Re: [Bug runtime/14026] inode based uprobes print_ubacktrace doesn't resolve the symbol name

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Mark Wielaard <mjw@redhat.com> [2012-07-23 10:26:07]:

> On Mon, 2012-07-23 at 11:27 +0530, Srikar Dronamraju wrote:
> > > --- Comment #24 from Frank Ch. Eigler <fche at redhat dot com> 2012-07-19 14:53:23 UTC ---
> > > This inode-uprobes shortcoming is biting other users too.  Perhaps we can work
> > > around it in stap land by using the mmap callbacks that check build-ids to also
> > > go and calculate run-time virtual addresses of the inode-uprobes, so they can
> > > be filled in around probe execution time.
> > > 
> > 
> > Can you please elaborate on whats the shortcoming?
> 
> The issue is that probe handlers (and various parts of the systemtap
> runtime) assume they have access to the exact REG_IP where the program
> was probed. At least on x86 architectures there is an off-by-one because
> the IP is increased before the probe handler is called. This results in
> the probe handler (and unwinder) not knowing exactly where they are. In
> some cases that leads to errors since the IP spilled over into the next
> function (in which case the runtime cannot find the correct CFI).
> 
> With kprobes and utrace-uprobes tapsets.cxx works around this by doing
> the following:
> 
>   // Make it look like the IP is set as it would in the actual user
>   // task when calling real probe handler. Reset IP regs on return, so
>   // we don't confuse uprobes. PR10458
>   s.op->newline() << "{";
>   s.op->indent(1);
>   s.op->newline() << "unsigned long uprobes_ip = REG_IP(c->uregs);";
>   s.op->newline() << "SET_REG_IP(regs, inst->vaddr);";
>   s.op->newline() << "(*sups->probe->ph) (c);";
>   s.op->newline() << "SET_REG_IP(regs, uprobes_ip);";
>   s.op->newline(-1) << "}";
> 
> We need something similar for inode-uprobes, because currently it has:
> 
>   // XXX: Can't set SET_REG_IP; we don't actually know the relocated address.
>   // ...  In some error cases, uprobes itself calls uprobes_get_bkpt_addr().
> 

uprobes_get_bkpt_addr() [ which is now called uprobe_get_swbp_addr ]
will only return the virtual address where a probe is placed (which is
probably what you are looking for). uprobe_get_swbp_addr() never
sets/resets IP. However its not a static function but its not exported
too.

The other option would be have something similar to
uprobe_get_swbp_addr() within stap. Including uprobes.h would
necessarily mean that UPROBE_SWBP_INSN_SIZE is always set.
But it means we have to depend on UPROBE_SWBP_INSN_SIZE.

May the later option is not all that nice. 

-- 
Thanks and regards
Srikar