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, 5 insertions, 1 deletions

diff --git a/sys/sys/user.h b/sys/sys/user.h
index ad574b1..fd362c2 100644
--- a/sys/sys/user.h
+++ b/sys/sys/user.h
@@ -541,6 +541,9 @@ struct kinfo_sigtramp {
 
 /* Flags for kern_proc_filedesc_out. */
 #define	KERN_FILEDESC_PACK_KINFO	0x00000001U
+
+/* Flags for kern_proc_vmmap_out. */
+#define	KERN_VMMAP_PACK_KINFO		0x00000001U
 struct sbuf;
 
 /*
@@ -556,7 +559,8 @@ int	kern_proc_filedesc_out(struct proc *p, struct sbuf *sb, ssize_t maxlen,
 	int flags);
 int	kern_proc_cwd_out(struct proc *p, struct sbuf *sb, ssize_t maxlen);
 int	kern_proc_out(struct proc *p, struct sbuf *sb, int flags);
-int	kern_proc_vmmap_out(struct proc *p, struct sbuf *sb);
+int	kern_proc_vmmap_out(struct proc *p, struct sbuf *sb, ssize_t maxlen,
+	int flags);
 
 int	vntype_to_kinfo(int vtype);
 #endif /* !_KERNEL */