dtrace pid provider on entry == Trace/BPT trap process

[Andreas Longwitz]

> Maybe the following is of interest for DTrace experts:
>>
>>>> I use FreeBSD 10.3-STABLE r317936 with updated /usr/lib/dtrace/ip.c and
>>>> see an issue that in my opinion should have been solved in r269342:
>>> 10.3 would be missing a number of other fixes for issues that might
>>> cause the symptom you're seeing. Unfortunately, userland DTrace is not
>>> very usable on FreeBSD before 11.0.
>> I have repeated my looptest on a server running FreeBSD 12.0-CURRENT #0
>> r324306 with GENERIC kernel. I saw the same problem as described before
>> for FreeBSD 10.3-Stable r317936:
>>
>> Nov  7 10:54:09 <kern.info> dssbkp2 kernel: pid 38041 (looptest), uid
>> 1003: exited on signal 5
>>
>> Further I saw following error message from dtrace:
>>
>>   dtrace: failed to grab pid 38622: Device busy.
>>
>> After some minutes playing around the server crashed with
>>
>> panic: PHOLD of exiting process 0xfffff80004c80540
> 
> Sorry for the delayed reply. I think this assertion is bogus: the
> process in question is already held at this point, so it can't continue
> exiting anyway. I think the patch below will address this and the
> SIGTRAP crashes. Could you give it a try?
> 
> diff --git a/sys/cddl/compat/opensolaris/kern/opensolaris_proc.c b/sys/cddl/compat/opensolaris/kern/opensolaris_proc.c
> index 652fa8928589..63db17199fb4 100644
> --- a/sys/cddl/compat/opensolaris/kern/opensolaris_proc.c
> +++ b/sys/cddl/compat/opensolaris/kern/opensolaris_proc.c
> @@ -35,9 +35,7 @@ uread(proc_t *p, void *kaddr, size_t len, uintptr_t uaddr)
>  {
>  	ssize_t n;
>  
> -	PHOLD(p);
>  	n = proc_readmem(curthread, p, uaddr, kaddr, len);
> -	PRELE(p);
>  	if (n != len)
>  		return (ENOMEM);
>  	return (0);
> @@ -48,9 +46,7 @@ uwrite(proc_t *p, void *kaddr, size_t len, uintptr_t uaddr)
>  {
>  	ssize_t n;
>  
> -	PHOLD(p);
>  	n = proc_writemem(curthread, p, uaddr, kaddr, len);
> -	PRELE(p);
>  	if (n != len)
>  		return (ENOMEM);
>  	return (0);
> diff --git a/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c b/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c
> index 82353cbb235d..d721270fd425 100644
> --- a/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c
> +++ b/sys/cddl/contrib/opensolaris/uts/common/dtrace/fasttrap.c
> @@ -64,6 +64,7 @@
>  #include <sys/dtrace_bsd.h>
>  #include <sys/eventhandler.h>
>  #include <sys/rmlock.h>
> +#include <sys/smp.h>
>  #include <sys/sysent.h>
>  #include <sys/sysctl.h>
>  #include <sys/u8_textprep.h>
> @@ -1097,6 +1098,8 @@ fasttrap_tracepoint_disable(proc_t *p, fasttrap_probe_t *probe, uint_t index)
>  		p->p_dtrace_count--;
>  	}
>  
> +	(void)quiesce_all_cpus("fasttrap", 0);
> +
>  	/*
>  	 * Remove the probe from the hash table of active tracepoints.
>  	 */
> diff --git a/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c b/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
> index d6655a0b3c55..0364aaf55094 100644
> --- a/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
> +++ b/sys/cddl/contrib/opensolaris/uts/intel/dtrace/fasttrap_isa.c
> @@ -1715,11 +1715,7 @@ fasttrap_pid_probe(struct reg *rp)
>  
>  		ASSERT(i <= sizeof (scratch));
>  
> -#ifdef illumos
>  		if (fasttrap_copyout(scratch, (char *)addr, i)) {
> -#else
> -		if (uwrite(p, scratch, i, addr)) {
> -#endif
>  			fasttrap_sigtrap(p, curthread, pc);
>  			new_pc = pc;
>  			break;

I have applied your patch, no crash, no core or error message so far.
But dtrace does not exit after CTRL C. After starting looptest I enter
the dtrace command and type CTRL C after some seconds:

 dtrace -n 'pid$target:libc.so.*::entry {@[probefunc] = count();}' -p
$(pgrep looptest)
dtrace: description 'pid$target:libc.so.*::entry ' matched 4068 probes
^C
  __vfprintf                                                   226824
  localeconv_l                                                 226824
  memset                                                       226824
  sprintf                                                      226824
  vsprintf                                                     226824
  __sfvwrite                                                   453647
  memcpy                                                       680469
^C^C^C^C^C^C

I could not stop dtrace, it does not exit. procstat gives

  PID  PPID  PGID   SID  TSID THR LOGIN    WCHAN    EMUL         COMM
28183 28031 28183 28025 28025   1 longwitz fasttrap FreeBSD ELF64 dtrace

If I stop looptest, then dtrace process stops immediately too.


-- 
Andreas Longwitz