dtrace execute action only when the function returns to a specific module - dtrace

Question

I'm tracking some libc functions with dtrace. I want to make a predicate that only executes the action when the function returns to an adress into a specific module given in the parameters.
copyin(uregs[R_ESP],1) on the return probe should give the return adress i think, i'm not entirely sure of it so it would be nice if someone can confirm.
But then i need a way to resolve that adress to a module, is this possible and how?

Answer 1

There is a ucaller variable which will give you the
saved program counter as a uint64_t and umod() will
translate it into the corresponding module name, e.g.
# dtrace -n 'pid$target:::entry {#[umod(ucaller)]=count()}' -p `pgrep -n xscreensaver`
dtrace: description 'pid$target:::entry ' matched 14278 probes
^C
xscreensaver 16
libXt.so.4 73
libX11.so.4 92
libxcb.so.1 141
libc.so.1 144
^C#
However, umod() is an action (as opposed to a subroutine); it
cannot be assigned to an lvalue and therefore cannot be used in
an expression (because the translation is deferred until the address
is received by the dtrace(1) user-land program).
Fortunately, there's nothing stopping you from finding the address
range occupied by libc in your process and comparing it with ucaller.
Here's an example on Solaris (where a hardware-specific libc is
mounted at boot time):
# mount | fgrep libc
/lib/libc.so.1 on /usr/lib/libc/libc_hwcap1.so.1 read/write/setuid/devices/rstchown/dev=30d0002 on Sat Jul 13 20:27:32 2013
# pmap `pgrep -n gedit` | fgrep libc_hwcap1.so.1
FEE10000 1356K r-x-- /usr/lib/libc/libc_hwcap1.so.1
FEF73000 44K rwx-- /usr/lib/libc/libc_hwcap1.so.1
FEF7E000 4K rwx-- /usr/lib/libc/libc_hwcap1.so.1
#
I'll assume that the text section is the one with only
read & execute permissions, but note that in some
circumstances the text section will be writeable.
# cat Vision.d
/*
* self->current is a boolean indicating whether or not execution is currently
* within the target range.
*
* self->next is a boolean indicating whether or not execution is about to
* return to the target range.
*/
BEGIN
{
self->current = 1;
}
pid$target:::entry
{
self->current = (uregs[R_PC] >= $1 && uregs[R_PC] < $2);
}
syscall:::return
/pid==$target/
{
self->next = self->current;
self->current = 0;
}
pid$target:::return
{
self->next = (ucaller >= $1 && ucaller < $2);
}
pid$target:::return,syscall:::return
/pid==$target && self->next && !self->current/
{
printf("Returning to target from %s:%s:%s:%s...\n",
probeprov, probemod, probefunc, probename);
ustack();
printf("\n");
}
pid$target:::return,syscall:::return
/pid==$target/
{
self->current = self->next;
}
# dtrace -qs Vision.d 0xFEE10000 0xFEF73000 -p `pgrep -n gedit`
This produces results like
Returning to target from pid2095:libcairo.so.2.10800.10:cairo_bo_event_compare:return...
libcairo.so.2.10800.10`cairo_bo_event_compare+0x158
libc.so.1`qsort+0x51c
libcairo.so.2.10800.10`_cairo_bo_event_queue_init+0x122
libcairo.so.2.10800.10`_cairo_bentley_ottmann_tessellate_bo_edges+0x2d
libcairo.so.2.10800.10`_cairo_bentley_ottmann_tessellate_polygon+0
.
.
.
Returning to target from syscall::pollsys:return...
libc.so.1`__pollsys+0x15
libc.so.1`poll+0x81
libxcb.so.1`_xcb_conn_wait+0xb5
libxcb.so.1`_xcb_out_send+0x3b
libxcb.so.1`xcb_writev+0x65
libX11.so.4`_XSend+0x17c
libX11.so.4`_XFlush+0x30
libX11.so.4`XFlush+0x37