linux-kernel - Re: [PATCH] tracing/user_events: Fix an erroneous usage of struct

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Date:   Mon, 21 Aug 2023 12:07:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/user_events: Fix an erroneous usage of
 struct_size()

On Sun, 20 Aug 2023 17:02:42 +0200
Christophe JAILLET <christophe.jaillet@...adoo.fr> wrote:

> If struct_size() returns a value that does not fit in a 'int', the size
> passed to kzalloc() is wrong.
> 
> Remove the intermediate 'size' variable and use struct_size() directly.
> 
> Fixes: 7f5a08c79df3 ("user_events: Add minimal support for trace_event into ftrace")
> Signed-off-by: Christophe JAILLET <christophe.jaillet@...adoo.fr>
> ---
> I don't know if 'size' can get bigger than a int in the real world, but the
> change looks safe in any cases.
> 
> On x86_64, looking at the .s files, the previous code had an extra:
>     movslq	%r13d, %r13
> which really looks wrong to me.

If size is bigger than int, then we have much bigger problems than this allocation.

That means count is over 2 billion, and the kzalloc() will fail regardless.

This is an unneeded change.

-- Steve


> ---
>  kernel/trace/trace_events_user.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c
> index 33cb6af31f39..67cc71a872b0 100644
> --- a/kernel/trace/trace_events_user.c
> +++ b/kernel/trace/trace_events_user.c
> @@ -2153,7 +2153,7 @@ static int user_events_ref_add(struct user_event_file_info *info,
>  {
>  	struct user_event_group *group = info->group;
>  	struct user_event_refs *refs, *new_refs;
> -	int i, size, count = 0;
> +	int i, count = 0;
>  
>  	refs = rcu_dereference_protected(info->refs,
>  					 lockdep_is_held(&group->reg_mutex));
> @@ -2166,10 +2166,8 @@ static int user_events_ref_add(struct user_event_file_info *info,
>  				return i;
>  	}
>  
> -	size = struct_size(refs, events, count + 1);
> -
> -	new_refs = kzalloc(size, GFP_KERNEL_ACCOUNT);
> -
> +	new_refs = kzalloc(struct_size(refs, events, count + 1),
> +			   GFP_KERNEL_ACCOUNT);
>  	if (!new_refs)
>  		return -ENOMEM;
>