netdev - Re: [PATCH v5 0/4] tracing: improve symbolic printing

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Message-ID: <2804ef964f4ce1c4689c1e741ef45e6b6d3f6422.camel@sipsolutions.net>
Date: Mon, 26 Aug 2024 20:24:08 +0200
From: Johannes Berg <johannes@...solutions.net>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	netdev@...r.kernel.org, linux-wireless@...r.kernel.org
Subject: Re: [PATCH v5 0/4] tracing: improve symbolic printing

Hi Steven,

> I finally got around to testing your patches.
> 
> I did the following:
> 
>  # cat /sys/kernel/tracing/events/*/*/format
> 
> and hit this:
> 
> BUG: unable to handle page fault for address: ffffffff8e6333d0

Ugh. That's ... unfortunate.

I couldn't reproduce this so far, do you happen to have the .config and
qemu command line perhaps?

> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 183c40067 P4D 183c40067 PUD 183c41063 PMD 1003ef063 PTE 800ffffe7b9cc062
> Oops: Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 7 UID: 0 PID: 893 Comm: cat Not tainted 6.11.0-rc4-test-00004-g4ce2836f008b #56 68afcee1248519f8b3b088836c40746e4a6b69d3

I guess that's just my four patches; reproducing that (rc4 + the
patches, seems there was a small conflict in skb tracing) ...

> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:f_show (kernel/trace/trace_events.c:1601 kernel/trace/trace_events.c:1651 kernel/trace/trace_events.c:1689)

means this was

        for (i = 0; i < n_sym_defs; i++) {
                unsigned int sym_len;

                if (!sym_defs[i])            // <--- HERE
                        continue;

which seems really strange to me.

Sadly the

> Code: [...]

was compiled so we don't see 'i' here, and the stop value is on the
stack...

> All code
> ========
>    0:   33 63 8e                xor    -0x72(%rbx),%esp
>    3:   48 2d d0 33 63 8e       sub    $0xffffffff8e6333d0,%rax

the big constant here must be __start_ftrace_sym_defs, being subtracted
from an already loaded __stop_ftrace_sym_defs in %rax ...

>    9:   48 c1 f8 03             sar    $0x3,%rax

also part of the size calculation (n_sym_defs) - we have 8 byte pointers
in each entry, so >>= 3 makes sense.

Given RBX is 0xffffffff8e6333d0 as well, that means
__start_ftrace_sym_defs isn't a valid pointer? But then what was
__stop_ftrace_sym_defs? And why would the section be non-empty but
invalid to dereference?

>    d:   85 c0                   test   %eax,%eax
>    f:   74 67                   je     0x78

Anyway this aborts if there are 0 entries so we get non-zero in
%eax/%rax ...

I've actually tried now with an empty section, and that also works OK.

Is there something special about your build here?

johannes