[Bug default/26646] unexpectedly declaration-only types

["dodji at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26646

--- Comment #10 from dodji at redhat dot com ---
Hello,

Here is my analysis on what is going on.

Let's look at the DWARF output and the abixml of the vmlinux3 binary.

From the abixml, we see that the struct ip_mc_list is used from the struct
in_device type.  For instance, the ip_mc_list::mc_list data member is of type
"pointer to decl-only struct ip_mc_list".

Moreover, the only use of struct ip_mc_list is through a "pointer to decl-only
struct ip_mc_list" use.

Let's look at the DWARF to see if we see the same.

[5ebb608]    structure_type       abbrev: 25
             name                 (strp) "in_device"
             byte_size            (data2) 352
             decl_file            (data1) inetdevice.h (122)
             decl_line            (data1) 25

This is the definition DIE of the struct in_device type.

[...]
     [5ebb641]      member               abbrev: 4
                   name                 (strp) "mc_list"
                   type                 (ref4) [5ebb7f7]
                   decl_file            (data1) inetdevice.h (122)
                   decl_line            (data1) 31
                   data_member_location (data1) 24

This is the definition of the "in_device::mc_list" data member.

Its type is the DIE [5ebb7f7] ...

[...]

[5ebb7f7]    pointer_type         abbrev: 5
             type                 (ref4) [5ebb7fc]

Here we see that the DIE [5ebb7f7] is a pointer ...

[5ebb7fc]    structure_type       abbrev: 12
            name                 (strp) "ip_mc_list"
            declaration          (flag_present) yes

... to a struct ip_mc_list that is declaration-only.

Looking through the DWARF, there no public exported function that refers
directly or indirectly to the full definition of ip_mc_list, as defined by this
DIE, for instance:

 [9b97e73]    structure_type       abbrev: 3
             name                 (strp) "ip_mc_list"
             byte_size            (data1) 144
             decl_file            (data1) igmp.h (228)
             decl_line            (data1) 67
[...]

So, this is why libabigail considers that the /definition/ of the struct
ip_mc_list is not used by any function that is publicly defined.  So, that
struct definition is not part of the ABI.

What am I missing?

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