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lsusb: Add -F flag to read descriptors from a file
Add a new flag that allows dumping a USB descriptor file in a human-readable format. This is particularly useful for analyzing descriptor dumps collected from systems where lsusb cannot be run directly, such as during hardware bring-up or on non-Linux platforms. The descriptor file is typically obtained from: /sys/bus/usb/devices/[device-bus-id]/descriptors The normal workflow will be something like: HOST # scp target:/sys/bus/usb/devices/X-X/descriptors . HOST # lsusb -F descriptors Signed-off-by: Ricardo Ribalda <ribalda@google.com>
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Lines changed: 448 additions & 50 deletions

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‎desc-file.c‎

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// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* USB descriptor file parsing
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*
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* Copyright (C) 2026 Google LLC
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*/
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#include <stdlib.h>
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#include <string.h>
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#include <stdio.h>
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#include <unistd.h>
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#include <libusb.h>
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#include "desc-file.h"
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/*
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* parse_config_descriptor - Parse a configuration descriptor from a buffer.
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* @buf: The buffer containing the configuration descriptor.
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* @size: The size of the buffer.
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*
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* This function parses a raw configuration descriptor from a buffer and
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* populates a libusb_config_descriptor structure.
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*
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* Returns a pointer to the allocated configuration descriptor, or NULL on failure.
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*/
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static struct libusb_config_descriptor *parse_config_descriptor(const unsigned char *buf, int size)
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{
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struct libusb_config_descriptor *cfg;
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struct libusb_interface *ifc = NULL;
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struct libusb_interface_descriptor *alt = NULL;
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struct libusb_endpoint_descriptor *ep = NULL;
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if (size < 9 || buf[1] != LIBUSB_DT_CONFIG)
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return NULL;
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cfg = calloc(1, sizeof(*cfg));
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if (!cfg) {
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perror("calloc");
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return NULL;
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}
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cfg->bLength = buf[0];
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cfg->bDescriptorType = buf[1];
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cfg->wTotalLength = buf[2] | (buf[3] << 8);
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cfg->bNumInterfaces = buf[4];
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cfg->bConfigurationValue = buf[5];
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cfg->iConfiguration = buf[6];
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cfg->bmAttributes = buf[7];
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cfg->MaxPower = buf[8];
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if (cfg->bNumInterfaces > 0) {
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cfg->interface = calloc(cfg->bNumInterfaces, sizeof(*cfg->interface));
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if (!cfg->interface) {
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perror("calloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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}
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buf += cfg->bLength;
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size -= cfg->bLength;
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while (size >= 2) {
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unsigned char len = buf[0];
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unsigned char type = buf[1];
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void *tmp;
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int ep_idx;
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if (len > size || len < 2)
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break;
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switch (type) {
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case LIBUSB_DT_INTERFACE:
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if (buf[2] >= cfg->bNumInterfaces)
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break;
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ifc = (struct libusb_interface *)&cfg->interface[buf[2]];
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ifc->num_altsetting++;
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tmp = realloc((void *)ifc->altsetting, ifc->num_altsetting * sizeof(*ifc->altsetting));
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if (!tmp) {
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perror("realloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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ifc->altsetting = tmp;
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alt = (struct libusb_interface_descriptor *)&ifc->altsetting[ifc->num_altsetting - 1];
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memset(alt, 0, sizeof(*alt));
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alt->bLength = buf[0];
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alt->bDescriptorType = buf[1];
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alt->bInterfaceNumber = buf[2];
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alt->bAlternateSetting = buf[3];
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alt->bNumEndpoints = buf[4];
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alt->bInterfaceClass = buf[5];
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alt->bInterfaceSubClass = buf[6];
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alt->bInterfaceProtocol = buf[7];
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alt->iInterface = buf[8];
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if (alt->bNumEndpoints > 0) {
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alt->endpoint = calloc(alt->bNumEndpoints, sizeof(*alt->endpoint));
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if (!alt->endpoint) {
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perror("calloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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}
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ep = NULL;
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break;
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case LIBUSB_DT_ENDPOINT:
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if (!(alt && alt->endpoint))
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break;
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ep_idx = 0;
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/* Find next free endpoint slot */
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while (ep_idx < alt->bNumEndpoints && alt->endpoint[ep_idx].bLength)
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ep_idx++;
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if (ep_idx < alt->bNumEndpoints) {
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ep = (struct libusb_endpoint_descriptor *)&alt->endpoint[ep_idx];
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ep->bLength = buf[0];
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ep->bDescriptorType = buf[1];
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ep->bEndpointAddress = buf[2];
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ep->bmAttributes = buf[3];
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ep->wMaxPacketSize = buf[4] | (buf[5] << 8);
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ep->bInterval = buf[6];
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ep->bRefresh = (len > 7) ? buf[7] : 0;
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ep->bSynchAddress = (len > 8) ? buf[8] : 0;
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}
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break;
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default:
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/* Extra descriptors */
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if (ep) {
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tmp = realloc((void *)ep->extra, ep->extra_length + len);
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if (!tmp) {
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perror("realloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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ep->extra = tmp;
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memcpy((void *)(ep->extra + ep->extra_length), buf, len);
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ep->extra_length += len;
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} else if (alt) {
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tmp = realloc((void *)alt->extra, alt->extra_length + len);
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if (!tmp) {
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perror("realloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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alt->extra = tmp;
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memcpy((void *)(alt->extra + alt->extra_length), buf, len);
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alt->extra_length += len;
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} else if (cfg) {
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tmp = realloc((void *)cfg->extra, cfg->extra_length + len);
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if (!tmp) {
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perror("realloc");
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desc_file_free_config_descriptor(cfg);
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return NULL;
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}
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cfg->extra = tmp;
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memcpy((void *)(cfg->extra + cfg->extra_length), buf, len);
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cfg->extra_length += len;
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}
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break;
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}
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buf += len;
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size -= len;
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}
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return cfg;
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}
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/*
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* desc_file_free_config_descriptor - Free a configuration descriptor.
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* @cfg: The configuration descriptor to free.
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*/
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void desc_file_free_config_descriptor(struct libusb_config_descriptor *cfg)
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{
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if (!cfg)
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return;
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for (int i = 0; i < cfg->bNumInterfaces; i++) {
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struct libusb_interface *ifc = (struct libusb_interface *)&cfg->interface[i];
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if (!ifc)
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continue;
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for (int j = 0; j < ifc->num_altsetting; j++) {
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struct libusb_interface_descriptor *alt =
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(struct libusb_interface_descriptor *)&ifc->altsetting[j];
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if (!alt)
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continue;
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for (int k = 0; k < alt->bNumEndpoints; k++) {
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struct libusb_endpoint_descriptor *ep =
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(struct libusb_endpoint_descriptor *)&alt->endpoint[k];
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if (!ep)
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continue;
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if (ep->extra)
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free((void *)ep->extra);
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}
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if (alt->endpoint)
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free((void *)alt->endpoint);
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if (alt->extra)
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free((void *)alt->extra);
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}
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if (ifc->altsetting)
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free((void *)ifc->altsetting);
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}
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if (cfg->interface)
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free((void *)cfg->interface);
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if (cfg->extra)
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free((void *)cfg->extra);
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free(cfg);
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}
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/*
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* desc_file_get_device_descriptor - Get a device descriptor from a file.
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* @fd: The file descriptor to read from.
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* @desc: The device descriptor to populate.
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*
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* Returns 0 on success, or -1 on failure.
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*/
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int desc_file_get_device_descriptor(int fd, struct libusb_device_descriptor *desc)
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{
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unsigned char buf[18];
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int n;
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n = read(fd, buf, sizeof(buf));
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if (n < (int)sizeof(buf)) {
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fprintf(stderr, "File too short for device descriptor\n");
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return -1;
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}
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/* Populate device descriptor */
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desc->bLength = buf[0];
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desc->bDescriptorType = buf[1];
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desc->bcdUSB = buf[2] | (buf[3] << 8);
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desc->bDeviceClass = buf[4];
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desc->bDeviceSubClass = buf[5];
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desc->bDeviceProtocol = buf[6];
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desc->bMaxPacketSize0 = buf[7];
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desc->idVendor = buf[8] | (buf[9] << 8);
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desc->idProduct = buf[10] | (buf[11] << 8);
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desc->bcdDevice = buf[12] | (buf[13] << 8);
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desc->iManufacturer = buf[14];
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desc->iProduct = buf[15];
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desc->iSerialNumber = buf[16];
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desc->bNumConfigurations = buf[17];
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return 0;
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}
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/*
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* desc_file_get_next_config_descriptor - Get the next config descriptor from a file.
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* @fd: The file descriptor to read from.
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*
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* Returns a pointer to the allocated configuration descriptor, or NULL on failure.
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*/
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struct libusb_config_descriptor *desc_file_get_next_config_descriptor(int fd)
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{
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unsigned char config_header[9];
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struct libusb_config_descriptor *config;
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unsigned char *config_buf;
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uint16_t total_length;
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ssize_t n;
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n = read(fd, config_header, sizeof(config_header));
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if (n < (ssize_t)sizeof(config_header)) {
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if (n > 0)
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fprintf(stderr, "File too short for config header\n");
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return NULL;
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}
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total_length = config_header[2] | (config_header[3] << 8);
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config_buf = malloc(total_length);
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if (!config_buf) {
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perror("malloc");
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return NULL;
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}
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memcpy(config_buf, config_header, sizeof(config_header));
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if (total_length > sizeof(config_header)) {
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n = read(fd, config_buf + sizeof(config_header), total_length - sizeof(config_header));
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if (n < (ssize_t)(total_length - sizeof(config_header))) {
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fprintf(stderr, "File too short for config\n");
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free(config_buf);
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return NULL;
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}
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}
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config = parse_config_descriptor(config_buf, total_length);
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if (!config) {
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fprintf(stderr, "Failed to parse config descriptor\n");
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free(config_buf);
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return NULL;
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}
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free(config_buf);
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return config;
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}

‎desc-file.h‎

Lines changed: 42 additions & 0 deletions
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// SPDX-License-Identifier: GPL-2.0-or-later
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/*
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* USB descriptor file parsing
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*
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* Copyright (C) 2026 Google LLC
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*/
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#ifndef _DESC_FILE_H
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#define _DESC_FILE_H
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#include <libusb.h>
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/**
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* desc_file_free_config_descriptor:
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* @cfg: The configuration descriptor to free.
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*
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* This function frees a configuration descriptor and all its sub-descriptors.
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*/
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void desc_file_free_config_descriptor(struct libusb_config_descriptor *cfg);
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/**
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* desc_file_get_device_descriptor:
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* @fd: File descriptor to read from.
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* @desc: Pointer to a device descriptor to populate.
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*
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* Reads a USB device descriptor from a file descriptor.
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*
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* Returns 0 on success, or -1 on failure.
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*/
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int desc_file_get_device_descriptor(int fd, struct libusb_device_descriptor *desc);
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/**
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* desc_file_get_next_config_descriptor:
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* @fd: File descriptor to read from.
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*
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* Reads the next configuration descriptor from a file descriptor.
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*
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* Returns a pointer to the allocated configuration descriptor, or NULL on failure.
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*/
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struct libusb_config_descriptor *desc_file_get_next_config_descriptor(int fd);
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#endif /* _DESC_FILE_H */

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