Re: [Patch 2/2] ksym_tracer:Handle machine stall when cat trace_pipe for ksym tracer

Frederic Weisbecker
2009-06-19T03:30:41Z

On Thu, Jun 18, 2009 at 10:55:22PM +0530, K.Prasad wrote:
> On Wed, Jun 17, 2009 at 07:12:36AM +0200, Frederic Weisbecker wrote:
> > On Wed, Jun 17, 2009 at 04:35:27AM +0530, K.Prasad wrote:
> > > 'trace_pipe' does not handle a TRACE_TYPE_PARTIAL_LINE well and causes the
> > > machine to stall.
> >
> >
> >
> > No, if it stalls here, it means it handles it well :)
> >
>
> Okay. As stated earlier, the cause is an incorrect
> TRACE_TYPE_PARTIAL_LINE return.
>
> > > While a TRACE_TYPE_UNHANDLED return causes the tracer to
> > > output unrelated data, a TRACE_TYPE_HANDLED return presents a clean output
> > > (minus all partial traces).
> > >
> > > Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
> > > ---
> > > kernel/trace/trace_ksym.c | 6 ++++++
> > > 1 file changed, 6 insertions(+)
> > >
> > > Index: linux-2.6-tip.hbkpt/kernel/trace/trace_ksym.c
> > > ===================================================================
> > > --- linux-2.6-tip.hbkpt.orig/kernel/trace/trace_ksym.c
> > > +++ linux-2.6-tip.hbkpt/kernel/trace/trace_ksym.c
> > > @@ -389,6 +389,12 @@ static enum print_line_t ksym_trace_outp
> > >
> > > trace_assign_type(field, entry);
> > >
> > > + /*
> > > + * Return early without any output if we don't have sufficient
> > > + * information
> > > + */
> > > + if ((!field->ksym_hbp->info.type) || (!field->ip))
> > > + return TRACE_TYPE_HANDLED;
> >
> >
> >
> > Yeah that seems a good fix. But such silent ignored traces may
> > hide bugs (current or futures).
> > Is it a common situation to have a zero ip or an undefined
> > breakpoint type? How can that happen?
> >
> > Thanks.
> >
>
> When a symbol is removed from ksym's trace list, it causes
> unregistration of the breakpoints and the breakpoint structure is
> kfree()-ed.
>
> Now, looking at the relevant code in ksym_hbp_handler() in trace_ksym.c
>
> 88 entry = ring_buffer_event_data(event);
> 89 strlcpy(entry->ksym_name, hbp->info.name, KSYM_SYMBOL_LEN);
> 90 entry->ksym_hbp = hbp;
> 91 entry->ip = instruction_pointer(regs);
> 92 strlcpy(entry->p_name, current->comm, TASK_COMM_LEN);
>
> While the ksym_name and p_name fields are copied into the ring buffer,
> only a pointer to the structure is provided (in line 90). So, when
> ksym_hbp is unregistered the fields contained in it, namely 'type' and
> 'ip' are lost (I'm not sure how the fields beyond 'field->ksym_hbp' is
> accessible in ksym_trace_output() when it is supposed to be already
> free).

If they are freed, they could still be available if the memory place
hasn't been touched by someone else since you kfree'ed it.

But that's buggy and dangerous :)

> This patch would cause an early return when such is the case. I also
> plan to remove the 'default case' in ksym_trace_output() as it was meant
> to handle a zero value for type (other values are eliminated at the time
> of breakpoint registration itself).
>
> Please find a patch below that eliminates the two issues of output
> concatenation and machine stall, excepting for one issue which I'm
> unable to reason out.
>
> A 'cat trace_pipe' done immediately after removal of an entry (without a
> preceding 'cat trace') terminates only with a SIGTERM (kill <pid>) and
> not a SIGINT (Ctrl+C). However, if the trace buffer receives any new
> data subsequently, it responds to the Ctrl+C signal. I would be glad to
> receive your suggestions in this regard.

Hmm, weird...

>
> ===================================================================
> --- linux-2.6-tip.hbkpt.orig/kernel/trace/trace_ksym.c
> +++ linux-2.6-tip.hbkpt/kernel/trace/trace_ksym.c
> @@ -388,6 +388,12 @@ static enum print_line_t ksym_trace_outp
> return TRACE_TYPE_UNHANDLED;
>
> trace_assign_type(field, entry);
> + /*
> + * Return early without any output if we don't have sufficient
> + * information
> + */
> + if ((!field->ksym_hbp->info.type) || (!field->ip))
> + return TRACE_TYPE_HANDLED;
>

No I still think it's dangerous, it may dereference a freed pointer.

> ret = trace_seq_printf(s, "%-15s %-5d %-3d %-20s ",
> field->p_name,
> entry->pid, iter->cpu,
> field->ksym_name);
> @@ -401,10 +407,7 @@ static enum print_line_t ksym_trace_outp
> case HW_BREAKPOINT_RW:
> ret = trace_seq_printf(s, " RW ");
> break;
> - default:
> - return TRACE_TYPE_PARTIAL_LINE;
> }
> -
> if (!ret)
> return TRACE_TYPE_PARTIAL_LINE;
>
> Thanks,
> K.Prasad
>

What do you think about the full copy I suggested before?
That would solve these issues in a simple way and would be
_much_ more safe.

If you try this, tell me if you still have such signal issue.

Thanks.