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, 14 insertions, 1 deletions

diff --git a/share/man/man5/core.5 b/share/man/man5/core.5
index 3ea88a1..dcc57ba 100644
--- a/share/man/man5/core.5
+++ b/share/man/man5/core.5
@@ -28,7 +28,7 @@
 .\"     @(#)core.5	8.3 (Berkeley) 12/11/93
 .\" $FreeBSD$
 .\"
-.Dd September 2, 2015
+.Dd October 5, 2015
 .Dt CORE 5
 .Os
 .Sh NAME
@@ -130,6 +130,19 @@ All file descriptor information can be preserved by disabling packing.
 This potentially wastes up to PATH_MAX bytes per open fd.
 Packing is disabled with
 .Dl sysctl kern.coredump_pack_fileinfo=0 .
+.Pp
+Similarly, corefiles are written with vmmap information as an ELF note, which
+contains file paths.
+By default, they are packed to only use as much space as
+needed.
+By the same mechanism as for the open files note, these paths can also
+change at any time and result in a truncated note.
+.Pp
+All vmmap information can be preserved by disabling packing.
+Like the file information, this potentially wastes up to PATH_MAX bytes per
+mapped object.
+Packing is disabled with
+.Dl sysctl kern.coredump_pack_vmmapinfo=0 .
 .Sh EXAMPLES
 In order to store all core images in per-user private areas under
 .Pa /var/coredumps ,