我最终也弄明白了这一点，所以这就是我必须做的事情（基于tools/usb/ffs-test. c）。
一个用户空间函数必须将所有描述符写入ep 0，所以诀窍是扩展描述符struct：

• 在ss_count之后添加__le32 os_count
• 在ss_descs后添加struct usb_os_desc_header os_header和struct usb_ext_compat_desc os_desc
• FUNCTIONFS_HAS_MS_OS_DESC需要添加到header.flags

下面是我如何初始化WinUSB兼容性的新字段，我假设WebUSB一般工作类似：

.os_count = cpu_to_le32(1),
    .os_header = {
        .interface = cpu_to_le32(1),
        .dwLength = cpu_to_le32(sizeof(descriptors.os_header) + sizeof(descriptors.os_desc)),
        .bcdVersion = cpu_to_le32(1),
        .wIndex = cpu_to_le32(4),
        .bCount = cpu_to_le32(1),
        .Reserved = cpu_to_le32(0),
    },
    .os_desc = {
        .bFirstInterfaceNumber = 0,
        .Reserved1 = cpu_to_le32(1),
        .CompatibleID = {'W', 'I', 'N', 'U', 'S', 'B', 0, 0},
        .SubCompatibleID = {0},
        .Reserved2 = {0},
    }

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