The following commands:
# echo 'read_max u64 size;' > synthetic_events
# echo 'hist:keys=common_pid:count=count:onmax($count).trace(read_max,count)' > events/syscalls/sys_enter_read/trigger
Causes:
BUG: kernel NULL pointer dereference, address:
0000000000000000
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP
CPU: 4 PID: 1763 Comm: bash Not tainted 5.14.0-rc2-test+ #155
Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01
v03.03 07/14/2016
RIP: 0010:strcmp+0xc/0x20
Code: 75 f7 31 c0 0f b6 0c 06 88 0c 02 48 83 c0 01 84 c9 75 f1 4c 89 c0
c3 0f 1f 80 00 00 00 00 31 c0 eb 08 48 83 c0 01 84 d2 74 0f <0f> b6 14 07
3a 14 06 74 ef 19 c0 83 c8 01 c3 31 c0 c3 66 90 48 89
RSP: 0018:
ffffb5fdc0963ca8 EFLAGS:
00010246
RAX:
0000000000000000 RBX:
ffffffffb3a4e040 RCX:
0000000000000000
RDX:
0000000000000000 RSI:
ffff9714c0d0b640 RDI:
0000000000000000
RBP:
0000000000000000 R08:
00000022986b7cde R09:
ffffffffb3a4dff8
R10:
0000000000000000 R11:
0000000000000000 R12:
ffff9714c50603c8
R13:
0000000000000000 R14:
ffff97143fdf9e48 R15:
ffff9714c01a2210
FS:
00007f1fa6785740(0000) GS:
ffff9714da400000(0000)
knlGS:
0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0:
0000000080050033
CR2:
0000000000000000 CR3:
000000002d863004 CR4:
00000000001706e0
Call Trace:
__find_event_file+0x4e/0x80
action_create+0x6b7/0xeb0
? kstrdup+0x44/0x60
event_hist_trigger_func+0x1a07/0x2130
trigger_process_regex+0xbd/0x110
event_trigger_write+0x71/0xd0
vfs_write+0xe9/0x310
ksys_write+0x68/0xe0
do_syscall_64+0x3b/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f1fa6879e87
The problem was the "trace(read_max,count)" where the "count" should be
"$count" as "onmax()" only handles variables (although it really should be
able to figure out that "count" is a field of sys_enter_read). But there's
a path that does not find the variable and ends up passing a NULL for the
event, which ends up getting passed to "strcmp()".
Add a check for NULL to return and error on the command with:
# cat error_log
hist:syscalls:sys_enter_read: error: Couldn't create or find variable
Command: hist:keys=common_pid:count=count:onmax($count).trace(read_max,count)
^
Link: https://lkml.kernel.org/r/20210808003011.4037f8d0@oasis.local.home
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 6c3959f282c95 tracing: Add 'onmax' hist trigger action support
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>