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From:   Sven Schnelle <svens@linux.ibm.com>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH] tracing: fix memcpy size when copying stack entries
References: <20230612160748.4082850-1-svens@linux.ibm.com>
        <20230612123407.5ebcabdf@gandalf.local.home>
Date:   Tue, 13 Jun 2023 07:19:14 +0200
In-Reply-To: <20230612123407.5ebcabdf@gandalf.local.home> (Steven Rostedt's
        message of "Mon, 12 Jun 2023 12:34:07 -0400")
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Steven Rostedt <rostedt@goodmis.org> writes:

> On Mon, 12 Jun 2023 18:07:48 +0200
> Sven Schnelle <svens@linux.ibm.com> wrote:
>
>> Noticed the following warning during boot:
>> 
>> [    2.316341] Testing tracer wakeup:
>> [    2.383512] ------------[ cut here ]------------
>> [    2.383517] memcpy: detected field-spanning write (size 104) of single field "&entry->caller" at kernel/trace/trace.c:3167 (size 64)
>> 
>> The reason seems to be that the maximum number of entries is calculated
>> from the size of the fstack->calls array which is 128. But later the same
>> size is used to memcpy() the entries to entry->callers, which has only
>> room for eight elements. Therefore use the minimum of both arrays as limit.
>> 
>> Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
>> ---
>>  kernel/trace/trace.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index 64a4dde073ef..988d664c13ec 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -3146,7 +3146,7 @@ static void __ftrace_trace_stack(struct trace_buffer *buffer,
>>  	barrier();
>>  
>>  	fstack = this_cpu_ptr(ftrace_stacks.stacks) + stackidx;
>> -	size = ARRAY_SIZE(fstack->calls);
>> +	size = min(ARRAY_SIZE(entry->caller), ARRAY_SIZE(fstack->calls));
>
> No, this is not how it works, and this breaks the stack tracing code.
> [..]
> The old way use to just record the 8 entries, but that was not very useful
> in real world analysis. Your patch takes that away. Might as well just
> record directly into the ring buffer again like it use to.
>
> Yes the above may be special, but your patch breaks it.

Indeed, i'm feeling a bit stupid for sending that patch, should have
used my brain during reading the source. Thanks for the explanation.