FreeBSD: Fix leaked strings in libspl mnttab

The FreeBSD implementations of various libspl functions for getting
mounted device information were found to leak several strings which
were being allocated in statfs2mnttab but never freed.

The Solaris getmntany(3C) and related interfaces are expected to return
strings residing in static buffers that need to be copied rather than
freed by the caller.

Use static thread-local storage to stash the mnttab structure strings
from FreeBSD's statfs info rather than strings allocated on the heap by
strdup(3).

While here, remove some stray commented out lines.

Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Rich Ercolani <[email protected]>
Signed-off-by: Ryan Moeller <[email protected]>
Closes openzfs#12961

lib/libspl/os/freebsd/mnttab.c

@@ -91,16 +91,28 @@ optadd(char *mntopts, size_t size, const char *opt)
 	strlcat(mntopts, opt, size);
 }
 
+static __thread char gfstypename[MFSNAMELEN];
+static __thread char gmntfromname[MNAMELEN];
+static __thread char gmntonname[MNAMELEN];
+static __thread char gmntopts[MNTMAXSTR];
+
 void
 statfs2mnttab(struct statfs *sfs, struct mnttab *mp)
 {
-	static char mntopts[MNTMAXSTR];
 	long flags;
 
-	mntopts[0] = '\0';
+	strlcpy(gfstypename, sfs->f_fstypename, sizeof (gfstypename));
+	mp->mnt_fstype = gfstypename;
+
+	strlcpy(gmntfromname, sfs->f_mntfromname, sizeof (gmntfromname));
+	mp->mnt_special = gmntfromname;
+
+	strlcpy(gmntonname, sfs->f_mntonname, sizeof (gmntonname));
+	mp->mnt_mountp = gmntonname;
 
 	flags = sfs->f_flags;
-#define	OPTADD(opt)	optadd(mntopts, sizeof (mntopts), (opt))
+	gmntopts[0] = '\0';
+#define	OPTADD(opt)	optadd(gmntopts, sizeof (gmntopts), (opt))
 	if (flags & MNT_RDONLY)
 		OPTADD(MNTOPT_RO);
 	else
@@ -121,10 +133,7 @@ statfs2mnttab(struct statfs *sfs, struct mnttab *mp)
 	else
 		OPTADD(MNTOPT_EXEC);
 #undef	OPTADD
-	mp->mnt_special = strdup(sfs->f_mntfromname);
-	mp->mnt_mountp = strdup(sfs->f_mntonname);
-	mp->mnt_fstype = strdup(sfs->f_fstypename);
-	mp->mnt_mntopts = strdup(mntopts);
+	mp->mnt_mntopts = gmntopts;
 }
 
 static struct statfs *gsfs = NULL;
@@ -166,7 +175,6 @@ statfs_init(void)
 int
 getmntany(FILE *fd __unused, struct mnttab *mgetp, struct mnttab *mrefp)
 {
-	//	struct statfs *sfs;
 	int i, error;
 
 	error = statfs_init();
@@ -195,7 +203,6 @@ getmntany(FILE *fd __unused, struct mnttab *mgetp, struct mnttab *mrefp)
 int
 getmntent(FILE *fp, struct mnttab *mp)
 {
-	//	struct statfs *sfs;
 	int error, nfs;
 
 	nfs = (int)lseek(fileno(fp), 0, SEEK_CUR);