Re: [PATCH dwarves v3] dwarf_loader: Fix skipped encoding of function BTF on 32-bit systems

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jun 30, 2025 at 11:01:19AM +0100, Alan Maguire wrote:
> On 24/06/2025 17:14, Alan Maguire wrote:
> > On 22/05/2025 07:37, Tony Ambardar wrote:
> >> I encountered an issue building BTF kernels for 32-bit armhf, where many
> >> functions are missing in BTF data:
> >>
> >>   LD      vmlinux
> >>   BTFIDS  vmlinux
> >> WARN: resolve_btfids: unresolved symbol vfs_truncate
> >> WARN: resolve_btfids: unresolved symbol vfs_fallocate
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_select_cpu_dfl
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_pick_idle_cpu_node
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_pick_idle_cpu
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_pick_any_cpu_node
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_pick_any_cpu
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_kick_cpu
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_exit_bstr
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_nr_queued
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move_vtime
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move_to_local
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_move
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_insert_vtime
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dsq_insert
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_vtime_from_dsq
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_vtime
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq_set_vtime
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq_set_slice
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch_from_dsq
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_dispatch
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_destroy_dsq
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_create_dsq
> >> WARN: resolve_btfids: unresolved symbol scx_bpf_consume
> >> WARN: resolve_btfids: unresolved symbol bpf_throw
> >> WARN: resolve_btfids: unresolved symbol bpf_sock_ops_enable_tx_tstamp
> >> WARN: resolve_btfids: unresolved symbol bpf_percpu_obj_new_impl
> >> WARN: resolve_btfids: unresolved symbol bpf_obj_new_impl
> >> WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
> >> WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
> >> WARN: resolve_btfids: unresolved symbol bpf_iter_task_vma_new
> >> WARN: resolve_btfids: unresolved symbol bpf_iter_scx_dsq_new
> >> WARN: resolve_btfids: unresolved symbol bpf_get_kmem_cache
> >> WARN: resolve_btfids: unresolved symbol bpf_dynptr_from_xdp
> >> WARN: resolve_btfids: unresolved symbol bpf_dynptr_from_skb
> >> WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
> >>   NM      System.map
> >>
> >> After further debugging this can be reproduced more simply:
> >>
> >> $ pahole -J -j --btf_features=decl_tag,consistent_func,decl_tag_kfuncs .tmp_vmlinux_armhf
> >> btf_encoder__tag_kfunc: failed to find kfunc 'scx_bpf_select_cpu_dfl' in BTF
> >> btf_encoder__tag_kfuncs: failed to tag kfunc 'scx_bpf_select_cpu_dfl'
> >>
> >> $ pfunct -Fbtf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
> >> <nothing>
> >>
> >> $ pfunct -Fdwarf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
> >> s32 scx_bpf_select_cpu_dfl(struct task_struct * p, s32 prev_cpu, u64 wake_flags, bool * is_idle);
> >>
> >> $ pahole -J -j --btf_features=decl_tag,decl_tag_kfuncs .tmp_vmlinux_armhf
> >>
> >> $ pfunct -Fbtf -E -f scx_bpf_select_cpu_dfl .tmp_vmlinux_armhf
> >> bpf_kfunc s32 scx_bpf_select_cpu_dfl(struct task_struct * p, s32 prev_cpu, u64 wake_flags, bool * is_idle);
> >>
> >> The key things to note are the pahole 'consistent_func' feature and the u64
> >> 'wake_flags' parameter vs. arm 32-bit registers. These point to existing
> >> code handling arguments larger than register-size, allowing them to be
> >> BTF encoded but only if structs.
> >>
> >> Generalize the code for any argument type larger than register size (i.e.
> >> size > cu->addr_size). This should work for integral or aggregate types,
> >> and also avoids a bug in the current code where a register-sized struct
> >> could be mistaken for larger. Note that zero-sized arguments will still
> >> be marked as inconsistent and not encoded.
> >>
> >> Fixes: a53c58158b76 ("dwarf_loader: Mark functions that do not use expected registers for params")
> >> Tested-by: Alexis Lothoré <alexis.lothore@bootlin.com>
> >> Tested-by: Alan Maguire <alan.maguire@oracle.com>
> >> Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
> > 
> > hi Tony,
> > 
> > I'm planning on landing this shortly unless anyone objects; and on that
> > topic if anyone has the cycles to test with this patch that would be
> > great! I ran it through the work-in-progress BTF comparison in github CI
> > and all looks good; see the "Compare functions generated" step in [1].
> > 
> > Thanks!
> >
> 
> In fact I spoke too soon; there was a bug in the function comparison.
> After that was fixed, I reran with this patch; see [1].
> 
> It shows that - as expected - functions with 0-sized params are left
> out, specifically
> 
> < int __io_run_local_work(struct io_ring_ctx * ctx, io_tw_token_t tw,
> int min_events, int max_events);
> < int __io_run_local_work_loop(struct llist_node * * node, io_tw_token_t
> tw, int events);
> 
> We expect this since io_tw_token_t is 0-sized. However on x86_64 it did
> show one _extra_ function that I didn't expect:
> 
> > int __vxlan_fdb_delete(struct vxlan_dev * vxlan, const unsigned char
> * addr, union vxlan_addr ip, __be16 port, __be32 src_vni, __be32 vni,
> u32 ifindex, bool swdev_notify);
> 
> It's not clear to me why that function was added with this change - I
> would have expected it either with or without the change. Any idea why
> that might be?

hi,
I can see that as well, IIUC the 'ip' argument is:

union vxlan_addr {
        struct sockaddr_in sin;
        struct sockaddr_in6 sin6;
        struct sockaddr sa;
};

so we have struct as 4th argument, which sets the has_wide_param condition
and won't set the fn->proto.unexpected_reg for the function, because of:

   if (!has_wide_param)
      fn->proto.unexpected_reg = 1;

I'm not sure it's correct.. if the ip struct is big enough that it's passed
on stack, why are the rest of the arguments marked with unexpected_reg
(in parameter__new) I think I'm missing something

jirka


> 
> [1]
> https://github.com/alan-maguire/dwarves/actions/runs/15872520906/job/44752273776
> 
> > Alan
> > 
> > [1] https://github.com/alan-maguire/dwarves/actions/runs/15854137212
> > 
> >> ---
> >> v2 -> v3:
> >>  - Added Tested-by: from Alexis and Alan.
> >>  - Revert support for encoding 0-sized structs (as v1) after discussion:
> >>    https://lore.kernel.org/dwarves/9a41b21f-c0ae-4298-bf95-09d0cdc3f3ab@oracle.com/
> >>  - Inline param__is_wide() and clarify some naming/wording.
> >>
> >> v1 -> v2:
> >>  - Update to preserve existing behaviour where zero-sized struct params
> >>    still permit the function to be encoded, as noted by Alan.
> >>
> >> ---
> >>  dwarf_loader.c | 37 ++++++++++++-------------------------
> >>  1 file changed, 12 insertions(+), 25 deletions(-)
> >>
> >> diff --git a/dwarf_loader.c b/dwarf_loader.c
> >> index e1ba7bc..134a76b 100644
> >> --- a/dwarf_loader.c
> >> +++ b/dwarf_loader.c
> >> @@ -2914,23 +2914,9 @@ out:
> >>  	return 0;
> >>  }
> >>  
> >> -static bool param__is_struct(struct cu *cu, struct tag *tag)
> >> +static inline bool param__is_wide(struct cu *cu, struct tag *tag)
> >>  {
> >> -	struct tag *type = cu__type(cu, tag->type);
> >> -
> >> -	if (!type)
> >> -		return false;
> >> -
> >> -	switch (type->tag) {
> >> -	case DW_TAG_structure_type:
> >> -		return true;
> >> -	case DW_TAG_const_type:
> >> -	case DW_TAG_typedef:
> >> -		/* handle "typedef struct", const parameter */
> >> -		return param__is_struct(cu, type);
> >> -	default:
> >> -		return false;
> >> -	}
> >> +	return tag__size(tag, cu) > cu->addr_size;
> >>  }
> >>  
> >>  static int cu__resolve_func_ret_types_optimized(struct cu *cu)
> >> @@ -2942,9 +2928,9 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
> >>  		struct tag *tag = pt->entries[i];
> >>  		struct parameter *pos;
> >>  		struct function *fn = tag__function(tag);
> >> -		bool has_unexpected_reg = false, has_struct_param = false;
> >> +		bool has_unexpected_reg = false, has_wide_param = false;
> >>  
> >> -		/* mark function as optimized if parameter is, or
> >> +		/* Mark function as optimized if parameter is, or
> >>  		 * if parameter does not have a location; at this
> >>  		 * point location presence has been marked in
> >>  		 * abstract origins for cases where a parameter
> >> @@ -2953,10 +2939,11 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
> >>  		 *
> >>  		 * Also mark functions which, due to optimization,
> >>  		 * use an unexpected register for a parameter.
> >> -		 * Exception is functions which have a struct
> >> -		 * as a parameter, as multiple registers may
> >> -		 * be used to represent it, throwing off register
> >> -		 * to parameter mapping.
> >> +		 * Exception is functions with a wide parameter,
> >> +		 * as single register won't be used to represent
> >> +		 * it, throwing off register to parameter mapping.
> >> +		 * Examples include large structs or 64-bit types
> >> +		 * on a 32-bit arch.
> >>  		 */
> >>  		ftype__for_each_parameter(&fn->proto, pos) {
> >>  			if (pos->optimized || !pos->has_loc)
> >> @@ -2967,11 +2954,11 @@ static int cu__resolve_func_ret_types_optimized(struct cu *cu)
> >>  		}
> >>  		if (has_unexpected_reg) {
> >>  			ftype__for_each_parameter(&fn->proto, pos) {
> >> -				has_struct_param = param__is_struct(cu, &pos->tag);
> >> -				if (has_struct_param)
> >> +				has_wide_param = param__is_wide(cu, &pos->tag);
> >> +				if (has_wide_param)
> >>  					break;
> >>  			}
> >> -			if (!has_struct_param)
> >> +			if (!has_wide_param)
> >>  				fn->proto.unexpected_reg = 1;
> >>  		}
> >>  
> > 
> > 
> 
>