mount: Add mount warning for impending timestamp expiry

The warning reuses the uptime max of 30 years used by
settimeofday().

Note that the warning is only emitted for writable filesystem mounts
through the mount syscall. Automounts do not have the same warning.

Print out the warning in human readable format using the struct tm.
After discussion with Arnd Bergmann, we chose to print only the year number.
The raw s_time_max is also displayed, and the user can easily decode
it e.g. "date -u -d @$((0x7fffffff))". We did not want to consolidate
struct rtc_tm and struct tm just to print the date using a format specifier
as part of this series.
Given that the rtc_tm is not compiled on all architectures, this is not a
trivial patch. This can be added in the future.

Signed-off-by: Deepa Dinamani <[e-mail]>
Acked-by: Jeff Layton <[e-mail]>

fs/namespace.c

···
 	unlock_mount_hash();
 }
 
+static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
+{
+	struct super_block *sb = mnt->mnt_sb;
+
+	if (!__mnt_is_readonly(mnt) &&
+	   (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
+		char *buf = (char *)__get_free_page(GFP_KERNEL);
+		char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
+		struct tm tm;
+
+		time64_to_tm(sb->s_time_max, 0, &tm);
+
+		pr_warn("Mounted %s file system at %s supports timestamps until %04ld (0x%llx)\n",
+			sb->s_type->name, mntpath,
+			tm.tm_year+1900, (unsigned long long)sb->s_time_max);
+
+		free_page((unsigned long)buf);
+	}
+}
+
 /*
  * Handle reconfiguration of the mountpoint only without alteration of the
  * superblock it refers to.  This is triggered by specifying MS_REMOUNT|MS_BIND
···
 	if (ret == 0)
 		set_mount_attributes(mnt, mnt_flags);
 	up_write(&sb->s_umount);
+
+	mnt_warn_timestamp_expiry(path, &mnt->mnt);
+
 	return ret;
 }
 
···
 		}
 		up_write(&sb->s_umount);
 	}
+
+	mnt_warn_timestamp_expiry(path, &mnt->mnt);
+
 	put_fs_context(fc);
 	return err;
 }
···
 		return PTR_ERR(mnt);
 
 	error = do_add_mount(real_mount(mnt), mountpoint, mnt_flags);
-	if (error < 0)
+	if (error < 0) {
 		mntput(mnt);
+		return error;
+	}
+
+	mnt_warn_timestamp_expiry(mountpoint, mnt);
+
 	return error;
 }