"args.retval" broken in kfuncs

[ajor]

Probe listing claims that k[ret]funcs have "retval" arguments:

# bpftrace -lv kfunc:finish_task_switch
kfunc:vmlinux:finish_task_switch
    struct task_struct * prev
    struct rq * retval
# bpftrace -lv kretfunc:finish_task_switch
kretfunc:vmlinux:finish_task_switch
    struct task_struct * prev
    struct rq * retval

But it's not possible to access them:

$ sudo bpftrace -e 'kfunc:finish_task_switch { print(args.retval) }'
stdin:1:28-39: ERROR: Can't find function parameter retval
kfunc:finish_task_switch { print(args.retval) }
                           ~~~~~~~~~~~
# bpftrace -e 'kretfunc:finish_task_switch { print(args.retval) }'
stdin:1:31-42: ERROR: Can't find function parameter retval
kretfunc:finish_task_switch { print(args.retval) }
                              ~~~~~~~~~~~

Printing the entire args struct shows no "retval" for kfuncs:

# bpftrace -e 'kfunc:finish_task_switch { print(args) }'
{ .prev = 0xffff88a086cc3280 }

But does for kretfuncs:

# bpftrace -e 'kretfunc:finish_task_switch { print(args) }'
{ .prev = 0xffff888107ac9940, .$retval = 0x0 }

[viktormalik]

IIRC, we use the retval builtin for these cases. But since retval is listed as a field in listing, I guess that we could support args.retval, too. Shouldn't be too hard to implement, we just need to handle the situation when the function has an argument called retval (should happen very rarely).

[ajor]

I think it'd also be fine to require use of the retval builtin and not allow args.retval to be used - it's just the inconsistencies between listing/field accesses/printing that need to be fixed.