usb: mon: Increase BUFF_MAX to 64 MiB to support multi-MB URBs

The usbmon binary interface currently truncates captures of large
transfers from higher-speed USB devices. Because a single event capture
is limited to one-fifth of the total buffer size, the current maximum
size of a captured URB is around 240 KiB. This is insufficient when
capturing traffic from modern devices that use transfers of several
hundred kilobytes or more, as truncated URBs can make it impossible for
user-space USB analysis tools like Wireshark to properly defragment and
reassemble higher-level protocol packets in the captured data.

The root cause of this issue is the 1200 KiB BUFF_MAX limit, which has
not been changed since the binary interface was introduced in 2006.

To resolve this issue, this patch increases BUFF_MAX to 64 MiB. The
original comment for BUFF_MAX based the limit's calculation on a
saturated 480 Mbit/s bus. Applying the same logic to a modern USB 3.2
Gen 2×2 20 Gbit/s bus (~2500 MB/s over a 20ms window) indicates the
buffer should be at least 50 MB. The new limit of 64 MiB covers that,
plus a little extra for any overhead.

With this change, both users and developers should now be able to debug
and reverse engineer modern USB devices even when running unmodified
distro kernels.

Please note that this change does not affect the default buffer size. A
larger buffer is only allocated when a user explicitly requests it via
the MON_IOCT_RING_SIZE ioctl, so the change to the maximum buffer size
should not unduly increase memory usage for users that don't
deliberately request a larger buffer.

Link: https://lore.kernel.org/CAO3ALPzdUkmMr0YMrODLeDSLZqNCkWcAP8NumuPHLjNJ8wC1kQ@mail.gmail.com
Signed-off-by: Forest Crossman <cyrozap@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://lore.kernel.org/r/CAO3ALPxU5RzcoueC454L=WZ1qGMfAcnxm+T+p+9D8O9mcrUbCQ@mail.gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions