Re: [PATCH 3/3] tracing/probes: Fix return value when "(fault)" is injected

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

On Thu, 6 Jul 2023 09:50:39 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Thu, 6 Jul 2023 13:40:36 +0900
> Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:
> 
> > On Wed, 5 Jul 2023 22:49:56 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > On Sun,  2 Jul 2023 23:47:35 +0900
> > > "Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:
> > >   
> > > > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > > 
> > > > When the "(fault)" is injected, the return value of fetch_store_string*()
> > > > should be the length of the "(fault)", but an error code is returned.
> > > > Fix it to return the correct length and update the data_loc according the
> > > > updated length.
> > > > This needs to update a ftracetest test case, which expects trace output
> > > > to appear as '(fault)' instead of '"(fault)"'.
> > > >   
> > > 
> > > Ah, because of patch 2, the ret < 0 makes it return without printing the
> > > "fault"?  
> > 
> > No, actually set_data_loc() updates the 'ret' argument, but it is just
> > disposed... (not returned to the caller)
> 
> That's not what I was talking about.
> 
> We have:
> 
> process_fetch_insn_bottom() {
> 	[..]
> 	case FETCH_OP_ST_STRING:
> 		loc = *(u32 *)dest;
> 		ret = fetch_store_string(val + code->offset, dest, base);
> 		break;
> 	[..]
> 
> // And from patch 2 we have:
> 
> @@ -193,6 +193,8 @@ process_fetch_insn_bottom(struct fetch_insn *code, unsigned long val,
>  	default:
>  		return -EILSEQ;
>  	}
> +	if (ret < 0)
> +		return ret;
>  	code++;
> 
> And now that the return value of fetch_store_string() is being checked, and
> if it returns negative, it ends the function before being processed
> further. And if there's a fault, it happens to return negative!
> 
> This patch now changes fetch_store_string() and fetch_store_string_user()
> to not return negative if there's a fault. As this patch has:
> 
> @@ -107,9 +106,7 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
>  	 * probing.
>  	 */
>  	ret = strncpy_from_kernel_nofault(__dest, (void *)addr, maxlen);
> -	set_data_loc(ret, dest, __dest, base, maxlen);
> -
> -	return ret;
> +	return set_data_loc(ret, dest, __dest, base, maxlen);
>  }
> 
> But to do that, you needed to update set_data_loc() to return a value.
> 
> *that's* what I meant by 
> 
> 'Ah, because of patch 2, the ret < 0 makes it return without printing the "fault"?'

Yes, that's correct. Actually, the data ("(fault)") is stored, but ignored
because data_loc is not updated.

But wait, it seems that the print function shows (fault), so commit 2e9906f84fc7
("tracing: Add "(fault)" name injection to kernel probes") may not needed?

----
/* Print type function for string type */
int PRINT_TYPE_FUNC_NAME(string)(struct trace_seq *s, void *data, void *ent)
{
        int len = *(u32 *)data >> 16;

        if (!len)
                trace_seq_puts(s, "(fault)");
        else
----

In this case, what we need is to set data_loc length = 0 if ret < 0.

Do you really need to get '"(fault)"' (with double quotation) or
just '(fault)' (no double quotation) is OK?

Thank you,
> 
> 
> -- Steve
> 
> > 
> > -static nokprobe_inline void set_data_loc(int ret, void *dest, void *__dest, void *base, int len)
> > +static nokprobe_inline int set_data_loc(int ret, void *dest, void *__dest, void *base, int len)
> >  {
> > -	if (ret >= 0) {
> > -		*(u32 *)dest = make_data_loc(ret, __dest - base);
> > -	} else {
> > +	if (ret < 0) {
> >  		strscpy(__dest, FAULT_STRING, len);
> >  		ret = strlen(__dest) + 1;
> >  	}
> > +
> > +	*(u32 *)dest = make_data_loc(ret, __dest - base);
> > +	return ret;
> >  }
> > 
> > So this returns updated 'ret', and also update data_loc to use the
> > updated 'ret' value (which is the length of the stored data).
> > 
> > > 
> > > Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>  
> > 
> > Thank you!
> > 
> > > 
> > > -- Steve
> > > 
> > >   
> > > > Fixes: 2e9906f84fc7 ("tracing: Add "(fault)" name injection to kernel probes")
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > > > ---  
> > 
> > 
> 


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