Re: core files

Todd Larason (jtl@molehill.org)
Thu, 31 Dec 1998 14:05:17 -0800

On 981230, Michael Elizabeth Chastain wrote:
> You could do a lot of *this* in user space by having a library function
> set the default value for all signals, or just SIGSEGV, to a function
> that uses some kind of IPC, such as writing to a fifo whose name includes
> the uid. I wonder if it's possible to use preloading to inject that
> snippet of code into an existing executable without relinking it and
> without touching libc.

Yes. The only hard part I see is getting the executable filename.
core.c:

---
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>


void init_libcore(void) __attribute__((constructor));


void
sighandler(int signo)
{
    struct {
        int signo;
        pid_t pid;
    } s;
    int fd;


    s.signo = signo;
    s.pid = getpid();
    fd = open(getenv("SIG_FIFO"), O_WRONLY);
    write(fd, &s, sizeof s);
    close(fd);
    pause();
}


void
init_libcore(void)
{
    struct sigaction act;


    act.sa_handler = sighandler;
    sigfillset(&act.sa_mask);
    act.sa_flags = SA_RESETHAND;
    sigaction(SIGSEGV, &act, NULL);
}
-----
% gcc -fPIC -o core.o -c core.c
% gcc -shared -o core.so core.o
% LD_PRELOAD=./core SIG_FIFO=$HOME/.crash_handler_fifo bash


A program to listen to the fifo should be straightforward.

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