net: Fix uninitialized data usage

e.g.
1109 15:16:20.151506 Uninitialized bytes in ioctl_common_pre at offset 0 inside [0x7ffc516af9b8, 4)
 1109 15:16:20.151659 ==588974==WARNING: MemorySanitizer: use-of-uninitialized-value
 1109 15:16:20.312923     #0 0x5639b88acb21 in tap_probe_vnet_hdr_len third_party/qemu/net/tap-linux.c:183:9
 1109 15:16:20.312952     #1 0x5639b88afd66 in net_tap_fd_init third_party/qemu/net/tap.c:409:9
 1109 15:16:20.312954     #2 0x5639b88b2d1b in net_init_tap_one third_party/qemu/net/tap.c:681:19
 1109 15:16:20.312956     #3 0x5639b88b16a8 in net_init_tap third_party/qemu/net/tap.c:912:13
 1109 15:16:20.312957     #4 0x5639b8890175 in net_client_init1 third_party/qemu/net/net.c:1110:9
 1109 15:16:20.312958     #5 0x5639b888f912 in net_client_init third_party/qemu/net/net.c:1208:15
 1109 15:16:20.312960     #6 0x5639b8894aa5 in net_param_nic third_party/qemu/net/net.c:1588:11
 1109 15:16:20.312961     #7 0x5639b900cd18 in qemu_opts_foreach third_party/qemu/util/qemu-option.c:1135:14
 1109 15:16:20.312962     #8 0x5639b889393c in net_init_clients third_party/qemu/net/net.c:1612:9
 1109 15:16:20.312964     #9 0x5639b717aaf3 in qemu_create_late_backends third_party/qemu/softmmu/vl.c:1962:5
 1109 15:16:20.312965     #10 0x5639b717aaf3 in qemu_init third_party/qemu/softmmu/vl.c:3694:5
 1109 15:16:20.312967     #11 0x5639b71083b8 in main third_party/qemu/softmmu/main.c:49:5
 1109 15:16:20.312968     #12 0x7f464de1d8d2 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x628d2)
 1109 15:16:20.312969     #13 0x5639b6bbd389 in _start /usr/grte/v5/debug-src/src/csu/../sysdeps/x86_64/start.S:120
 1109 15:16:20.312970
 1109 15:16:20.312975   Uninitialized value was stored to memory at
 1109 15:16:20.313393     #0 0x5639b88acbee in tap_probe_vnet_hdr_len third_party/qemu/net/tap-linux.c
 1109 15:16:20.313396     #1 0x5639b88afd66 in net_tap_fd_init third_party/qemu/net/tap.c:409:9
 1109 15:16:20.313398     #2 0x5639b88b2d1b in net_init_tap_one third_party/qemu/net/tap.c:681:19
 1109 15:16:20.313399     #3 0x5639b88b16a8 in net_init_tap third_party/qemu/net/tap.c:912:13
 1109 15:16:20.313400     #4 0x5639b8890175 in net_client_init1 third_party/qemu/net/net.c:1110:9
 1109 15:16:20.313401     #5 0x5639b888f912 in net_client_init third_party/qemu/net/net.c:1208:15
 1109 15:16:20.313403     #6 0x5639b8894aa5 in net_param_nic third_party/qemu/net/net.c:1588:11
 1109 15:16:20.313404     #7 0x5639b900cd18 in qemu_opts_foreach third_party/qemu/util/qemu-option.c:1135:14
 1109 15:16:20.313405     #8 0x5639b889393c in net_init_clients third_party/qemu/net/net.c:1612:9
 1109 15:16:20.313407     #9 0x5639b717aaf3 in qemu_create_late_backends third_party/qemu/softmmu/vl.c:1962:5
 1109 15:16:20.313408     #10 0x5639b717aaf3 in qemu_init third_party/qemu/softmmu/vl.c:3694:5
 1109 15:16:20.313409     #11 0x5639b71083b8 in main third_party/qemu/softmmu/main.c:49:5
 1109 15:16:20.313410     #12 0x7f464de1d8d2 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x628d2)
 1109 15:16:20.313412     #13 0x5639b6bbd389 in _start /usr/grte/v5/debug-src/src/csu/../sysdeps/x86_64/start.S:120
 1109 15:16:20.313413
 1109 15:16:20.313417   Uninitialized value was stored to memory at
 1109 15:16:20.313791     #0 0x5639b88affbd in net_tap_fd_init third_party/qemu/net/tap.c:400:26
 1109 15:16:20.313826     #1 0x5639b88b2d1b in net_init_tap_one third_party/qemu/net/tap.c:681:19
 1109 15:16:20.313829     #2 0x5639b88b16a8 in net_init_tap third_party/qemu/net/tap.c:912:13
 1109 15:16:20.313831     #3 0x5639b8890175 in net_client_init1 third_party/qemu/net/net.c:1110:9
 1109 15:16:20.313836     #4 0x5639b888f912 in net_client_init third_party/qemu/net/net.c:1208:15
 1109 15:16:20.313838     #5 0x5639b8894aa5 in net_param_nic third_party/qemu/net/net.c:1588:11
 1109 15:16:20.313839     #6 0x5639b900cd18 in qemu_opts_foreach third_party/qemu/util/qemu-option.c:1135:14
 1109 15:16:20.313841     #7 0x5639b889393c in net_init_clients third_party/qemu/net/net.c:1612:9
 1109 15:16:20.313843     #8 0x5639b717aaf3 in qemu_create_late_backends third_party/qemu/softmmu/vl.c:1962:5
 1109 15:16:20.313844     #9 0x5639b717aaf3 in qemu_init third_party/qemu/softmmu/vl.c:3694:5
 1109 15:16:20.313845     #10 0x5639b71083b8 in main third_party/qemu/softmmu/main.c:49:5
 1109 15:16:20.313846     #11 0x7f464de1d8d2 in __libc_start_main (/usr/grte/v5/lib64/libc.so.6+0x628d2)
 1109 15:16:20.313847     #12 0x5639b6bbd389 in _start /usr/grte/v5/debug-src/src/csu/../sysdeps/x86_64/start.S:120
 1109 15:16:20.313849
 1109 15:16:20.313851   Uninitialized value was created by an allocation of 'ifr' in the stack frame of function 'tap_probe_vnet_hdr'
 1109 15:16:20.313855     #0 0x5639b88ac680 in tap_probe_vnet_hdr third_party/qemu/net/tap-linux.c:151
 1109 15:16:20.313856
 1109 15:16:20.313878 SUMMARY: MemorySanitizer: use-of-uninitialized-value third_party/qemu/net/tap-linux.c:183:9 in tap_probe_vnet_hdr_len

Fixes: dc69004c7d ("net: move tap_probe_vnet_hdr() to tap-linux.c")
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Foley <pefoley@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/tap-linux.c

@@ -150,6 +150,7 @@ void tap_set_sndbuf(int fd, const NetdevTapOptions *tap, Error **errp)
 int tap_probe_vnet_hdr(int fd, Error **errp)
 {
     struct ifreq ifr;
+    memset(&ifr, 0, sizeof(ifr));
 
     if (ioctl(fd, TUNGETIFF, &ifr) != 0) {
         /* TUNGETIFF is available since kernel v2.6.27 */