Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf_kprobe' PMU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 17 Aug 2023 13:06:20 +0200
Francis Laniel <flaniel@linux.microsoft.com> wrote:

> Hi.
> 
> Le jeudi 17 août 2023, 09:50:57 CEST Masami Hiramatsu a écrit :
> > Hi,
> > 
> > On Wed, 16 Aug 2023 18:35:17 +0200
> > 
> > Francis Laniel <flaniel@linux.microsoft.com> wrote:
> > > When using sysfs, it is possible to create kprobe for several kernel
> > > functions sharing the same name, but of course with different addresses,
> > > by writing their addresses in kprobe_events file.
> > > 
> > > When using PMU, if only the symbol name is given, the event will be
> > > created for the first address which matches the symbol, as returned by
> > > kallsyms_lookup_name().
> > 
> > Do you mean probing the same name symbols? Yes, it is intended behavior,
> > since it is not always true that the same name function has the same
> > prototype (it is mostly true but is not ensured), it is better to leave
> > user to decide which one is what you want to probe.
> 
> This is what I meant.
> I also share your mind regarding leaving the users deciding which one they 
> want to probe but in my case (which I agree is a bit a corner one) it leaded 
> me to misunderstanding as the PMU kprobe was only added to the first 
> ntfs_file_write_iter() which is not the one for ntfs3.

Hmm, OK. I think in that case (multiple same-name symbols exist) the default
behavior is rejecting with error message. And optionally, it will probe all
or them like your patch.

> 
> > Have you used 'perf probe' tool? It tries to find the appropriate function
> > by line number and creates the probe by 'text+OFFSET' style, not by symbol.
> > I think this is the correct way to do that, because user will not know
> > which 'address' of the symbol is what the user want.
> 
> 'perf probe' perfectly does the trick, as it would find all the kernel 
> addresses which correspond to the symbol name and create as many probes as 
> corresponding symbols [1]:
> root@vm-amd64:~# perf probe --add ntfs_file_write_iter

If you can specify the (last part of) file path as below,

perf probe --add ntfs_file_write_iter@ntfs3/file.c

Then it will choose correct one. :)

> 
> Added new events:
>   probe:ntfs_file_write_iter (on ntfs_file_write_iter)
>   probe:ntfs_file_write_iter (on ntfs_file_write_iter)
> 
> You can now use it in all perf tools, such as:
> 
>         perf record -e probe:ntfs_file_write_iter -aR sleep 1
> root@vm-amd64:~# cat /sys/kernel/tracing/kprobe_events
> p:probe/ntfs_file_write_iter _text+5088544
> p:probe/ntfs_file_write_iter _text+5278560
> 
> > Thought?
> 
> This contribution is basically here to sort of mimic what perf does but with 
> PMU kprobes, as this is not possible to write in a sysfs file with this type of 
> probe.

OK, I see it is for BPF only. Maybe BPF program can filter correct one
to access the argument etc. 

Thank you,

> 
> > 
> > Thank you,
> > 
> > > The idea here is to search all kernel functions which match this symbol
> > > and
> > > create a trace_kprobe for each of them.
> > > All these trace_kprobes are linked together by sharing the same
> > > trace_probe.
> > > 
> > > Signed-off-by: Francis Laniel <flaniel@linux.microsoft.com>
> > > ---
> > > 
> > >  kernel/trace/trace_kprobe.c | 86 +++++++++++++++++++++++++++++++++++++
> > >  1 file changed, 86 insertions(+)
> > > 
> > > diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> > > index 1b3fa7b854aa..08580f1466c7 100644
> > > --- a/kernel/trace/trace_kprobe.c
> > > +++ b/kernel/trace/trace_kprobe.c
> > > @@ -1682,13 +1682,42 @@ static int unregister_kprobe_event(struct
> > > trace_kprobe *tk)> 
> > >  }
> > >  
> > >  #ifdef CONFIG_PERF_EVENTS
> > > 
> > > +
> > > +struct address_array {
> > > +	unsigned long *addrs;
> > > +	size_t size;
> > > +};
> > > +
> > > +static int add_addr(void *data, unsigned long addr)
> > > +{
> > > +	struct address_array *array = data;
> > > +	unsigned long *p;
> > > +
> > > +	array->size++;
> > > +	p = krealloc(array->addrs,
> > > +				sizeof(*array->addrs) * array->size,
> > > +				GFP_KERNEL);
> > > +	if (!p) {
> > > +		kfree(array->addrs);
> > > +		return -ENOMEM;
> > > +	}
> > > +
> > > +	array->addrs = p;
> > > +	array->addrs[array->size - 1] = addr;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > 
> > >  /* create a trace_kprobe, but don't add it to global lists */
> > >  struct trace_event_call *
> > >  create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
> > >  
> > >  			  bool is_return)
> > >  
> > >  {
> > >  
> > >  	enum probe_print_type ptype;
> > > 
> > > +	struct address_array array;
> > > 
> > >  	struct trace_kprobe *tk;
> > > 
> > > +	unsigned long func_addr;
> > > +	unsigned int i;
> > > 
> > >  	int ret;
> > >  	char *event;
> > > 
> > > @@ -1722,7 +1751,64 @@ create_local_trace_kprobe(char *func, void *addr,
> > > unsigned long offs,> 
> > >  	if (ret < 0)
> > >  	
> > >  		goto error;
> > > 
> > > +	array.addrs = NULL;
> > > +	array.size = 0;
> > > +	ret = kallsyms_on_each_match_symbol(add_addr, func, &array);
> > > +	if (ret)
> > > +		goto error_free;
> > > +
> > > +	if (array.size == 1)
> > > +		goto end;
> > > +
> > > +	/*
> > > +	 * Below loop allocates a trace_kprobe for each function with the same
> > > +	 * name in kernel source code.
> > > +	 * All this differente trace_kprobes will be linked together through
> > > +	 * append_trace_kprobe().
> > > +	 * NOTE append_trace_kprobe() is called in register_trace_kprobe() 
> which
> > > +	 * is called when a kprobe is added through sysfs.
> > > +	 */
> > > +	func_addr = kallsyms_lookup_name(func);
> > > +	for (i = 0; i < array.size; i++) {
> > > +		struct trace_kprobe *tk_same_name;
> > > +		unsigned long address;
> > > +
> > > +		address = array.addrs[i];
> > > +		/* Skip the function address as we already registered it. */
> > > +		if (address == func_addr)
> > > +			continue;
> > > +
> > > +		/*
> > > +		 * alloc_trace_kprobe() first considers symbol name, so we set
> > > +		 * this to NULL to allocate this kprobe on the given address.
> > > +		 */
> > > +		tk_same_name = alloc_trace_kprobe(KPROBE_EVENT_SYSTEM, event,
> > > +						  (void *)address, NULL, offs,
> > > +						  0 /* maxactive */,
> > > +						  0 /* nargs */, is_return);
> > > +
> > > +		if (IS_ERR(tk_same_name)) {
> > > +			ret = -ENOMEM;
> > > +			goto error_free;
> > > +		}
> > > +
> > > +		init_trace_event_call(tk_same_name);
> > > +
> > > +		if (traceprobe_set_print_fmt(&tk_same_name->tp, ptype) < 0) {
> > > +			ret = -ENOMEM;
> > > +			goto error_free;
> > > +		}
> > > +
> > > +		ret = append_trace_kprobe(tk_same_name, tk);
> > > +		if (ret)
> > > +			goto error_free;
> > > +	}
> > > +
> > > +end:
> > > +	kfree(array.addrs);
> > > 
> > >  	return trace_probe_event_call(&tk->tp);
> > > 
> > > +error_free:
> > > +	kfree(array.addrs);
> > > 
> > >  error:
> > >  	free_trace_kprobe(tk);
> > >  	return ERR_PTR(ret);
> 
> ---
> [1]: https://github.com/torvalds/linux/blob/
> 57012c57536f8814dec92e74197ee96c3498d24e/tools/perf/util/probe-event.c#L2989-
> L2993
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>