Category Archives: Dtrace
When Dtrace Fails – Spectacularly
So, I’ve been spending some time looking at Dtrace today. At first, I created a proof of concept on OS X, and then went on to try it in production on FreeBSD.
No such luck. After several hours of trying to figure out what the heck was going wrong, I tried the following experiment, on OS X.
% uname -a Darwin foo 14.5.0 Darwin Kernel Version 14.5.0: Tue Sep 1 21:23:09 PDT 2015; root:xnu-2782.50.1~1/RELEASE_X86_64 x86_64 % cat hello.c #include <stdio.h> int main(int argc, char *argv[]) { printf( "Hello\n" ); return 0; } % dtrace -n 'pid$target::main:entry{printf("%#p\n",uregs[R_RBP]);}' -c ./hello dtrace: description 'pid$target::main:entry' matched 1 probe Hello dtrace: pid 9224 has exited CPU ID FUNCTION:NAME 0 67470 main:entry 0x7fff583c1c20
And then again on FreeBSD.
% uname -a FreeBSD bar 10.2-RELEASE FreeBSD 10.2-RELEASE #0 r286666: Wed Aug 12 15:26:37 UTC 2015 root@releng1.nyi.freebsd.org:/usr/obj/usr/src/sys/GENERIC amd64 % cat hello.c #include <stdio.h> int main(int argc, char *argv[]) { printf( "Hello\n" ); return 0; } % dtrace -n 'pid$target::main:entry{printf("%#p\n",uregs[R_RBP]);}' -c ./hello dtrace: description 'pid$target::main:entry' matched 1 probe Hello dtrace: pid 84313 has exited CPU ID FUNCTION:NAME 2 54008 main:entry 0
As you can see, the printed value of the %rbp register is zero on FreeBSD. In my experiments, trying to read that register always yields zero. Similarly, I do not trust it for other registers.
This seems to be a bug in FreeBSD’s Dtrace. At the time of this writing, I have not tried it on recent Illumos.