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Adding custom ("Hello") system call: help

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# 1  
Old 01-09-2011
Adding custom ("Hello") system call: help
I'm trying to add a custom ("Hello world" Smilie) system call.
In /usr/src/linux/hello/ I put simple hello.c ...
Code:
#include "linux/linkage.h" // for linking a system call
#include "linux/kernel.h" // for "printk"

asmlinkage int sys_hello() 
{
	printk(KERN_ALERT "Hello!");
	return 1;
}

... and in home directory helloTest.c:
Code:
#include <stdio.h>
#include <sys/syscall.h>
#include <linux/unistd.h>

#define __NR_hello	338

int main()
{
	syscall(__NR_hello);
	return 0;
}

I compiled helloTest.c and ran ./a.out: in /var/log/messages nothing gets written! Smilie

Before "compile & install & reboot" I did these steps (linux is symbolic link to linux-2.6.35.10):
  1. In /usr/src/linux/arch/x86/include/asm/unistd_32.h, I've added/changed:
    Code:
    #define __NR_hello	 338
#define __NR_syscalls	 339

  2. In /usr/src/linux/arch/x86/kernel/syscall_table.S, I've added (on the last line):
    Code:
    .long sys_hello

  3. In /usr/src/linux/hello, I've created a Makefile:
    Code:
    obj-y += hello.o

    ... and in /usr/src/linux, I've modified Makefile and added hello/ directory:
    Code:
    core-y += kernel/ mm/ fs/ ipc/ security/ crypto/ block/ hello/

  4. In /usr/src/linux/include/linux/syscalls.h, I've added (on the last line):
    Code:
    asmlinkage long sys_hello(void)

Last edited by courteous; 01-09-2011 at 04:41 PM..
# 2  
Old 01-10-2011
check if 'hello' actually ended up in System.map

And just to rule out the obvious you did reboot into your new kernel yes?
This User Gave Thanks to Corona688 For This Post:
courteous
# 3  
Old 01-13-2011
"Hello!" IS in dmesg but NOT in /var/log/messages
Yes, 'hello' is in System.map (in /boot directory):
Code:
c015c3c0 T sys_hello

Yes, I did reboot (into the new kernel). Smilie

As 'hello' is in System.map, does that mean that something is wrong with the test function helloTest.c? Smilie

EDIT: dmesg* does print "Hello!" on the last line, but there is nothing in /var/log/messages file.


* dmesg - prints or controls the kernel ring buffer


EDIT2: I've compared output of dmesg and tail -f /var/log/messages commands: equal lines are in bold red, Hello! is the last line of dmesg.
Code:
blaise@blaise-E520 ~/Documents/seminar $ dmesg
[ 5364.127763] [... snipped... ]
[ 5379.250373] usb 2-3: USB disconnect, address 4
[10619.880053] usb 2-3: new high speed USB device using ehci_hcd and address 5
[10620.016622] scsi6 : usb-storage 2-3:1.0
[10621.889492] scsi 6:0:0:0: Direct-Access     USB      FLASH DRIVE      1100 PQ: 0 ANSI: 0 CCS
[10621.891282] sd 6:0:0:0: Attached scsi generic sg2 type 0
[10621.897073] sd 6:0:0:0: [sdb] 7831552 512-byte logical blocks: (4.00 GB/3.73 GiB)
[10621.897837] sd 6:0:0:0: [sdb] Write Protect is off
[10621.897841] sd 6:0:0:0: [sdb] Mode Sense: 43 00 00 00
[10621.897844] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[10621.901956] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[10621.901964]  sdb: sdb1
[10621.905319] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[10621.905324] sd 6:0:0:0: [sdb] Attached SCSI removable disk
[10645.191905] usb 2-3: USB disconnect, address 5
[11121.593000] Hello!
blaise@blaise-E520 ~/Documents/seminar $ tail -f /var/log/messages
Jan 13 14:56:59 blaise-E520 sudo: pam_sm_authenticate: username = [blaise]
Jan 13 15:15:04 blaise-E520 kernel: [10619.880053] usb 2-3: new high speed USB device using ehci_hcd and address 5
Jan 13 15:15:04 blaise-E520 kernel: [10620.016622] scsi6 : usb-storage 2-3:1.0
Jan 13 15:15:06 blaise-E520 kernel: [10621.889492] scsi 6:0:0:0: Direct-Access     USB      FLASH DRIVE      1100 PQ: 0 ANSI: 0 CCS
Jan 13 15:15:06 blaise-E520 kernel: [10621.891282] sd 6:0:0:0: Attached scsi generic sg2 type 0
Jan 13 15:15:06 blaise-E520 kernel: [10621.897073] sd 6:0:0:0: [sdb] 7831552 512-byte logical blocks: (4.00 GB/3.73 GiB)
Jan 13 15:15:06 blaise-E520 kernel: [10621.897837] sd 6:0:0:0: [sdb] Write Protect is off
Jan 13 15:15:06 blaise-E520 kernel: [10621.901964]  sdb: sdb1
Jan 13 15:15:06 blaise-E520 kernel: [10621.905324] sd 6:0:0:0: [sdb] Attached SCSI removable disk
Jan 13 15:15:29 blaise-E520 kernel: [10645.191905] usb 2-3: USB disconnect, address 5

Why isn't Hello! also printed in /var/log/messages? Smilie
Last edited by courteous; 01-13-2011 at 10:40 AM..
# 4  
Old 01-13-2011
What ends up in /var/log/messages is controlled in software by your system logger. It may or may not be syslog-ng. dmesg does end up there in my systems but that's by no means mandatory!

You seem to be getting some parts of dmesg in messages, but not your 'hello', possibly because it's just an 'alert' and not anything critical. the system logger is able to tell the different levels of criticality apart.

So, it's working. Smilie All the really tough stuff actually went as planned. Be happy.
# 5  
Old 01-13-2011
Quote:
Originally Posted by Corona688
You seem to be getting some parts of dmesg in messages, but not your 'hello', possibly because it's just an 'alert' and not anything critical. the system logger is able to tell the different levels of criticality apart.
But KERN_ALERT is on a higher, 2nd, log-level than is KERN_CRIT! Smilie

Snippet from kernel.h:
Code:
#define	KERN_EMERG	"<0>"	/* system is unusable			*/
#define	KERN_ALERT	"<1>"	/* action must be taken immediately	*/
#define	KERN_CRIT	"<2>"	/* critical conditions			*/
#define	KERN_ERR	"<3>"	/* error conditions			*/
#define	KERN_WARNING	"<4>"	/* warning conditions			*/
#define	KERN_NOTICE	"<5>"	/* normal but significant condition	*/
#define	KERN_INFO	"<6>"	/* informational			*/
#define	KERN_DEBUG	"<7>"	/* debug-level messages		        */

# 6  
Old 01-13-2011
I'm afraid I don't know then Smilie
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