[Bug default/29322] New: Missing type information for common symbols

[Bug default/29322] New: Missing type information for common symbols

[maennich at android dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=29322

            Bug ID: 29322
           Summary: Missing type information for common symbols
           Product: libabigail
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: default
          Assignee: dodji at redhat dot com
          Reporter: maennich at android dot com
                CC: libabigail at sourceware dot org
  Target Milestone: ---

Consider the following example

test.c
-------
   int arg_common1[2];
   int arg_common2[2];
-------

Compiled with 

  $ gcc -fcommon test.c -o test.o -c -g


abidw is not able to connect the DWARF type information with the symbols.

That is because of two issues:

The section index of the symbol is 'COM'

$ readelf -sW test.o

Symbol table '.symtab' contains 9 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS test.c
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    4 .debug_info
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    6 .debug_abbrev
     4: 0000000000000000     0 SECTION LOCAL  DEFAULT    9 .debug_line
     5: 0000000000000000     0 SECTION LOCAL  DEFAULT   11 .debug_str
     6: 0000000000000000     0 SECTION LOCAL  DEFAULT   12 .debug_line_str
     7: 0000000000000008     8 OBJECT  GLOBAL DEFAULT  COM arg_common1
     8: 0000000000000008     8 OBJECT  GLOBAL DEFAULT  COM arg_common2

That makes the lookup of the section via elfutils fail and the address
0000000000000008 is used.
The address as such can't be used as it might collide with non-common symbols
actually being defined at the same address.

But even then, the lookup of the symbol fails and is also ambiguous as common
symbols might be located at the same address. In order to address one would
need to make the symbol lookup also carry a name in the key. That is quite
intrusive for libabigail.

$ dwarfdump test.o

.debug_info

COMPILE_UNIT<header overall offset = 0x00000000>:
< 0><0x0000000c>  DW_TAG_compile_unit
                    DW_AT_producer              GNU C17 11.3.0 -mtune=generic
-march=x86-64 -g -fcommon -fasynchronous-unwind-tables
                    DW_AT_language              DW_LANG_C11
                    DW_AT_name                  test.c
                    DW_AT_comp_dir              /ws/contrib/libabigail
                    DW_AT_stmt_list             0x00000000

LOCAL_SYMBOLS:
< 1><0x0000001e>    DW_TAG_array_type
                      DW_AT_type                  <0x00000035>
                      DW_AT_sibling               <0x0000002e>
< 2><0x00000027>      DW_TAG_subrange_type
                        DW_AT_type                  <0x0000002e>
                        DW_AT_upper_bound           1
< 1><0x0000002e>    DW_TAG_base_type
                      DW_AT_byte_size             0x00000008
                      DW_AT_encoding              DW_ATE_unsigned
                      DW_AT_name                  long unsigned int
< 1><0x00000035>    DW_TAG_base_type
                      DW_AT_byte_size             0x00000004
                      DW_AT_encoding              DW_ATE_signed
                      DW_AT_name                  int
< 1><0x0000003c>    DW_TAG_variable
                      DW_AT_name                  arg_common1
                      DW_AT_decl_file             0x00000001
                      DW_AT_decl_line             0x00000001
                      DW_AT_decl_column           0x00000005
                      DW_AT_type                  <0x0000001e>
                      DW_AT_external              yes(1)
                      DW_AT_location              len 0x0009:
0x030800000000000000:
                          DW_OP_addr 0x00000008
< 1><0x00000050>    DW_TAG_variable
                      DW_AT_name                  arg_common2
                      DW_AT_decl_file             0x00000001
                      DW_AT_decl_line             0x00000002
                      DW_AT_decl_column           0x00000005
                      DW_AT_type                  <0x0000001e>
                      DW_AT_external              yes(1)
                      DW_AT_location              len 0x0009:
0x030800000000000000:
                          DW_OP_addr 0x00000008

Given that -fcommon is not a default flag, is rarely used (apparently) and
particularly not in any of my use cases, I do not intend to work on this. I
just reported the bug for posterity.

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