memset() to 0 interfaces array before reusing
usb_configuration structure.

This commit fix bug:

ln -s functions/acm.1 configs/c.1
ln -s functions/acm.2 configs/c.1
ln -s functions/acm.3 configs/c.1
echo "UDC name" > UDC
echo "" > UDC
rm configs/c.1/acm.*
rmdir functions/*
mkdir functions/ecm.usb0
ln -s functions/ecm.usb0 configs/c.1
echo "UDC name" > UDC

[   82.220969] Unable to handle kernel NULL pointer dereference at virtual address 00000000
[   82.229009] pgd = c0004000
[   82.231698] [00000000] *pgd=00000000
[   82.235260] Internal error: Oops: 17 [#1] PREEMPT SMP ARM
[   82.240638] Modules linked in:
[   82.243681] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.0.0-rc2 #39
[   82.249926] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[   82.256003] task: c07cd2f0 ti: c07c8000 task.ti: c07c8000
[   82.261393] PC is at composite_setup+0xe3c/0x1674
[   82.266073] LR is at composite_setup+0xf20/0x1674
[   82.270760] pc : [<c03510d4>]    lr : [<c03511b8>]    psr: 600001d3
[   82.270760] sp : c07c9df0  ip : c0806448  fp : ed8c9c9c
[   82.282216] r10: 00000001  r9 : 00000000  r8 : edaae918
[   82.287425] r7 : ed551cc0  r6 : 00007fff  r5 : 00000000  r4 : ed799634
[   82.293934] r3 : 00000003  r2 : 00010002  r1 : edaae918  r0 : 0000002e
[   82.300446] Flags: nZCv  IRQs off  FIQs off  Mode SVC_32  ISA ARM  Segment kernel
[   82.307910] Control: 10c5387d  Table: 6bc1804a  DAC: 00000015
[   82.313638] Process swapper/0 (pid: 0, stack limit = 0xc07c8210)
[   82.319627] Stack: (0xc07c9df0 to 0xc07ca000)
[   82.323969] 9de0:                                     00000000 c06e65f4 00000000 c07c9f68
[   82.332130] 9e00: 00000067 c07c59ac 000003f7 edaae918 ed8c9c98 ed799690 eca2f140 200001d3
[   82.340289] 9e20: ee79a2d8 c07c9e88 c07c5304 ffff55db 00010002 edaae810 edaae860 eda96d50
[   82.348448] 9e40: 00000009 ee264510 00000007 c07ca444 edaae860 c0340890 c0827a40 ffff55e0
[   82.356607] 9e60: c0827a40 eda96e40 ee264510 edaae810 00000000 edaae860 00000007 c07ca444
[   82.364766] 9e80: edaae860 c0354170 c03407dc c033db4c edaae810 00000000 00000000 00000010
[   82.372925] 9ea0: 00000032 c0341670 00000000 00000000 00000001 eda96e00 00000000 00000000
[   82.381084] 9ec0: 00000000 00000032 c0803a23 ee1aa840 00000001 c005d54c 249e2450 00000000
[   82.389244] 9ee0: 200001d3 ee1aa840 ee1aa8a0 ed84f4c0 00000000 c07c9f68 00000067 c07c59ac
[   82.397403] 9f00: 00000000 c005d688 ee1aa840 ee1aa8a0 c07db4b4 c006009c 00000032 00000000
[   82.405562] 9f20: 00000001 c005ce20 c07c59ac c005cf34 f002000c c07ca780 c07c9f68 00000057
[   82.413722] 9f40: f0020000 413fc090 00000001 c00086b4 c000f804 60000053 ffffffff c07c9f9c
[   82.421880] 9f60: c0803a20 c0011fc0 00000000 00000000 c07c9fb8 c001bee0 c07ca4f0 c057004c
[   82.430040] 9f80: c07ca4fc c0803a20 c0803a20 413fc090 00000001 00000000 01000000 c07c9fb0
[   82.438199] 9fa0: c000f800 c000f804 60000053 ffffffff 00000000 c0050e70 c0803bc0 c0783bd8
[   82.446358] 9fc0: ffffffff ffffffff c0783664 00000000 00000000 c07b13e8 00000000 c0803e54
[   82.454517] 9fe0: c07ca480 c07b13e4 c07ce40c 4000406a 00000000 40008074 00000000 00000000
[   82.462689] [<c03510d4>] (composite_setup) from [<c0340890>] (s3c_hsotg_complete_setup+0xb4/0x418)
[   82.471626] [<c0340890>] (s3c_hsotg_complete_setup) from [<c0354170>] (usb_gadget_giveback_request+0xc/0x10)
[   82.481429] [<c0354170>] (usb_gadget_giveback_request) from [<c033db4c>] (s3c_hsotg_complete_request+0xcc/0x12c)
[   82.491583] [<c033db4c>] (s3c_hsotg_complete_request) from [<c0341670>] (s3c_hsotg_irq+0x4fc/0x558)
[   82.500614] [<c0341670>] (s3c_hsotg_irq) from [<c005d54c>] (handle_irq_event_percpu+0x50/0x150)
[   82.509291] [<c005d54c>] (handle_irq_event_percpu) from [<c005d688>] (handle_irq_event+0x3c/0x5c)
[   82.518145] [<c005d688>] (handle_irq_event) from [<c006009c>] (handle_fasteoi_irq+0xd4/0x18c)
[   82.526650] [<c006009c>] (handle_fasteoi_irq) from [<c005ce20>] (generic_handle_irq+0x20/0x30)
[   82.535242] [<c005ce20>] (generic_handle_irq) from [<c005cf34>] (__handle_domain_irq+0x6c/0xdc)
[   82.543923] [<c005cf34>] (__handle_domain_irq) from [<c00086b4>] (gic_handle_irq+0x2c/0x6c)
[   82.552256] [<c00086b4>] (gic_handle_irq) from [<c0011fc0>] (__irq_svc+0x40/0x74)
[   82.559716] Exception stack(0xc07c9f68 to 0xc07c9fb0)
[   82.564753] 9f60:                   00000000 00000000 c07c9fb8 c001bee0 c07ca4f0 c057004c
[   82.572913] 9f80: c07ca4fc c0803a20 c0803a20 413fc090 00000001 00000000 01000000 c07c9fb0
[   82.581069] 9fa0: c000f800 c000f804 60000053 ffffffff
[   82.586113] [<c0011fc0>] (__irq_svc) from [<c000f804>] (arch_cpu_idle+0x30/0x3c)
[   82.593491] [<c000f804>] (arch_cpu_idle) from [<c0050e70>] (cpu_startup_entry+0x128/0x1a4)
[   82.601740] [<c0050e70>] (cpu_startup_entry) from [<c0783bd8>] (start_kernel+0x350/0x3bc)
[   82.609890] Code: 0a000002 e3530005 05975010 15975008 (e5953000)
[   82.615965] ---[ end trace f57d5f599a5f1bfa ]---

Most of kernel code assume that interface array in
struct usb_configuration is NULL terminated.

When gadget is composed with configfs configuration
structure may be reused for different functions set.

This bug happens because purge_configs_funcs() sets
only next_interface_id to 0. Interface array still
contains pointers to already freed interfaces. If in
second try we add less interfaces than earlier we
may access unallocated memory when trying to get
interface descriptors.
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Cc: <stable@vger.kernel.org> # 3.10+
Signed-off-by: Felipe Balbi <balbi@ti.com>