On Thu, 6 Jun 2024 10:37:46 -0500
Yan Zhai [off-list ref] wrote:

quoted

name: kfree_skb
ID: 1799
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:void * skbaddr;   offset:8;       size:8; signed:0;
        field:void * location;  offset:16;      size:8; signed:0;
        field:unsigned short protocol;  offset:24;      size:2; signed:0;
        field:enum skb_drop_reason reason;      offset:28;      size:4; signed:0;

Notice that "protocol" is 2 bytes in size at offset 24, but "reason" starts
at offset 28. This means at offset 26, there's a 2 byte hole.
 

The reason I added the pointer as the last argument is trying to
minimize the surprise to existing TP users, because for common ABIs
it's fine to omit later arguments when defining a function, but it
needs change and recompilation if the order of arguments changed.

Nothing should be hard coding the offsets of the fields. This is
exported to user space so that tools can see where the fields are.
That's the purpose of libtraceevent. The fields should be movable and
not affect anything. There should be no need to recompile.

Looking at the actual format after the change, it does not add a new
hole since protocol and reason are already packed into the same 8-byte
block, so rx_skaddr starts at 8-byte aligned offset:

# cat /sys/kernel/debug/tracing/events/skb/kfree_skb/format
name: kfree_skb
ID: 2260
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:void * skbaddr;   offset:8;       size:8; signed:0;
        field:void * location;  offset:16;      size:8; signed:0;
        field:unsigned short protocol;  offset:24;      size:2; signed:0;
        field:enum skb_drop_reason reason;      offset:28;
size:4; signed:0;
        field:void * rx_skaddr; offset:32;      size:8; signed:0;

Do you think we still need to change the order?

Up to you, just wanted to point it out.

-- Steve