tree: Fix potential NULL pointer dereference in nvme_host_get_ids()

[stephenchengCloud]

The nvme_host_get_hostid() and nvme_host_get_hostnqn() functions can return NULL when the host configuration is incomplete or invalid. Using strdup() directly on these return values causes a segmentation fault when NULL is passed to strdup().

Replace strdup() calls with xstrdup() to safely handle NULL input values.

[stephenchengCloud]

Here is the crash I observed:

(gdb) bt
#0  __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:76
#1  0x00007fd6be066933 in __GI___strdup (s=0x0) at strdup.c:41
#2  0x00007fd6be209786 in nvme_host_get_ids (r=r@entry=0x55d846da9540, hostnqn_arg=0x0, hostid_arg=<optimized out>, hostnqn=hostnqn@entry=0x7ffd51ef5a20,
    hostid=hostid@entry=0x7ffd51ef5a18) at ../src/nvme/tree.c:150
#3  0x000055d82ae43a18 in nvmf_discover (desc=0x55d82af35b80 "Discover NVMeoF subsystems and connect to them", argc=<optimized out>, argv=<optimized out>,
    connect=<optimized out>) at ../fabrics.c:763
#4  0x000055d82ae8c7a4 in handle_plugin (argc=8, argv=0x7ffd51ef85c0, plugin=0x55d82af7f820 <builtin>) at ../plugin.c:171
#5  0x000055d82ae4077c in main (argc=9, argv=0x7ffd51ef85b8) at ../nvme.c:9932
(gdb) frame 2
#2  0x00007fd6be209786 in nvme_host_get_ids (r=r@entry=0x55d846da9540, hostnqn_arg=0x0, hostid_arg=<optimized out>, hostnqn=hostnqn@entry=0x7ffd51ef5a20,
    hostid=hostid@entry=0x7ffd51ef5a18) at ../src/nvme/tree.c:150
150                             hid = strdup(nvme_host_get_hostid(h));

(gdb) info locals
nqn = 0x0
hid = <optimized out>
hnqn = 0x0
h = 0x55d846da9670

(gdb) x/gx 0x55d846da9670+0x30
0x55d846da96a0: 0x0000000000000000

[igaw]

Thanks!