2018-03-13 19:54:18

by Salvatore Mesoraca

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

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 <[email protected]>
---
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

2018-03-26 23:17:33

by Kees Cook

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

On Tue, Mar 13, 2018 at 12:51 PM, Salvatore Mesoraca
<[email protected]> 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 <[email protected]>
> ---
> 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

2018-03-28 10:58:06

by Salvatore Mesoraca

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

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