Re: Encoding BTF information from DWARF causes "has void type" error.

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Tue, Feb 06, 2024 at 01:02:26PM +0100, Sebastian Andrzej Siewior wrote:
> with linux kernel v6.8-rc2-RT and the following file as testcase
> 
> ----->8--------
> 
> #include <linux/spinlock.h>
> #include <linux/local_lock.h>
> 
> struct per_cpu_struct {
>         local_lock_t lock;
>         unsigned int something;
> };
> 
> static DEFINE_PER_CPU(struct per_cpu_struct, per_cpu_struct) = {
>         .lock = INIT_LOCAL_LOCK(lock),
> };
> 
> DEFINE_GUARD(ll_lock, local_lock_t __percpu*,
>              local_lock(_T),
>              local_unlock(_T))
> 
> void function(void);
> void function(void)
> {
>         struct per_cpu_struct *pcs = this_cpu_ptr(&per_cpu_struct);
> 
>         guard(ll_lock)(&per_cpu_struct.lock);
>         pcs->something++;
> }
> 
> -----8<--------
> 
> compiling and running pahole afterwards:
> | make kernel/pahole-tc.o && pahole -J --btf_gen_floats -j --lang_exclude=rust  kernel/pahole-tc.o
> |   CC      kernel/pahole-tc.o
> | error: found variable 'per_cpu_struct' in CU 'kernel/pahole-tc.c' that has void type
> 
> This doesn't look good. If I swap the order of "lock" and "something"
> within per_cpu_struct then it goes away.
> The dwarf/die object it complains about has only DW_AT_abstract_origin
> and DW_AT_location set.
> At this point I am not sure if gcc wrongly created the dwarf information
> or of pahole accidentally matches the internal local_lock_t member as
> per_cpu_struct because it starts the same address.
> 
> >From dumpwarf the problematic entry is:
> | 0x0000085b:       DW_TAG_variable
> |                     DW_AT_abstract_origin       (0x0000099e "t")
> |                     DW_AT_location      (DW_OP_addr 0x0, DW_OP_stack_value)
> 
> which looks slightly different in the good case:
> | 0x00000852:         DW_TAG_variable
> |                       DW_AT_abstract_origin     (0x00000980 "t")
> |                       DW_AT_location    (0x00000040: 
> |                          [0x0000000000000046, 0x000000000000005b): DW_OP_reg3 RBX
> |                          [0x000000000000005b, 0x0000000000000061): DW_OP_addr 0x0, DW_OP_stack_value)
> |                       DW_AT_GNU_locviews        (0x0000003c)
> 
> I made an archive with c file, the compiled version in bad and good case
> (swapped the first two members), the dumpwarf output and uploaded to
> 	https://breakpoint.cc/pahole-tc.tar.xz

Lots of great info, I'm on it.

- Arnaldo