Re: [PATCH] time/fs - file's time race with vgettimeofday

[Oleg Nesterov]

In no way I can review this patch, but I am curious and have the questions.
Also, I think it makes sense to cc the fs/ developers, I've added Al.

On 07/02, Jiri Olsa wrote:
>
> there's a race among calling gettimeofday(2) and a file's time
> updates.  Following test program expose the race.
>
> run it in the while loop
> 	while [ 1 ]; do ./test1 || break; done
>
> --- SNIP ---
> #include <stdio.h>
> #include <stdlib.h>
> #include <fcntl.h>
>
> int main (void)
> {
> 	struct stat st;
> 	struct timeval tv;
>
> 	unlink("./file");
>
> 	gettimeofday(&tv, NULL);
>
> 	if (-1 == creat("./file", O_RDWR)) {
> 		perror("creat");
> 		return -1;
> 	}
>
> 	if (stat("./file", &st) != 0) {
> 		perror("stat");
> 		return -1;
> 	}
>
> 	printf("USER gtod: %ld\n", (long)tv.tv_sec);
> 	printf("USER file: %ld.%09u\n",
> 			(long) st.st_mtime,
> 			(unsigned) st.st_mtim.tv_nsec);
>
> 	return tv.tv_sec <= st.st_mtime ? 0 : -1;
> }

Interesting. To the point, I actually compiled this code and yes,
it triggers the problem on ext3 ;)

> The following patch will prevent the race by adding the
> CURRENT_TIME_SEC_REAL macro, which will return seconds from
> the getnstimeofday call, ensuring it's computed on current tick.
> It fixes the 'creat' case for ext4.

What about other filesystems? Perhaps it makes sense to change
CURRENT_TIME_SEC instead of adding CURRENT_TIME_SEC_REAL?

Once again, I am asking. It is not that I suggest to change your patch.



But there is something I can't understand.

We have

	#define CURRENT_TIME_SEC	((struct timespec) { get_seconds(), 0 })

and your patch adds

	#define CURRENT_TIME_SEC_REAL	((struct timespec) { get_seconds_real(), 0 })

which fixes the problem for ext4.

But, get_seconds() is also used by sys_time(), and we should have the
same problem with another trivial test-case:

	#include <stdio.h>
	#include <sys/time.h>
	#include <time.h>

	int main(void)
	{
		struct timeval tv;
		int sec;

		do {
			gettimeofday(&tv, NULL);
			sec = time(NULL);
		} while (tv.tv_sec <= sec);

		printf("gtod: %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
		printf("time: %d.000000\n", sec);
		return 0;
	}

However, this test-case can't trigger the problem. Confused.

Oleg.

>
> I'm not sure how much trouble is having this race unfixed compared
> to the performance impact the fix might have. Maybe there're
> better ways to fix this.
>
>
> thanks for any ideas,
> jirka
>
>
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> diff --git a/fs/ext2/ialloc.c b/fs/ext2/ialloc.c
> index 938dbc7..7a0a2fc 100644
> --- a/fs/ext2/ialloc.c
> +++ b/fs/ext2/ialloc.c
> @@ -558,7 +558,7 @@ got:
>
>  	inode->i_ino = ino;
>  	inode->i_blocks = 0;
> -	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC;
> +	inode->i_mtime = inode->i_atime = inode->i_ctime = CURRENT_TIME_SEC_REAL;
>  	memset(ei->i_data, 0, sizeof(ei->i_data));
>  	ei->i_flags =
>  		ext2_mask_flags(mode, EXT2_I(dir)->i_flags & EXT2_FL_INHERITED);
> diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> index 19a4de5..2c2925c 100644
> --- a/fs/ext4/ext4.h
> +++ b/fs/ext4/ext4.h
> @@ -1157,7 +1157,7 @@ static inline struct ext4_inode_info *EXT4_I(struct inode *inode)
>  static inline struct timespec ext4_current_time(struct inode *inode)
>  {
>  	return (inode->i_sb->s_time_gran < NSEC_PER_SEC) ?
> -		current_fs_time(inode->i_sb) : CURRENT_TIME_SEC;
> +		current_fs_time(inode->i_sb) : CURRENT_TIME_SEC_REAL;
>  }
>
>  static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
> diff --git a/include/linux/time.h b/include/linux/time.h
> index ea3559f..f390687 100644
> --- a/include/linux/time.h
> +++ b/include/linux/time.h
> @@ -109,12 +109,14 @@ void timekeeping_init(void);
>  extern int timekeeping_suspended;
>
>  unsigned long get_seconds(void);
> +unsigned long get_seconds_real(void);
>  struct timespec current_kernel_time(void);
>  struct timespec __current_kernel_time(void); /* does not hold xtime_lock */
>  struct timespec get_monotonic_coarse(void);
>
>  #define CURRENT_TIME		(current_kernel_time())
>  #define CURRENT_TIME_SEC	((struct timespec) { get_seconds(), 0 })
> +#define CURRENT_TIME_SEC_REAL	((struct timespec) { get_seconds_real(), 0 })
>
>  /* Some architectures do not supply their own clocksource.
>   * This is mainly the case in architectures that get their
> diff --git a/kernel/time.c b/kernel/time.c
> index 848b1c2..ce10dae 100644
> --- a/kernel/time.c
> +++ b/kernel/time.c
> @@ -227,7 +227,8 @@ SYSCALL_DEFINE1(adjtimex, struct timex __user *, txc_p)
>   */
>  struct timespec current_fs_time(struct super_block *sb)
>  {
> -	struct timespec now = current_kernel_time();
> +	struct timespec now;
> +	getnstimeofday(&now);
>  	return timespec_trunc(now, sb->s_time_gran);
>  }
>  EXPORT_SYMBOL(current_fs_time);
> diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
> index caf8d4d..7ebfe23 100644
> --- a/kernel/time/timekeeping.c
> +++ b/kernel/time/timekeeping.c
> @@ -897,6 +897,14 @@ unsigned long get_seconds(void)
>  }
>  EXPORT_SYMBOL(get_seconds);
>
> +unsigned long get_seconds_real(void)
> +{
> +	struct timespec ts;
> +	getnstimeofday(&ts);
> +	return ts.tv_sec;
> +}
> +EXPORT_SYMBOL(get_seconds_real);
> +
>  struct timespec __current_kernel_time(void)
>  {
>  	return xtime;

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