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Date: Fri, 4 Apr 2025 09:38:37 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Devaansh Kumar <devaanshk840@gmail.com>, mhiramat@kernel.org,
 linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org,
 skhan@linuxfoundation.org, linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [PATCH] tracing: Replace deprecated strncpy() with memcpy() for
 stack_trace_filter_buf
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On Fri, 4 Apr 2025 08:54:33 -0400
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> >>> -     if ((len = str_has_prefix(str, "_filter=")))
> >>> -             strncpy(stack_trace_filter_buf, str + len, COMMAND_LINE_SIZE);
> >>> +     len = str_has_prefix(str, "_filter=");
> >>> +
> >>> +     if (len)
> >>> +             memcpy(stack_trace_filter_buf, str + len, sizeof(stack_trace_filter_buf));  
> >>
> >> Hmm, this location looks like it can just use strscpy().  
> > 
> > Yes strscpy() also works. But since stack_trace_filter_buf is length
> > bounded, shouldn't memcpy be the right choice?  
> 
> It's not only about the destination, but also about the source length.

Correct.

> 
> AFAIU, turning a strncpy into a memcpy here will overflow reading the
> input @str if the input string is smaller than
> sizeof(stack_trace_filter_buf) + len.

The old code just read str + len and what was after it until it hit a '\0'
or the COMMAND_LINE_SIZE limit.

memcpy() always reads COMMAND_LINE_SIZE (which is sizeof(stack_trace_filter_buf))
and will read more of the source "str" than may exist. Which as Mathieu
pointed out, is a bug.

-- Steve