MFC r315686,r315688:

r315686:

kvm_geterr: handle `kd` == NULL in a deterministic/graceful manner

Return a NUL string instead of just working by accident with kvm_geterr(3)
when MALLOC_PRODUCTION is disabled (I didn't confirm the MALLOC_PRODUCTION
being enabled path).

Document the new explicit return behavior for kvm_geterr(3), as well
as the previous implicit behavior, i.e., the buffer attached to
returned via kvm_geterr(3) would be empty if a previous error hadn't been
stored in `kd`.

r315688:

kvm_write: fix -Wcast-qual warning in pointer arithmetic argument

Cast buf to `const char *` when doing arithmetic operation to match
`cp`'s type [1].

2 files changed, 16 insertions, 2 deletions

diff --git a/lib/libkvm/kvm.c b/lib/libkvm/kvm.c
index 2f95a20..d6cc7d8 100644
--- a/lib/libkvm/kvm.c
+++ b/lib/libkvm/kvm.c
@@ -66,6 +66,8 @@ static char sccsid[] = "@(#)kvm.c	8.2 (Berkeley) 2/13/94";
 
 SET_DECLARE(kvm_arch, struct kvm_arch);
 
+static char _kd_is_null[] = "";
+
 /* from src/lib/libc/gen/nlist.c */
 int __fdnlist(int, struct nlist *);
 
@@ -113,6 +115,9 @@ kvm_fdnlist(kvm_t *kd, struct kvm_nlist *list)
 char *
 kvm_geterr(kvm_t *kd)
 {
+
+	if (kd == NULL)
+		return (_kd_is_null);
 	return (kd->errbuf);
 }
 
@@ -932,7 +937,7 @@ kvm_write(kvm_t *kd, u_long kva, const void *buf, size_t len)
 		len -= cw;
 	}
 
-	return (cp - (char *)buf);
+	return (cp - (const char *)buf);
 }
 
 int
diff --git a/lib/libkvm/kvm_geterr.3 b/lib/libkvm/kvm_geterr.3
index 3ce5c72..4aa9cc8 100644
--- a/lib/libkvm/kvm_geterr.3
+++ b/lib/libkvm/kvm_geterr.3
@@ -32,7 +32,7 @@
 .\"     @(#)kvm_geterr.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd June 4, 1993
+.Dd March 21, 2017
 .Dt KVM_GETERR 3
 .Os
 .Sh NAME
@@ -54,6 +54,15 @@ library call did not produce an error.
 The string returned is stored in memory owned by
 .Xr kvm 3
 so the message should be copied out and saved elsewhere if necessary.
+.Sh RETURN VALUES
+The function
+.Fn kvm_geterr
+will return "" if
+.Fa kd
+is
+.Dv NULL
+or an error has not been captured for
+.Fa kd .
 .Sh SEE ALSO
 .Xr kvm 3 ,
 .Xr kvm_close 3 ,