linux-kernel - Re: linux-next: build failure after merge of the kspp tree

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Date:   Tue, 25 Jan 2022 23:31:54 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Next Mailing List <linux-next@...r.kernel.org>
Subject: Re: linux-next: build failure after merge of the kspp tree

On Tue, 25 Jan 2022 22:27:32 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

> > /*
> >  * struct trace_event_data_offsets_<call> {
> >  *      u32                             <item1>;
> >  *      u32                             <item2>;
> >  *      [...]
> >  * };
> >  *
> >  * The __dynamic_array() macro will create each u32 <item>, this is
> >  * to keep the offset of each array from the beginning of the event.
> >  * The size of an array is also encoded, in the higher 16 bits of
> >  * <item>.
> >  */
> > 
> > So, I think -Warray-bounds is refusing to see the destination as
> > anything except a u32, but being accessed at 4 (sizeof(u32)) + 8
> > (address && 0xffff) (?)
> 
> Ah, I got it. Yes, that's right. __data_loc() will access the data
> from the __entry, but the __rel_loc() points the same address from
> the encoded field ("__rel_loc_foo" in this case) itself.
> This is introduced for the user application event, which doesn't
> know the actual __entry size because the __entry includes some
> kernel internal defined fields.
> 
> > But if this is true, I would imagine there would be plenty of other
> > warnings? I'm currently stumped.
> 
> That is because __rel_loc is used only in the sample code in the kernel
> for testing. Other use-cases comes from user-space.
> Hmm, can we skip this boundary check for this example?

If the -Warray-bounds determines the destination array size from
the type of given pointer, we can just change the macro as below;

#define __get_rel_dynamic_array(field) 
			((void *)__entry +                                 \
			 offsetof(typeof(*__entry), __rel_loc_##field) +   \
			 sizeof(__entry->__rel_loc_##field) +              \
			 (__entry->__rel_loc_##field & 0xffff))

This must works same as __get_dynamic_array() macro.

Could you try this patch?

>From 2982ba01367ec1f746a4f128512436e5325a7f9d Mon Sep 17 00:00:00 2001
From: Masami Hiramatsu <mhiramat@...nel.org>
Date: Tue, 25 Jan 2022 23:19:30 +0900
Subject: [PATCH] tracing: Avoid -Warray-bounds warning for __rel_loc macro

Since -Warray-bounds checks the destination size from the
type of given pointer, __assign_rel_str() macro gets warned
because it passes the pointer to the 'u32' field instead of
'trace_event_raw_*' data structure.
Pass the data address calculated from the 'trace_event_raw_*'
instead of 'u32' __rel_loc field.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Kees Cook <keescook@...omium.org>
---
 include/trace/trace_events.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/trace/trace_events.h b/include/trace/trace_events.h
index 8c6f7c433518..65d927e059d3 100644
--- a/include/trace/trace_events.h
+++ b/include/trace/trace_events.h
@@ -318,9 +318,10 @@ TRACE_MAKE_SYSTEM_STR();
 #define __get_str(field) ((char *)__get_dynamic_array(field))
 
 #undef __get_rel_dynamic_array
-#define __get_rel_dynamic_array(field)	\
-		((void *)(&__entry->__rel_loc_##field) +	\
-		 sizeof(__entry->__rel_loc_##field) +		\
+#define __get_rel_dynamic_array(field)					\
+		((void *)__entry + 					\
+		 offsetof(typeof(*__entry), __rel_loc_##field) +	\
+		 sizeof(__entry->__rel_loc_##field) +			\
 		 (__entry->__rel_loc_##field & 0xffff))
 
 #undef __get_rel_dynamic_array_len
-- 
2.25.1


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