bpf: Don't move a MAP_FIXED address in arena_get_unmapped_area()

The user range of an arena must not cross a 4 GiB boundary. Thus
arena_get_unmapped_area() asks the mm for 2 * len bytes and rounds the
result up if necessary. This is correct only if the mm finds a free range.
With MAP_FIXED the mm does no search and returns addr. For
MAP_FIXED_NOREPLACE, do_mmap() also sets MAP_FIXED.

With MAP_FIXED the arena is not at addr after round_up(). Thus mmap()
removes mappings outside the range from addr to addr + len. It keeps a part
of those in this range. Also, round_up() can wrap to address 0, and the
test for address 0 is before it.

The arch code gets twice the length of the mapping. Thus it can refuse a
valid MAP_FIXED range. On s390 the arch code also grows the page table for
the range. With the double length it can omit this step for a valid range.

The test for a crossing is not exact. Because ret is a long, ret >> 32
extends the sign, but the other side is unsigned. The sides are never equal
for an address with bit 63 set, and round_up() always runs. Such user
addresses exist on s390. arena_map_alloc() does the same test with a u64.

With MAP_FIXED, pass len to the mm. Return -EINVAL if the range crosses a
boundary, as for address 0. Make ret unsigned, so that only these ranges
get the error. On x86-64, if round_up() gives address 0 today, mmap() now
fails with -EINVAL or with -ENOMEM from the mm. Today mmap() can also fail
for the address after round_up(), for example with -ENOMEM, -EEXIST, -EPERM
or -EBUSY.

Without MAP_FIXED the mm must still find a free range of 2 * len bytes
inside the address space of the task. The range after round_up() is inside
it, and round_up() cannot wrap. An address with bit 63 set now moves only
if its range crosses a boundary.

For an arena, libbpf uses MAP_FIXED only with map_extra, and
arena_map_alloc() refuses a map_extra range that crosses a boundary. A
subsequent mmap() must use the range of the first mmap(). For mmap() of the
map fd, this range comes from arena_get_unmapped_area(), thus it crosses no
boundary. arena_map_alloc() still accepts a map_extra above TASK_SIZE, and
mmap() of such an arena fails.

Fixes: 317460317a02 ("bpf: Introduce bpf_arena.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261102064231.2153879-2-alexei.starovoitov@gmail.com

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