tracing: fprobe: do not zero out unused fgraph_data

[PATCH v2] tracing: fprobe: do not zero out unused fgraph_data

Posted by Martin Kaiser 1 week, 3 days ago

If fprobe_entry does not fill the allocated fgraph_data completely, the
unused part does not have to be zeroed.

fgraph_data is a short-lived part of the shadow stack. The preceding
length field allows locating the end regardless of the content.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
---
v2:
 - remove the memset instead of fixing the length

 kernel/trace/fprobe.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
index dcadf1d23b8a..56d145017902 100644
--- a/kernel/trace/fprobe.c
+++ b/kernel/trace/fprobe.c
@@ -450,8 +450,6 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
 				used += FPROBE_HEADER_SIZE_IN_LONG + size_words;
 		}
 	}
-	if (used < reserved_words)
-		memset(fgraph_data + used, 0, reserved_words - used);
 
 	/* If any exit_handler is set, data must be used. */
 	return used != 0;
-- 
2.43.7

Re: [PATCH v2] tracing: fprobe: do not zero out unused fgraph_data

Posted by Masami Hiramatsu (Google) 1 week, 2 days ago

On Tue, 24 Mar 2026 09:47:08 +0100
Martin Kaiser <martin@kaiser.cx> wrote:

> If fprobe_entry does not fill the allocated fgraph_data completely, the
> unused part does not have to be zeroed.
> 
> fgraph_data is a short-lived part of the shadow stack. The preceding
> length field allows locating the end regardless of the content.
> 
> Signed-off-by: Martin Kaiser <martin@kaiser.cx>

OK, thanks for update. Let me pick it.

Thanks,

> ---
> v2:
>  - remove the memset instead of fixing the length
> 
>  kernel/trace/fprobe.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index dcadf1d23b8a..56d145017902 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -450,8 +450,6 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
>  				used += FPROBE_HEADER_SIZE_IN_LONG + size_words;
>  		}
>  	}
> -	if (used < reserved_words)
> -		memset(fgraph_data + used, 0, reserved_words - used);
>  
>  	/* If any exit_handler is set, data must be used. */
>  	return used != 0;
> -- 
> 2.43.7
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH v2] tracing: fprobe: do not zero out unused fgraph_data

Posted by Steven Rostedt 1 week, 2 days ago

On Tue, 24 Mar 2026 09:47:08 +0100
Martin Kaiser <martin@kaiser.cx> wrote:

Hi Martin,

First, please do not send a v2 as a reply to v1. A new version should
always start a new mail thread.

> If fprobe_entry does not fill the allocated fgraph_data completely, the
> unused part does not have to be zeroed.
> 
> fgraph_data is a short-lived part of the shadow stack. The preceding
> length field allows locating the end regardless of the content.
> 
> Signed-off-by: Martin Kaiser <martin@kaiser.cx>
> ---
> v2:

But to maintain a link to the previous version, I recommend adding here:

Changes since v1: https://lore.kernel.org/all/20260324084804.375764-1-martin@kaiser.cx


>  - remove the memset instead of fixing the length
> 
>  kernel/trace/fprobe.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index dcadf1d23b8a..56d145017902 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -450,8 +450,6 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
>  				used += FPROBE_HEADER_SIZE_IN_LONG + size_words;
>  		}
>  	}
> -	if (used < reserved_words)
> -		memset(fgraph_data + used, 0, reserved_words - used);
>  
>  	/* If any exit_handler is set, data must be used. */
>  	return used != 0;

As for the patch,

Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

Re: [PATCH v2] tracing: fprobe: do not zero out unused fgraph_data

Posted by Masami Hiramatsu (Google) 1 week, 2 days ago

On Tue, 24 Mar 2026 08:39:03 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 24 Mar 2026 09:47:08 +0100
> Martin Kaiser <martin@kaiser.cx> wrote:
> 
> Hi Martin,
> 
> First, please do not send a v2 as a reply to v1. A new version should
> always start a new mail thread.
> 
> > If fprobe_entry does not fill the allocated fgraph_data completely, the
> > unused part does not have to be zeroed.
> > 
> > fgraph_data is a short-lived part of the shadow stack. The preceding
> > length field allows locating the end regardless of the content.
> > 
> > Signed-off-by: Martin Kaiser <martin@kaiser.cx>
> > ---
> > v2:
> 
> But to maintain a link to the previous version, I recommend adding here:
> 
> Changes since v1: https://lore.kernel.org/all/20260324084804.375764-1-martin@kaiser.cx

Ah, this is a good idea for the patch which does not have cover mail.

Thanks :)

> 
> 
> >  - remove the memset instead of fixing the length
> > 
> >  kernel/trace/fprobe.c | 2 --
> >  1 file changed, 2 deletions(-)
> > 
> > diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> > index dcadf1d23b8a..56d145017902 100644
> > --- a/kernel/trace/fprobe.c
> > +++ b/kernel/trace/fprobe.c
> > @@ -450,8 +450,6 @@ static int fprobe_fgraph_entry(struct ftrace_graph_ent *trace, struct fgraph_ops
> >  				used += FPROBE_HEADER_SIZE_IN_LONG + size_words;
> >  		}
> >  	}
> > -	if (used < reserved_words)
> > -		memset(fgraph_data + used, 0, reserved_words - used);
> >  
> >  	/* If any exit_handler is set, data must be used. */
> >  	return used != 0;
> 
> As for the patch,
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> 
> -- Steve
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>