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

[Salvatore Mesoraca]

2018-03-27 1:16 GMT+02:00 Kees Cook :
> On Tue, Mar 13, 2018 at 12:51 PM, Salvatore Mesoraca
>  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 
>> ---
>>  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

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

[Kees Cook]

On Tue, Mar 13, 2018 at 12:51 PM, Salvatore Mesoraca
 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 
> ---
>  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

[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 
---
 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