getting false SIGTRAP breakpoints in kernel i.e. kernel hung unlessgdb remotely attached on x86 & cont is issued

[Denis Joseph Barrow]

Hi ladies/gentlemen,
The kernel I'm running gdb with is 2.6.27-rc4
The false sigtrap is occuring in ia32_sysenter_target in arch/x86/kernel/entry_32.S:303
when gdb is stepped from the user process as described below

To reproduce
compile kernel with kgdb support 
compile my randsleep program attached using the .mk script
as root
attach randsleep to an idle serial port e.g. /dev/ttyS0 by typing
randsleep /dev/ttyS0
from another bash shell type
ps -aux | grep randsleep

gdb ./randsleep
attach <pid of randsleep>

You should get messages from gdb like
Attaching to program: /home/djbarrow/devel2/randsleep/randsleep, process 6397
Reading symbols from /lib/tls/i686/cmov/libc.so.6...done.
Loaded symbols for /lib/tls/i686/cmov/libc.so.6
Reading symbols from /lib/ld-linux.so.2...done.
Loaded symbols for /lib/ld-linux.so.2
0xb7fda430 in __kernel_vsyscall ()

Now type step.

The machine is now hung until gdb attaches remotely.


-- 
best regards,
D.J. Barrow
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <time.h>  
#include <signal.h>
#include <unistd.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>

#define READLEN (128)
void sigalrm(int signal)
{
	fprintf(stderr,"atcmd_response_read timed out.\n");
	exit(-1);
}
void atcmd_response_read(int fd)
{
	int i,count;
	char tmpbuf;
	//alarm(5);
	for (i=0;;i++) {
		count=read(fd,&tmpbuf,1);
		if (count < 0) {
			perror("read failed\n");
			exit(-1);
		}

		if (count == 1 )
		{
			printf("%c",tmpbuf);
			if( i > 2&& (tmpbuf==0xd || tmpbuf==0xa))
				break;
		}
	}
	fflush(stdout);
	alarm(0);
}

void atcmd_write(int fd, char *buf)
{
	ssize_t written=0,retval;
	size_t count=strlen(buf);
	
	while (written < count) {
		retval=write(fd, &buf[written], 1);
		if (retval < 0) {
			perror("write failed\n");
			exit(-1);
		}
		if(retval==1)
			printf("%c",buf[written]);
		written+=retval;
	}
}


double getrand()
{
	int rnd;
	double randfloat;
	rnd=rand();
	return (double)rnd/(double)RAND_MAX;

	
}
int main(int argc,char *argv[])
{
	int fd;
	int count;
	char *devname;
	char tmpbuff[READLEN];
	int  i;
	if (argc != 2) {
		fprintf(stderr,"Usage:\n"
			"randsleep <hso application port>\n");
		return -1;
	}
	devname=argv[1];

	fd=open(devname,O_RDWR);
	if (fd<0) {
		perror("open failed");
		return -1;
	}
	signal(SIGALRM,sigalrm);
	atcmd_write(fd,"ATE0\r\n");
	atcmd_response_read(fd);
	for(i=0;;i++)
	{
		printf("\nLoop %d\n",i);
	        
		atcmd_write(fd,"AT\r\n");
		usleep(getrand()*(double)4000000);
		atcmd_response_read(fd);
		usleep(getrand()*(double)4000000);
	}

}
gcc randsleep.c -g -o randsleep