[PATCH v2] cgroup: align the comm length with TASK_COMM_LEN

[Kassey Li]

__string could get a dst string with length less than
TASK_COMM_LEN.

A task->comm may change that can cause out of bounds access
for the dst string buffer, e.g in the call trace of below:

Call trace:

    dump_backtrace.cfi_jt+0x0/0x4
    show_stack+0x14/0x1c
    dump_stack+0xa0/0xd8
    die_callback+0x248/0x24c
    notify_die+0x7c/0xf8
    die+0xac/0x290
    die_kernel_fault+0x88/0x98
    die_kernel_fault+0x0/0x98
    do_page_fault+0xa0/0x544
    do_mem_abort+0x60/0x10c
    el1_da+0x1c/0xc4
    trace_event_raw_event_cgroup_migrate+0x124/0x170
    cgroup_attach_task+0x2e8/0x41c
    __cgroup1_procs_write+0x114/0x1ec
    cgroup1_tasks_write+0x10/0x18
    cgroup_file_write+0xa4/0x208
    kernfs_fop_write+0x1f0/0x2f4
    __vfs_write+0x5c/0x200
    vfs_write+0xe0/0x1a0
    ksys_write+0x74/0xdc
    __arm64_sys_write+0x18/0x20
    el0_svc_common+0xc0/0x1a4
    el0_svc_compat_handler+0x18/0x20
    el0_svc_compat+0x8/0x2c

Change it as arrary with same length TASK_COMM_LEN,
This idea is from commit d1eb650ff413 ("tracepoint: Move signal sending
tracepoint to events/signal.h").

Signed-off-by: Kassey Li <quic_yingangl@xxxxxxxxxxx>
---
 include/trace/events/cgroup.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
index dd7d7c9efecd..b4ef0ffa38a4 100644
--- a/include/trace/events/cgroup.h
+++ b/include/trace/events/cgroup.h
@@ -130,7 +130,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
 		__field(	u64,		dst_id			)
 		__field(	int,		pid			)
 		__string(	dst_path,	path			)
-		__string(	comm,		task->comm		)
+		__array(char, comm, TASK_COMM_LEN)
 	),
 
 	TP_fast_assign(
@@ -139,12 +139,12 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
 		__entry->dst_level = dst_cgrp->level;
 		__assign_str(dst_path, path);
 		__entry->pid = task->pid;
-		__assign_str(comm, task->comm);
+		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
 	),
 
 	TP_printk("dst_root=%d dst_id=%llu dst_level=%d dst_path=%s pid=%d comm=%s",
 		  __entry->dst_root, __entry->dst_id, __entry->dst_level,
-		  __get_str(dst_path), __entry->pid, __get_str(comm))
+		  __get_str(dst_path), __entry->pid, __entry->comm)
 );
 
 DEFINE_EVENT(cgroup_migrate, cgroup_attach_task,
-- 
2.17.1