linuxlist: fix false-positive UBSan misaligned-access reports

llist_for_each_entry() and its variants terminate by comparing
&pos->member against head.  Once a full traversal reaches the end of a
non-empty list (or the list is empty to begin with), pos becomes a pure
container_of()-computed sentinel address that was never a real object
of typeof(*pos), only ever the plain 'struct llist_head *' passed in as
head.  Forming 'pos->member' on that address requires pos to satisfy
the alignment of typeof(*pos), which the sentinel does not necessarily
provide (e.g. embedded struct osmo_timer_list/gprs_nsvc on 32-bit ARM
with a 64-bit time_t need 8-byte alignment, while head itself is only
pointer-aligned) - tripping -fsanitize=alignment even though no real
object is ever misaligned or dereferenced.

Add __llist_member(), computing the same address via 'char *'
pointer arithmetic and an explicit cast to 'struct llist_head *', which
carries no alignment requirement of its own.  Use it for the loop
termination check, the "next" pointer computation, and the prefetch()
calls (dereferencing the returned struct llist_head * still only
requires pointer alignment, so prefetch() keeps working exactly
as before without reintroducing the false positive).

Change-Id: I0424e76e76d8aa9402bd1a5aefe789de16e72fae
Related: OS#7036, OS#6858
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M include/osmocom/core/linuxlist.h
1 file changed, 33 insertions(+), 19 deletions(-)