Merge tag 'vfs-7.4-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull coredump updates from Christian Brauner:
 "This adds sparse coredumps and per-request memory type selection to
  the coredump socket protocol.

  A coredump sent over the socket transfers every zero of every hole in
  every mapping. For a process with 128 threads that's 1 GB of mostly
  thread stacks, 8 MB reservations with a few pages touched, and the
  holes sit in the middle of the dump rather than at the end. The
  coredump_filter bit that was proposed for this stops each PT_LOAD at
  the last populated page. But that mask says which kinds of memory to
  include and propagates across fork and exec, whereas what is selected
  here is an encoding. The usermodehelper isn't salvagable for this
  either. The coredump socket already has a handshake for exactly this.

  A server that raises COREDUMP_RECORDS in coredump_ack->mask gets the
  coredump as a sequence of records, each a struct
  coredump_record_header followed by what it describes. The stream ends
  explicitly with an end record that reports the size, so a missing one
  means truncation.

  With COREDUMP_SPARSE holes cross the socket as a bare header that says
  how many zero bytes were skipped. Reassembling the records gives back
  the same coredump, so a debugger and everything else still see an
  ordinary core file. Numbers from the selftests: the 128 thread process
  sends 1424153 bytes for a coredump of 1075150848 bytes, a 256 MB
  mapping with the first and the last page touched sends 188793 bytes
  for 268890112 bytes, and with COREDUMP_RECORDS alone the record
  overhead is under one percent.

  The selftests also simulate a server uploading to a blob store that
  can't seek and rewrites the program headers so the object is an
  ordinary ELF core file without the holes.

  COREDUMP_MEMORY_TYPES lets the server choose what memory to dump per
  request instead of relying on the static /proc/<pid>/coredump_filter
  of the crashing task. The request carries the default types and a mask
  of every type the kernel knows about, the ack carries the selection.
  Zero is valid and dumps the program headers and notes and nothing but
  the mappings that are always dumped.

  Preparatory cleanups refuse negative skips in dump_skip_to(), chunk
  every write at a page so a pipe or a socket never takes a short write
  in the middle of a record, make the dump helpers return bool and
  replace the core_dumped integer with COREDUMP_STATE_STARTED and
  COREDUMP_STATE_TRUNCATED flags.

  Also included is a fix for coredump_parse() reading core_pattern while
  it is being modified. The sysctl handler now validates a copy and
  publishes it under a spinlock and the parser works on a snapshot, so a
  rejected pattern is never visible and the rollback logic goes away.
  And to_kthread()'s lockless flags read is annotated with READ_ONCE()"

* tag 'vfs-7.4-rc1.coredump' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs: (29 commits)
  kthread: Annotate lockless to_kthread() flags access
  coredump: parse a snapshot of core_pattern
  selftests/coredump: test failed handshakes
  selftests/coredump: improve coredump size negotiation tests
  selftests/coredump: test COREDUMP_MEMORY_TYPES
  selftests/coredump: simplify the refusal tests
  tools: sync coredump.h header
  coredump: select memory types to include
  selftests/coredump: show how to inspect the task to decide how the coredump should be sent
  selftests/coredump: simulate a blob store
  selftests/coredump: put a hole in the middle of a sparse mapping
  selftests/coredump: hand the record stream to a sink
  selftests/coredump: test COREDUMP_RECORDS and COREDUMP_SPARSE
  coredump: describe the holes when COREDUMP_SPARSE is negotiated
  coredump: send the coredump in records if requested
  tools: sync coredump.h header
  coredump: add COREDUMP_SPARSE to the coredump socket protocol
  coredump: add COREDUMP_RECORDS to the coredump socket protocol
  coredump: clean up coredump state handling
  coredump: always chunk writes
  ...

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