[v2] ftrace: drop a VLA in module_exists()

[v2] ftrace: drop a VLA in module_exists()

[Salvatore Mesoraca]

Avoid a VLA[1] by using a real constant expression instead of a variable.
The compiler should be able to optimize the original code and avoid using
an actual VLA. Anyway this change is useful because it will avoid a false
positive with -Wvla, it might also help the compiler generating better
code.

[1] https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
---
 kernel/trace/ftrace.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index eac9ce2..16bbf06 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -3902,14 +3902,13 @@ static bool module_exists(const char *module)
 {
 	/* All modules have the symbol __this_module */
 	const char this_mod[] = "__this_module";
-	const int modname_size = MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 1;
-	char modname[modname_size + 1];
+	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
 	unsigned long val;
 	int n;
 
-	n = snprintf(modname, modname_size + 1, "%s:%s", module, this_mod);
+	n = snprintf(modname, sizeof(modname), "%s:%s", module, this_mod);
 
-	if (n > modname_size)
+	if (n > sizeof(modname) - 1)
 		return false;
 
 	val = module_kallsyms_lookup_name(modname);
-- 
1.9.1

Re: [v2] ftrace: drop a VLA in module_exists()

[Steven Rostedt]

On Fri, 30 Mar 2018 10:53:08 +0200
Salvatore Mesoraca <s.mesoraca16@gmail.com> wrote:

Couple of things. First, "PATCH" was dropped from the subject. If my
inbox was busy today, I probably would have missed this email.

> Avoid a VLA[1] by using a real constant expression instead of a variable.
> The compiler should be able to optimize the original code and avoid using
> an actual VLA. Anyway this change is useful because it will avoid a false
> positive with -Wvla, it might also help the compiler generating better
> code.
> 
> [1] https://lkml.org/lkml/2018/3/7/621

Second, do not use lkml.org, use http://lkml.kernel.org/r/<messageid>

Search for "Link:" tags in git log to see examples.

Other than that, the patch looks fine to me and I can pull it for the
next merge window.

-- Steve

> 
> Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
> ---
>  kernel/trace/ftrace.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index eac9ce2..16bbf06 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3902,14 +3902,13 @@ static bool module_exists(const char *module)
>  {
>  	/* All modules have the symbol __this_module */
>  	const char this_mod[] = "__this_module";
> -	const int modname_size = MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 1;
> -	char modname[modname_size + 1];
> +	char modname[MAX_PARAM_PREFIX_LEN + sizeof(this_mod) + 2];
>  	unsigned long val;
>  	int n;
>  
> -	n = snprintf(modname, modname_size + 1, "%s:%s", module, this_mod);
> +	n = snprintf(modname, sizeof(modname), "%s:%s", module, this_mod);
>  
> -	if (n > modname_size)
> +	if (n > sizeof(modname) - 1)
>  		return false;
>  
>  	val = module_kallsyms_lookup_name(modname);

Re: [v2] ftrace: drop a VLA in module_exists()

[Salvatore Mesoraca]

2018-03-30 16:34 GMT+02:00 Steven Rostedt <rostedt@goodmis.org>:
> On Fri, 30 Mar 2018 10:53:08 +0200
> Salvatore Mesoraca <s.mesoraca16@gmail.com> wrote:
>
> Couple of things. First, "PATCH" was dropped from the subject. If my
> inbox was busy today, I probably would have missed this email.

Aargh, I'm sorry for this, I didn't notice it.

>> Avoid a VLA[1] by using a real constant expression instead of a variable.
>> The compiler should be able to optimize the original code and avoid using
>> an actual VLA. Anyway this change is useful because it will avoid a false
>> positive with -Wvla, it might also help the compiler generating better
>> code.
>>
>> [1] https://lkml.org/lkml/2018/3/7/621
>
> Second, do not use lkml.org, use http://lkml.kernel.org/r/<messageid>
>
> Search for "Link:" tags in git log to see examples.

Thank you for the suggestion.

> Other than that, the patch looks fine to me and I can pull it for the
> next merge window.

Thank you very much,

Salvatore