acct: stop using get_seconds()

In 'struct acct', 'struct acct_v3', and 'struct taskstats' we have
a 32-bit 'ac_btime' field containing an absolute time value, which
will overflow in year 2106.

There are two possible ways to deal with it:

a) let it overflow and have user space code deal with reconstructing
   the data based on the current time, or
b) truncate the times based on the range of the u32 type.

Neither of them solves the actual problem. Pick the second
one to best document what the issue is, and have someone
fix it in a future version.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/uapi/linux/acct.h

@@ -49,6 +49,7 @@ struct acct
 	__u16		ac_uid16;		/* LSB of Real User ID */
 	__u16		ac_gid16;		/* LSB of Real Group ID */
 	__u16		ac_tty;			/* Control Terminal */
+	/* __u32 range means times from 1970 to 2106 */
 	__u32		ac_btime;		/* Process Creation Time */
 	comp_t		ac_utime;		/* User Time */
 	comp_t		ac_stime;		/* System Time */
@@ -81,6 +82,7 @@ struct acct_v3
 	__u32		ac_gid;			/* Real Group ID */
 	__u32		ac_pid;			/* Process ID */
 	__u32		ac_ppid;		/* Parent Process ID */
+	/* __u32 range means times from 1970 to 2106 */
 	__u32		ac_btime;		/* Process Creation Time */
 #ifdef __KERNEL__
 	__u32		ac_etime;		/* Elapsed Time */

include/uapi/linux/taskstats.h

@@ -112,6 +112,7 @@ struct taskstats {
 	__u32	ac_gid;			/* Group ID */
 	__u32	ac_pid;			/* Process ID */
 	__u32	ac_ppid;		/* Parent process ID */
+	/* __u32 range means times from 1970 to 2106 */
 	__u32	ac_btime;		/* Begin time [sec since 1970] */
 	__u64	ac_etime __attribute__((aligned(8)));
 					/* Elapsed time [usec] */

kernel/acct.c

@@ -416,6 +416,7 @@ static void fill_ac(acct_t *ac)
 {
 	struct pacct_struct *pacct = &current->signal->pacct;
 	u64 elapsed, run_time;
+	time64_t btime;
 	struct tty_struct *tty;
 
 	/*
@@ -448,7 +449,8 @@ static void fill_ac(acct_t *ac)
 	}
 #endif
 	do_div(elapsed, AHZ);
-	ac->ac_btime = get_seconds() - elapsed;
+	btime = ktime_get_real_seconds() - elapsed;
+	ac->ac_btime = clamp_t(time64_t, btime, 0, U32_MAX);
 #if ACCT_VERSION==2
 	ac->ac_ahz = AHZ;
 #endif

kernel/tsacct.c

@@ -24,6 +24,7 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	const struct cred *tcred;
 	u64 utime, stime, utimescaled, stimescaled;
 	u64 delta;
+	time64_t btime;
 
 	BUILD_BUG_ON(TS_COMM_LEN < TASK_COMM_LEN);
 
@@ -32,9 +33,10 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	/* Convert to micro seconds */
 	do_div(delta, NSEC_PER_USEC);
 	stats->ac_etime = delta;
-	/* Convert to seconds for btime */
-	do_div(delta, USEC_PER_SEC);
-	stats->ac_btime = get_seconds() - delta;
+	/* Convert to seconds for btime (note y2106 limit) */
+	btime = ktime_get_real_seconds() - div_u64(delta, USEC_PER_SEC);
+	stats->ac_btime = clamp_t(time64_t, btime, 0, U32_MAX);
+
 	if (thread_group_leader(tsk)) {
 		stats->ac_exitcode = tsk->exit_code;
 		if (tsk->flags & PF_FORKNOEXEC)