Re: [PATCH] tracing: eprobe: read the complete FILTER_PTR_STRING pointer

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 15 Jun 2026 16:54:12 +0200
Martin Kaiser <martin@kaiser.cx> wrote:

> For a char * element in an event, the FILTER_PTR_STRING filter type is
> used. When the event occurs, a pointer is stored in the ringbuffer.
> 
> If an eprobe references such a char * element of a "base event" and
> decodes the pointer as string, the pointer cannot be dereferenced.
> 
> $ echo 'e syscalls.sys_enter_openat $filename:string' > \
> 		/sys/kernel/tracing/dynamic_events
> $ trace-cmd start -e eprobes
> $ trace-cmd show
>     ... : sys_enter_openat: (syscalls.sys_enter_openat) arg1=(fault)
> 
> The problem is in get_event_field
> 
> 	val = (unsigned long)(*(char *)addr);
> 
> addr points to the position in the ringbuffer where the pointer was
> stored. We must read the complete pointer, not just the lowest byte.
> 
> Fix the assignment, make the example above work.
> 

Ah, this is a bit complicated. It seems to work with sched_switch event
as commit f04dec93466a ("tracing/eprobes: Fix reading of string fields"):

echo 'e:sw sched/sched_switch comm=$next_comm:string' > dynamic_events

#           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
#              | |         |   |||||     |         |
              sh-162     [002] d..3.    54.027213: sw: (sched.sched_switch) comm="swapper/2"
          <idle>-0       [007] d..3.    54.034573: sw: (sched.sched_switch) comm="rcu_preempt"
     rcu_preempt-15      [007] d..3.    54.034589: sw: (sched.sched_switch) comm="swapper/7"

Maybe comm is stored as a fixed string information in the event record?

/sys/kernel/tracing # cat events/sched/sched_switch/format 
name: sched_switch
ID: 254
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:char prev_comm[16];	offset:8;	size:16;	signed:0;
	field:pid_t prev_pid;	offset:24;	size:4;	signed:1;
	field:int prev_prio;	offset:28;	size:4;	signed:1;
	field:long prev_state;	offset:32;	size:8;	signed:1;
	field:char next_comm[16];	offset:40;	size:16;	signed:0;
	field:pid_t next_pid;	offset:56;	size:4;	signed:1;
	field:int next_prio;	offset:60;	size:4;	signed:1;

But the filename is a pointer.

/sys/kernel/tracing # cat events/syscalls/sys_enter_openat/format 
name: sys_enter_openat
ID: 705
format:
	field:unsigned short common_type;	offset:0;	size:2;	signed:0;
	field:unsigned char common_flags;	offset:2;	size:1;	signed:0;
	field:unsigned char common_preempt_count;	offset:3;	size:1;	signed:0;
	field:int common_pid;	offset:4;	size:4;	signed:1;

	field:int __syscall_nr;	offset:8;	size:4;	signed:1;
	field:int dfd;	offset:16;	size:8;	signed:0;
	field:const char * filename;	offset:24;	size:8;	signed:0;
	field:int flags;	offset:32;	size:8;	signed:0;
	field:umode_t mode;	offset:40;	size:8;	signed:0;
	field:__data_loc char[] __filename_val;	offset:48;	size:4;	signed:0;

In this case, the filename field should use __data_loc directly instead of
pointing data on the ring buffer.

Can you try 

echo 'e syscalls.sys_enter_openat $__filename_val:string' > \
 		/sys/kernel/tracing/dynamic_events

Instead?

I think better solution is fixing sycall tracer.

Thanks,

-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>