linux-kernel - Re: [BUGFIX PATCH] tracing/kprobes: Fix strpbrk() argument order

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Date:   Sat, 3 Nov 2018 20:54:48 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX PATCH] tracing/kprobes: Fix strpbrk() argument order

On Fri, 2 Nov 2018 09:54:24 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 2 Nov 2018 16:14:59 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > On Thu, 1 Nov 2018 15:10:14 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > On Thu,  1 Nov 2018 23:29:28 +0900
> > > Masami Hiramatsu <mhiramat@...nel.org> wrote:
> > >   
> > > > Fix strpbrk()'s argument order, it must pass acceptable string
> > > > in 2nd argument. Note that this can cause a kernel panic where
> > > > it recovers backup character to code->data.
> > > > 
> > > > Fixes: a6682814f371 ("tracing/kprobes: Allow kprobe-events to record module symbol")
> > > > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>  
> > > 
> > > Thanks Masami,
> > > 
> > > I'm pulling this and starting to test it.  
> > 
> > Thank you! I still couldn't believe how this bug passed through the tests...
> 
> I am too. I'm running tests with and without this patch, and the patch
> doesn't appear to be making much difference.

Maybe traceprobe_free_probe_arg() is silently failed.

> 
> Then I tested with this:
> 
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 3ef15a6683c0..4ddafddf1343 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -516,6 +516,7 @@ void traceprobe_free_probe_arg(struct probe_arg
> *arg) kfree(code->data);
>  		code++;
>  	}
> +	printk("free arg->code %s\n", arg->code);
>  	kfree(arg->code);
>  	kfree(arg->name);
>  	kfree(arg->comm);
> @@ -535,11 +536,15 @@ int traceprobe_update_arg(struct probe_arg *arg)
>  			if (code[1].op != FETCH_OP_IMM)
>  				return -EINVAL;
>  
> +			tmp = strpbrk(code->data, "+-");
> +			printk("first tmp tmp=%s\n", tmp);
>  			tmp = strpbrk("+-", code->data);
> +			printk("second tmp=%s data=%s\n", tmp,
> code->data); if (tmp)
>  				c = *tmp;
>  			ret =
> traceprobe_split_symbol_offset(code->data, &offset);
> +			printk("third data=%s\n", code->data);
>  			if (ret)
>  				return ret;
>  
> @@ -547,6 +552,7 @@ int traceprobe_update_arg(struct probe_arg *arg)
>  				(unsigned
> long)kallsyms_lookup_name(code->data); if (tmp)
>  				*tmp = c;
> +			printk("forth data=%s\n", code->data);
>  			if (!code[1].immediate)
>  				return -ENOENT;
>  			code[1].immediate += offset;
> 
> And I don't see where that code->data is used elsewhere. That is, why
> even bother saving the character?

Would you mean parsing the symbol+offs every time is useless?
It needs to solve the symbol address always because  traceprobe_update_arg
is called when new symbols added on the kernel (by loading modules).

Hmm, maybe I can introduce a struct like 

struct symbol_offset {
	long offset;
	char symbol[];
};

and use it instead of parsing the symbol+offset always.

Thanks,

-- 
Masami Hiramatsu <mhiramat@...nel.org>