autofs 5.1.9 snprintf() crash with -D_FORTIFY_SOURCE=3

Hi,

in Ubuntu we are building most packages with -D_FORTIFY_SOURCE=3
nowadays, and we just got a bug report that 5.1.9 was crashing with a
buffer overflow warning. When rebuilt with -D_FORTIFY_SOURCE=2, it
does not crash.

Here is a small reproducer using a loop device. This is on kernel 6.8.0:

/etc/auto.master:
/- file,sun:/etc/auto.mp strictexpire

"strictexpire" is what triggers the crash.

/etc/auto.mp:
/mp defaults :/dev/loop0

# automount -f -d3
Starting automounter version 5.1.9, master map /etc/auto.master
using kernel protocol version 5.05
lookup_nss_read_master: reading master file /etc/auto.master
do_init: parse(sun): init gathered global options: (null)
lookup_read_master: lookup(file): read entry /-
master_do_mount: mounting /-
reading file map /etc/auto.mp
do_init: parse(sun): init gathered global options: (null)
*** buffer overflow detected ***: terminated
Aborted (core dumped)

gdb show this being in the snprintf call in lib/mounts.c when
",strictexpire" is being added to the autofs mount options string:
#9  0x00007ffff7dbaab4 in snprintf (__fmt=0x7ffff7dca232 "%s", __n=93,
__s=0x7fffec002c1c "") at
/usr/include/x86_64-linux-gnu/bits/stdio2.h:54
No locals.
#10 make_options_string (path=0x5555555b7d50 "/-", pipefd=6,
type=type@entry=0x7ffff7dca02b "direct", flags=2560) at
/usr/src/autofs-5.1.9-1ubuntu3/lib/mounts.c:764
        kver_major = <optimized out>
        kver_minor = 5
        options = 0x7fffec002bf0
"fd=6,pgrp=22935,minproto=5,maxproto=5,direct"
        max_len = 93
        len = 44
        new = <optimized out>
        __FUNCTION__ = "make_options_string"

lib/mounts.c:760
    /* maybe add ",strictexpire" */
    if (flags & MOUNT_FLAG_STRICTEXPIRE) {
        new = snprintf(options + len,
                   max_len, "%s", ",strictexpire");

I don't think this is actually overflowing options in this particular
case, but the max_len argument doesn't seem right, as that was the
original max size for options.

This patch stopped the crash, but there is a lot of C string
manipulation going on here and I would appreciate more eyes on this. I
 *think* the truncated and error checks are still ok after this:

--- a/lib/mounts.c
+++ b/lib/mounts.c
@@ -760,7 +760,7 @@ char *make_options_string(char *path, int pipefd,
    /* maybe add ",strictexpire" */
    if (flags & MOUNT_FLAG_STRICTEXPIRE) {
        new = snprintf(options + len,
-                  max_len, "%s", ",strictexpire");
+                  max_len - len, "%s", ",strictexpire");
        if (new < 0)
               goto error_out;
        len += new;
@@ -774,7 +774,7 @@ char *make_options_string(char *path, int pipefd,
    /* maybe add ",ignore" */
    if (flags & MOUNT_FLAG_IGNORE) {
        new = snprintf(options + len,
-                  max_len, "%s", ",ignore");
+                  max_len - len, "%s", ",ignore");
        if (new < 0)
               goto error_out;
        len += new;