[alsa-devel] [PATCH] ALSA: Use scnprintf() instead of snprintf()

[Takashi Sakamoto]

Hi,

On Feb 27 2018 11:15, Jaejoong Kim wrote:
> The show() method should use scnprintf() not snprintf() because snprintf()
> may returns a value that exceeds its second argument.
> 
> Signed-off-by: Jaejoong Kim <climbbb.kim at gmail.com>
> ---
>   sound/core/init.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/sound/core/init.c b/sound/core/init.c
> index 168ae03..4a51a06 100644
> --- a/sound/core/init.c
> +++ b/sound/core/init.c
> @@ -670,7 +670,7 @@ card_id_show_attr(struct device *dev,
>   		  struct device_attribute *attr, char *buf)
>   {
>   	struct snd_card *card = container_of(dev, struct snd_card, card_dev);
> -	return snprintf(buf, PAGE_SIZE, "%s\n", card->id);
> +	return scnprintf(buf, PAGE_SIZE, "%s\n", card->id);
>   }
>   
>   static ssize_t
> @@ -710,7 +710,7 @@ card_number_show_attr(struct device *dev,
>   		     struct device_attribute *attr, char *buf)
>   {
>   	struct snd_card *card = container_of(dev, struct snd_card, card_dev);
> -	return snprintf(buf, PAGE_SIZE, "%i\n", card->number);
> +	return scnprintf(buf, PAGE_SIZE, "%i\n", card->number);
>   }
>   
>   static DEVICE_ATTR(number, S_IRUGO, card_number_show_attr, NULL);

In my opinion, original codes are good because it has a trailing '\n'
instead of '\0'.

We can see below explanation in 'Documentation/filesystems
/sysfs.txt'[1]:

```
- show() methods should return the number of bytes printed into the
   buffer. This is the return value of scnprintf().

- show() must not use snprintf() when formatting the value to be
   returned to user space. If you can guarantee that an overflow
   will never happen you can use sprintf() otherwise you must use
   scnprintf().
```

Actually, 'snprintf()' returns written bytes excluding trailing _null_
byte for end output to strings and we need to use 'scnprintf()'.

However, for our cases, the trailing letter is '\n' and 'snprintf()'
counts correctly including it.

Let's test:

```
#include <stdio.h>
#include <stdlib.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <unistd.h>

#include <string.h>
#include <errno.h>

int main(int argc, char **argv)
{
     int fd;
     char buf[8];
     ssize_t len;

     if (argc < 2) {
         printf("At least, one argument is required for path to sysfs 
node.\n");
         return EXIT_FAILURE;
     }

     fd = open(*(argv + 1), O_RDONLY);
     if (fd < 0) {
         printf("open(2): %s\n", strerror(errno));
         return EXIT_FAILURE;
     }

     // 'C' is a canary.
     memset(buf, 'C', sizeof(buf));

     len = read(fd, buf, sizeof(buf));
     if (len < 0) {
         printf("read(2): %s\n", strerror(errno));
         close(fd);
         return EXIT_FAILURE;
     }

     printf("len = %zd\n", len);
     for (int i = 0; i < sizeof(buf); ++i) {
             printf("%02x\n", buf[i]);
     }

     close(fd);

     return EXIT_SUCCESS;
}
```

```
$ cat /proc/asound/version
Advanced Linux Sound Architecture Driver Version k4.13.0-36-generic.
$ /tmp/test 
'/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/fw1/fw1.0/sound/card2/number'
len = 2
32
0a
43
43
43
43
43
43
$ /tmp/test 
'/sys/devices/pci0000:00/0000:00:01.0/0000:01:00.0/0000:02:00.0/fw1/fw1.0/sound/card2/id'
len = 5
53
6f
6c
6f
0a
43
43
43
```

Looks good.


[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/sysfs.txt


Thanks

Takashi Sakamoto