Daemon to print kernel uevents

[barryk]

I have a C program that waits for Linux kernel uevents and prints them. Uevents are messages that the kernel generates whenever something happens such as a USB stick plugged in or removed. The udevd daemon in the udev (now in systemd) package reads these messages.

Converting the C program to BaCon has been a very interesting exercise. I ran into a bug with RECORD, that Peter fixed:
basic-converter.proboards.com/index.cgi?board=bugs&action=display&thread=498

So, BaCon 2.1.7 is required to compile my program.

I ran into more problems with RECORD, that I reported here:
basic-converter.proboards.com/index.cgi?board=bugs&action=display&thread=500

...however, I was able to work around those, and my program works fine.

I was using IMPORT, until I read bigbass's excellent posts about PROTO:
basic-converter.proboards.com/index.cgi?board=doc&action=display&thread=497

...that is superb!

The "thinness" of BaCon above C, lazy translation, sure does make it easy to convert from C to BaCon. I experimented and found that putting in C syntax here and there gets through OK.

Anyway, here is my program:
REM (c) Copyright Barry Kauler, June 2013, bkhome.org
REM License GPL3 (refer: /usr/share/doc/legal)
REM libc ref: http://www.gnu.org/software/libc/manual/html_node/Function-Index.html#Function-Index
REM this little daemon reads kernel uevents.

REM from linux/netlink.h...
CONST NETLINK_KOBJECT_UEVENT=15

REM linux/poll.h, only includes asm/poll.h, which only includes asm-generic/poll.h...
CONST POLLIN=1

REM C functions, in libc.so. note, I originally used IMPORT, but PROTO is simpler...
PROTO getpid, socket, bind, poll, recv

REM struct sockaddr_nl, defined in linux/netlink.h...
RECORD nls
LOCAL nl_family TYPE unsigned short
LOCAL nl_pad TYPE unsigned short
LOCAL nl_pid TYPE unsigned int
LOCAL nl_groups TYPE unsigned int
END RECORD

REM struct pollfd, defined in asm-generic/poll.h...
RECORD pfd
LOCAL fd TYPE int
LOCAL events TYPE short
LOCAL revents TYPE short
END RECORD

DECLARE buf TYPE char ARRAY 512
DECLARE eventstatus TYPE int

size_char=SIZEOF(char)
size_buf=size_char*512
size_us=SIZEOF(unsigned short)
size_ui=SIZEOF(unsigned int)
size_nls=(2*size_us)+(2*size_ui)

REM initialise the nls structure...
nls.nl_family = AF_NETLINK
nls.nl_pad = 0
nls.nl_pid = getpid()
nls.nl_groups = -1

REM Open hotplug event netlink socket...
pfd.events = POLLIN
pfd.fd = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_KOBJECT_UEVENT)
IF pfd.fd == -1 THEN END 1

REM listen to netlink socket...
retval=bind(pfd.fd, (void *)&nls.nl_family, size_nls)
IF retval==-1 THEN END 2

WHILE TRUE DO
REM 2 second timeout... note, -1 is wait indefinitely.
eventstatus=poll(&pfd.fd, 1, 2000)
IF eventstatus==-1 THEN END 3
IF eventstatus==0 THEN
  PRINT "Two second timeout"
 CONTINUE
END IF

REM get the uevent...
len = recv(pfd.fd, buf, size_buf, MSG_DONTWAIT)
IF len==-1 THEN END 4

REM process the uevent...
REM only respond to add@ and remove@ uevents...
IF (buf[0]!='a' & buf[0]!='r') THEN CONTINUE
IF (buf[1]!='d' & buf[1]!='e') THEN CONTINUE
IF (buf[2]!='d' & buf[2]!='m') THEN CONTINUE

PRINT "Next uevent..."
buf[size_buf]=0
str$=buf
PRINT str$

WEND


After compiling, all you have to do is run it in a terminal, then plug in a USB stick. CTRL-C to kill it.

After stripping, it is 34KB, which is bigger than I expected.

[vovchik]

Dear Barry,

If you use my compile line:


bacon -o -s -o -Os -o -fdata-sections -o -ffunction-sections -o -Wl,--gc-sections "$myfile"


you'll end up with a 9_k prog. If you UPX, it will be about 6.5k

With kind regards,
vovchik

[Pjot]

Nice! Works perfect on my 64bit Linux Mint.

Thanks!
Peter

[barryk]

Correction, I didn't realise, when I was converting from C, that 'buf' actually gets filled with a sequence of zero-delimited strings, and variable 'len' is the total length.

Modify my program like this:

PRINT "Next uevent..."
'buf[size_buf]=0
'str$=buf
'PRINT str$
REM it seems that buf has a sequence of zero-delimited strings, print on one line...
i=0
WHILE i<len DO
 printf("%s ", buf+i);
 i = i+strlen(buf+i)+1;
WEND
printf("\n");


...look, I have been lazy, created PROTO for printf and strlen, and just put the C code straight in. This could get to be a very bad habit!

Furthermore, I accidentally left the semicolons in, and it still compiled!

Thanks for the info on making the executable smaller!

[bigbass]

Hey Mr.barryk !

I really like your excellent example code for a kernel-uevent
this is very useful and quite complex getting those low level signals
congratulations!

Thank you for posting the code

I included it to a collection of code examples for BaCon users
sourceforge.net/p/baconproto/wiki/kernel-uevents/

Joe