re: usb: gadget: configfs: OS Extended Compatibility descriptors support

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Hello Andrzej Pietrasiewicz,

The patch da4243145fb1: "usb: gadget: configfs: OS Extended
Compatibility descriptors support" from May 8, 2014, leads to the
following Smatch warning:

	drivers/usb/gadget/configfs.c:1195 interf_grp_sub_compatible_id_store()
	error: buffer overflow 'desc->ext_compat_id' 16 <= 16

drivers/usb/gadget/configfs.c
  1184  static ssize_t interf_grp_sub_compatible_id_store(struct usb_os_desc *desc,
  1185                                                    const char *page, size_t len)
  1186  {
  1187          int l;
  1188  
  1189          l = min_t(int, 8, len);

Let's assume l is 8.

  1190          if (page[l - 1] == '\n')
  1191                  --l;
  1192          if (desc->opts_mutex)
  1193                  mutex_lock(desc->opts_mutex);
  1194          memcpy(desc->ext_compat_id + 8, page, l);
  1195          desc->ext_compat_id[l + 8] = '\0';

Then we are putting the NULL terminator one space beyond the end of the
array.  ->ext_compat_id is set in rndis_alloc_inst().

This is not a false postive, but I'm not positive how we should fix it.

  1196  
  1197          if (desc->opts_mutex)
  1198                  mutex_unlock(desc->opts_mutex);
  1199  
  1200          return len;
  1201  }

regards,
dan carpenter
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