how to debug (printk/printf/uart?) inside vdso (arch/x86/vdso/vclock_gettime.c)

goblinhack · 12-03-2015, 12:44 PM

I have an issue where clock_gettime is stuck in a loop. I've seen
a few threads on workarounds, but I'm more interested in simply how
to debug inside vdso (arch/x86/vdso/vclock_gettime.c)

I cannot use printk as this is not a true part of the kernel.
I don't think I can use printf - at least I've not been able to find
a way how. And even directly printing to the serial uart e.g.:

static cycle_t vread_pvclock(int *mode)
...
outb('x', 0x3f8);

gives me a kernel crash with nothing specific to this function in
the traceback. In fact doing almost any change in here seems to lead
to a crash.

Are there any ways to debug in here ? I'm really just wanting to
print some output to the serial port when I find clock_gettime in
a loop - and I can't easily use gdb in this embedded platform

any advice welcome

tx

neil

goblinhack · 12-10-2015, 07:16 AM

Just an update, I've decided to try and invoke the syslog system call
inside vdso. It seems this *should* work

Code:

   notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
    {
    + my_syslog(0, "NEIL getttime", 10);

but I think I was crafting it wrong and instead of

Code:

   static ssize_t my_syslog(int fd, const void *buf, int size)
    {
        ssize_t ret;
        asm volatile
        (
            "syscall"
            // outputs
            : "=a" (ret)

            // inputs
            : "0"(103), "D"(fd), "S"(buf), "d"(size)

            : "cc", "rcx", "memory"
        );
        return ret;
    }

should be maybe:

Code:

    static notrace long my_syslog(long fd, const void *buf, long size)
    {
        long ret;

        asm(
                "call VDSO32_vsyscall \n"
                : "=a" (ret)
                : "0" (103), "g" (fd), "c" (buf), "d" (size)
                : "memory", "edx");
        return ret;
    }

will update if I get any joy out of this. There has to be some way to get
some debug out of vdso... 8)

tx

neil