linux-kernel - Re: [PATCH] ring buffer: propagate __rb_map

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Message-ID: <20251008121110.4e7c3671@gandalf.local.home>
Date: Wed, 8 Oct 2025 12:11:10 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 david.hunter.linux@...il.com, skhan@...uxfoundation.org,
 linux-kernel-mentees@...ts.linuxfoundation.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 syzbot+ddc001b92c083dbf2b97@...kaller.appspotmail.com
Subject: Re: [PATCH] ring buffer: propagate __rb_map_vma return value to
 caller

On Tue,  7 Oct 2025 22:42:56 +0530
Ankit Khushwaha <ankitkhushwaha.linux@...il.com> wrote:

Note, tracing subsystem expects the subject to start with a capital letter:

 ring buffer: Propagate __rb_map_vma return value to caller

> The return value from `__rb_map_vma()`, which rejects writable or
> executable mappings (VM_WRITE, VM_EXEC, or !VM_MAYSHARE), was being
> ignored. As a result the caller of `__rb_map_vma` always returned 0 
> even when the mapping had actually failed, allowing it to proceed
> with an invalid VMA.
> 
> Reported-by: syzbot+ddc001b92c083dbf2b97@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?id=194151be8eaebd826005329b2e123aecae714bdb
> Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@...il.com>
> ---
> 
> #syz test: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 4ff71af020ae
> 
> ---
>  kernel/trace/ring_buffer.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 43460949ad3f..4efb90364f48 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -7271,6 +7271,8 @@ int ring_buffer_map(struct trace_buffer *buffer, int cpu,
>  		cpu_buffer->subbuf_ids = NULL;
>  		rb_free_meta_page(cpu_buffer);
>  		atomic_dec(&cpu_buffer->resize_disabled);
> +		/* VM failed to be mapped */

No need to add the comment. It's obvious what happened.

> +		return err;

Don't return here.

>  	}
>  
>  	return 0;

Change this to:

	return err;

as after that if statement, err will be 0 on success or the value you want
to return.

-- Steve