Fix core corruption caused by race in note_procstat_vmmap

This fix is spiritually similar to r287442 and was discovered thanks to
the KASSERT added in that revision.

NT_PROCSTAT_VMMAP output length, when packing kinfo structs, is tied to
the length of filenames corresponding to vnodes in the process' vm map
via vn_fullpath.  As vnodes may move during coredump, this is racy.

We do not remove the race, only prevent it from causing coredump
corruption.

- Add a sysctl, kern.coredump_pack_vmmapinfo, to allow users to disable
  kinfo packing for PROCSTAT_VMMAP notes.  This avoids VMMAP corruption
  and truncation, even if names change, at the cost of up to PATH_MAX
  bytes per mapped object.  The new sysctl is documented in core.5.

- Fix note_procstat_vmmap to self-limit in the second pass.  This
  addresses corruption, at the cost of sometimes producing a truncated
  result.

- Fix PROCSTAT_VMMAP consumers libutil (and libprocstat, via copy-paste)
  to grok the new zero padding.

Reported by:	pho (https://people.freebsd.org/~pho/stress/log/datamove4-2.txt)
Relnotes:	yes
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D3824

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/sys/exec.h b/sys/sys/exec.h
index a13c1ac..0ccba24 100644
--- a/sys/sys/exec.h
+++ b/sys/sys/exec.h
@@ -84,6 +84,7 @@ int exec_register(const struct execsw *);
 int exec_unregister(const struct execsw *);
 
 extern int coredump_pack_fileinfo;
+extern int coredump_pack_vmmapinfo;
 
 /*
  * note: name##_mod cannot be const storage because the