trace: fix potenial dangerous pointer

The bdi_dev_name() returns a char [64], and
the __entry->name is a char [32].

It maybe dangerous to TP_printk("%s", __entry->name)
after the strncpy().

CC: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201124165205.GA23937@rlkAcked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Jan Kara <jack@suse.cz>

include/trace/events/writeback.h

@@ -190,7 +190,7 @@ TRACE_EVENT(inode_foreign_history,
 	),
 
 	TP_fast_assign(
-		strncpy(__entry->name, bdi_dev_name(inode_to_bdi(inode)), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(inode_to_bdi(inode)), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->cgroup_ino	= __trace_wbc_assign_cgroup(wbc);
 		__entry->history	= history;
@@ -219,7 +219,7 @@ TRACE_EVENT(inode_switch_wbs,
 	),
 
 	TP_fast_assign(
-		strncpy(__entry->name,	bdi_dev_name(old_wb->bdi), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(old_wb->bdi), 32);
 		__entry->ino		= inode->i_ino;
 		__entry->old_cgroup_ino	= __trace_wb_assign_cgroup(old_wb);
 		__entry->new_cgroup_ino	= __trace_wb_assign_cgroup(new_wb);
@@ -252,7 +252,7 @@ TRACE_EVENT(track_foreign_dirty,
 		struct address_space *mapping = page_mapping(page);
 		struct inode *inode = mapping ? mapping->host : NULL;
 
-		strncpy(__entry->name,	bdi_dev_name(wb->bdi), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
 		__entry->bdi_id		= wb->bdi->id;
 		__entry->ino		= inode ? inode->i_ino : 0;
 		__entry->memcg_id	= wb->memcg_css->id;
@@ -285,7 +285,7 @@ TRACE_EVENT(flush_foreign,
 	),
 
 	TP_fast_assign(
-		strncpy(__entry->name,	bdi_dev_name(wb->bdi), 32);
+		strscpy_pad(__entry->name, bdi_dev_name(wb->bdi), 32);
 		__entry->cgroup_ino	= __trace_wb_assign_cgroup(wb);
 		__entry->frn_bdi_id	= frn_bdi_id;
 		__entry->frn_memcg_id	= frn_memcg_id;