PROBLEM: brcmfmac's DMI-based fw file names break built-in fw loader

[Victor Bravo <1905@spmblk.com>]

The brcmfmac driver seems to have partially fixed problems which
prevented it to be used in shared system/kernel images for multiple
hardware by trying to load it's <config>.txt as
<config>.<dmi_sys_vendor>.<dmi_product_name>.txt first and then
falling back to <config>.txt. Real-life example:

brcmfmac mmc1:0001:1: Direct firmware load for brcm/brcmfmac43340-sdio.ASUSTeK COMPUTER INC.-T100HAN.txt failed with
error -2
brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43340-sdio for chip
BCM43340/2

Unfortunately this doesn't really help on systems which use static
kernel with firmware blobs (and also text configuration files in case of
brcmfmac) built-in using CONFIG_EXTRA_FIRMWARE, as CONFIG_EXTRA_FIRMWARE
doesn't support spaces in file names - kernel build fails with

CONFIG_EXTRA_FIRMWARE="brcm/brcmfmac43340-sdio.bin brcm/brcmfmac43340-sdio.ASUSTeK COMPUTER INC.-T100HAN.txt"

for obvious reasons. So the only way here is to stay with good old
brcmfmac43340-sdio.txt and support at most one brcmfmac-equipped machine
per kernel image.

Please consider filtering the DMI strings and replacing spaces and
possibly other invalid characters with underscores, and/or adding module
parameter to allow passing the string from command line (using
brcmfmac.tag=t100 or brcmfmac.board=t100 to make the module load
brcmfmac43340-sdio.t100.txt seems nicer to me, and isn't prone to
breaking when DMI strings change on BIOS update).

My brief grep-based research also suggest that strings retrieved
by dmi_get_system_info() are passed to firmware loader without any
checks for special character, /../ etc. I'm not sure whether this is
considered to be proper & safe use, but if it's not, it may also have
some security implications, as it allows attacker with access to DMI
strings (using root rights/other OS/BIOS/physical access) to mess
with kernel space or secure boot.

I would also really appreciate not allowing future brcm (and other)
drivers to leave staging area before they fully support =y.

Regards,
v.b.