[OpenWrt-Devel] [PATCH] [libubox] ulog: check if /dev/kmsg is special file

[John Crispin]

Hi,

On 16/08/2015 13:08, Sergiy Kibrik wrote:
> Race is possible in ulog_kmsg: if no /dev/kmsg exists
> (e.g. while /dev gets re-mounted) regular file created instead.
> From this point system goes without kernel logger:
> special character file can't be created anymore, all clients keep
> overwriting single message in regular file.
> 
> To avoid this we check if file we're going to write to is really
> character special and ensure it doesn't go away while we check.
> 
> Signed-off-by: Sergiy Kibrik <sakib at meta.ua>


the patch is quite invasive. how about just using "r+"

from the man page -->  "r+     Open for reading and writing.  The stream
is positioned at the beginning of the file."

could you try the following and not apply any other changes ?

	John

-	if ((kmsg = fopen("/dev/kmsg", "w")) != NULL) {
+	if ((kmsg = fopen("/dev/kmsg", "r+")) != NULL) {






> ---
>  ulog.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/ulog.c b/ulog.c
> index 776a0c4..10d61da 100644
> --- a/ulog.c
> +++ b/ulog.c
> @@ -23,6 +23,9 @@
>  #include <stdlib.h>
>  #include <unistd.h>
>  #include <string.h>
> +#include <sys/types.h>
> +#include <sys/stat.h>
> +#include <fcntl.h>
>  
>  static int _ulog_channels = -1;
>  static int _ulog_facility = -1;
> @@ -89,8 +92,18 @@ static void ulog_defaults(void)
>  static void ulog_kmsg(int priority, const char *fmt, va_list ap)
>  {
>  	FILE *kmsg;
> +	int fd;
> +	struct stat s;
> +
> +	/* extra checks not to accidentally create regular file */
> +	fd = open("/dev/kmsg", O_WRONLY);
> +	if (fd < 0)
> +		return;
> +
> +	if (fstat(fd, &s) == 0 &&
> +	    s.st_mode & S_IFCHR &&
> +	    (kmsg = fdopen(fd, "w")) != NULL) {
>  
> -	if ((kmsg = fopen("/dev/kmsg", "w")) != NULL) {
>  		fprintf(kmsg, "<%u>", priority);
>  
>  		if (_ulog_ident)
> @@ -99,6 +112,8 @@ static void ulog_kmsg(int priority, const char *fmt, va_list ap)
>  		vfprintf(kmsg, fmt, ap);
>  		fclose(kmsg);
>  	}
> +
> +	close(fd);
>  }
>  
>  static void ulog_stdio(int priority, const char *fmt, va_list ap)
> 
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