the result of the 0xffff mask with the exit status could have bit 15
set, in which case multiplying by 0x10001 overflows 32-bit signed int.
making the multiply unsigned avoids the overflow. it also changes the
sign extension behavior of the subsequent >> operation, but the
affected bits are all unwanted anyway and all discarded by the cast to
short.

these badly pollute the namespace with macros whenever _GNU_SOURCE is
defined, which is always the case with g++, and especially tends to
interfere with C++ constructs.

as our implementation of these was macro-only, their removal cannot
affect any existing binaries. at the source level, portable software
should be prepared for them not to exist.

for now, they are left in place with explicit _LARGEFILE64_SOURCE.
this provides an easy temporary path for integrators/distributions to
get packages building again right away if they break while working on
a proper, upstreamable fix. the intent is that this be a very
short-term measure and that the macros be removed entirely in the next
release cycle.

see

  linux commit 90f093fa8ea48e5d991332cee160b761423d55c1
  rseq, ptrace: Add PTRACE_GET_RSEQ_CONFIGURATION request

the struct type got __ prefix to follow existing practice.

see

  linux commit 201698626fbca1cf1a3b686ba14cf2a056500716
  arm64: Introduce prctl(PR_PAC_{SET,GET}_ENABLED_KEYS)

see

  linux commit 7fd3253a7de6a317a0683f83739479fb880bffc8
  net: Introduce preferred busy-polling

  linux commit 7c951cafc0cb2e575f1d58677b95ac387ac0a5bd
  net: Add SO_BUSY_POLL_BUDGET socket option

see

  linux commit 1446e1df9eb183fdf81c3f0715402f1d7595d4cb
  kernel: Implement selective syscall userspace redirection

  linux commit 36a6c843fd0d8e02506681577e96dabd203dd8e8
  entry: Use different define for selector variable in SUD

redirect syscalls to a userspace handler via SIGSYS, except for a specific
range of code. can be toggled via a memory write to a selector variable.
mainly for wine.

these are for the aarch64 MTE (memory tagging extension), see

  linux commit 1c101da8b971a36695319dce7a24711dc567a0dd
  arm64: mte: Allow user control of the tag check mode via prctl()

  linux commit af5ce95282dc99d08a27a407a02c763dde1c5558
  arm64: mte: Allow user control of the generated random tags via prctl()

see

  linux commit e47168f3d1b14af5281cf50c59561d59d28201f9
  powerpc/8xx: Support 16k hugepages with 4k pages

path resolution does not follow symlinks on nosymfollow mounts (but
readlink still does), see

  linux commit dab741e0e02bd3c4f5e2e97be74b39df2523fc6e
  Add a "nosymfollow" mount option.

can cause rseq restart on another cpu to synchronize with global
memory access from rseq critical sections, see

  linux commit 2a36ab717e8fe678d98f81c14a0b124712719840
  rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ