Detect badly behaved coredump note helpers

Coredump notes depend on being able to invoke dump routines twice; once
in a dry-run mode to get the size of the note, and another to actually
emit the note to the corefile.

When a note helper emits a different length section the second time
around than the length it requested the first time, the kernel produces
a corrupt coredump.

NT_PROCSTAT_FILES output length, when packing kinfo structs, is tied to
the length of filenames corresponding to vnodes in the process' fd table
via vn_fullpath.  As vnodes may move around during dump, this is racy.

So:

 - Detect badly behaved notes in putnote() and pad underfilled notes.

 - Add a fail point, debug.fail_point.fill_kinfo_vnode__random_path to
   exercise the NT_PROCSTAT_FILES corruption.  It simply picks random
   lengths to expand or truncate paths to in fo_fill_kinfo_vnode().

 - Add a sysctl, kern.coredump_pack_fileinfo, to allow users to
   disable kinfo packing for PROCSTAT_FILES notes.  This should avoid
   both FILES note corruption and truncation, even if filenames change,
   at the cost of about 1 kiB in padding bloat per open fd.  Document
   the new sysctl in core.5.

 - Fix note_procstat_files to self-limit in the 2nd pass.  Since
   sometimes this will result in a short write, pad up to our advertised
   size.  This addresses note corruption, at the risk of sometimes
   truncating the last several fd info entries.

 - Fix NT_PROCSTAT_FILES consumers libutil and libprocstat to grok the
   zero padding.

With suggestions from:	bjk, jhb, kib, wblock
Approved by:	markj (mentor)
Relnotes:	yes
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D3548

1 files changed, 4 insertions, 1 deletions

diff --git a/sys/sys/user.h b/sys/sys/user.h
index e831a37..ad574b1 100644
--- a/sys/sys/user.h
+++ b/sys/sys/user.h
@@ -539,6 +539,8 @@ struct kinfo_sigtramp {
 #define KERN_PROC_NOTHREADS	0x1
 #define KERN_PROC_MASK32	0x2
 
+/* Flags for kern_proc_filedesc_out. */
+#define	KERN_FILEDESC_PACK_KINFO	0x00000001U
 struct sbuf;
 
 /*
@@ -550,7 +552,8 @@ struct sbuf;
  * to be locked on enter.  On return the process is unlocked.
  */
 
-int	kern_proc_filedesc_out(struct proc *p, struct sbuf *sb, ssize_t maxlen);
+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);