGE_SIZE. Why? -#define VM_MIN_ADDRESS (0x0000000000000000UL) +#define VM_MIN_ADDRESS ((vaddr_t)PAGE_SIZE) #define VM_MAX_ADDRESS (0xffffffffffffffffUL) #define VM_MIN_KERNEL_ADDRESS (0xffffffc000000000UL)
