Hi,
Running the attached C program results in this output:
$ uname -mprsv
OpenBSD 7.8 GENERIC.MP#1 amd64 amd64
$ cc bug.c
$ ./a.out
0xffffffffffffffff (Cannot allocate memory)
By my reading of POSIX, this seems bad; the kernel is of course well within its
rights to reject mappings past VM_MAXUSER_ADDRESS, but since MAP_FIXED is not
used, I would expect the address hint to simply be ignored. AFAICT, every other
POSIX-ish OS (other BSDs, Linux, macOS, etc) does exactly that.
For context, I ran into this on Zig's OpenBSD CI machine while fixing some bugs
in our page allocator. We use address hints in mmap() to minimize address reuse
with the goal of making use-after-frees easier to debug. For that to work
reliably, we expect the kernel to just ignore any address hint it doesn't like
and place the mapping somewhere actually sensible, after which we'll base our
hints on that location.
For now, we're working around this by doing an extra mmap() without an address
hint if we get ENOMEM on OpenBSD, but of course we'd prefer to not do that.
Regards,
Alex#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <sys/mman.h>
#include <unistd.h>
int main() {
long pgsz = sysconf(_SC_PAGE_SIZE);
void *addr = mmap((void *)0x00007f7fffffc000, pgsz, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
printf("%p (%s)\n", addr, strerror(errno));
}
