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

[PATCH] 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..adebb92 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) + 1];
 	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: [PATCH] ftrace: drop a VLA in module_exists()

[Kees Cook]

On Tue, Mar 13, 2018 at 12:51 PM, Salvatore Mesoraca
<s.mesoraca16@gmail.com> wrote:
> 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..adebb92 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) + 1];

Actually, I think this needs to be "+ 2" (":" and NULL).

>         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);

Otherwise, looks good!

-Kees

-- 
Kees Cook
Pixel Security

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

[Salvatore Mesoraca]

2018-03-27 1:16 GMT+02:00 Kees Cook <keescook@chromium.org>:
> On Tue, Mar 13, 2018 at 12:51 PM, Salvatore Mesoraca
> <s.mesoraca16@gmail.com> wrote:
>> 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..adebb92 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) + 1];
>
> Actually, I think this needs to be "+ 2" (":" and NULL).

Ah, right! I'll fix it ASAP :)

>
>>         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);
>
> Otherwise, looks good!

Thank you!

Salvatore