[PATCH][gdb/symtab] Handle PU without import in "save gdb-index"

[Tom de Vries]

On 07-04-2020 11:46, Tom de Vries wrote:
> Hi,
> 
> Consider the test-case added in this patch, with resulting dwarf:
> ...
>   Compilation Unit @ offset 0xc7:
>    Length:        0x2c (32-bit)
>    Version:       4
>    Abbrev Offset: 0x64
>    Pointer Size:  8
>  <0><d2>: Abbrev Number: 2 (DW_TAG_partial_unit)
>     <d3>   DW_AT_language    : 2        (non-ANSI C)
>     <d4>   DW_AT_name        : imported_unit.c
>  <1><e4>: Abbrev Number: 3 (DW_TAG_base_type)
>     <e5>   DW_AT_byte_size   : 4
>     <e6>   DW_AT_encoding    : 5        (signed)
>     <e7>   DW_AT_name        : int
>  <1><eb>: Abbrev Number: 4 (DW_TAG_subprogram)
>     <ec>   DW_AT_name        : main
>     <f1>   DW_AT_type        : <0xe4>
>     <f5>   DW_AT_external    : 1
>  <1><f6>: Abbrev Number: 0
>   Compilation Unit @ offset 0xf7:
>    Length:        0x2c (32-bit)
>    Version:       4
>    Abbrev Offset: 0x85
>    Pointer Size:  8
>  <0><102>: Abbrev Number: 2 (DW_TAG_compile_unit)
>     <103>   DW_AT_language    : 2       (non-ANSI C)
>     <104>   DW_AT_name        : <artificial>
>  <1><111>: Abbrev Number: 3 (DW_TAG_subprogram)
>     <112>   DW_AT_abstract_origin: <0xeb>
>     <116>   DW_AT_low_pc      : 0x4004a7
>     <11e>   DW_AT_high_pc     : 0x4004b2
>  <1><126>: Abbrev Number: 0
> ...
> 
> When run with target board cc-with-gdb-index, we run into:
> ...
> (gdb) break main
> warning: (Internal error: pc 0x4004a7 in read in CU, but not in symtab.)
> <repeat>
> warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
> <repeat>
> Breakpoint 1 at 0x4004ab
> (gdb) PASS: gdb.dwarf2/imported-unit-runto-main.exp: setting breakpoint at main
> run
> Starting program: /data/gdb_versions/devel/build/gdb/testsuite/outputs/gdb.dwarf2/imported-unit-runto-main/imported-unit-runto-main
> warning: (Internal error: pc 0x4004a7 in read in CU, but not in symtab.)
> <repeat>
> warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
> <repeat>
> 
> Breakpoint 1, warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
> warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
> <repeat>
> 0x00000000004004ab in main ()
> warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
> <repeat>
> (gdb) FAIL: gdb.dwarf2/imported-unit-runto-main.exp: running to main in runto
> ...
> 
> Looking at the .gdb_index section contents using objdump --dwarf=gdb_index, we
> have:
> ...
> CU table:
> [  0] 0x0 - 0x2d
> [  1] 0x2e - 0xa4
> [  2] 0xa5 - 0xc6
> [  3] 0xf7 - 0x126
> [  4] 0x127 - 0x2de
> [  5] 0x2df - 0x300
> 
> Address table:
> 00000000004004a7 00000000004004b2 4
> 
> Symbol table:
> [489] main: 4 [global, function]
> ...
> We see that both the main symbol, and main address range map to CU 4, which has
> offset range 0x127 - 0x2de, while main actually is contained in CU 3 at offset
> range 0xf7 - 0x126.
> 
> This is caused by this continue in write_gdbindex, which triggers for the PU:
> ...
>       /* CU of a shared file from 'dwz -m' may be unused by this main file.
>         It may be referenced from a local scope but in such case it does not
>         need to be present in .gdb_index.  */
>       if (psymtab == NULL)
>        continue;
> ...
> The continue causes the PU to be skipped in the CU table (we can see that the
> PU offset range 0xc7-0xf6 is missing) but the references are not taking that
> into account.
> 
> I've tried fixing this in the optimal way, by updating the references, but ran
> into trouble when follow_die_offset tries to find the CU for the inter-CU
> ref.  Because the PU is missing from the CU table,
> dwarf2_find_containing_comp_unit bisects to the wrong CU.
> 
> Fix this by not skipping the PU in the CU table.
> 
> Build and reg-tested on x86_64-linux, with native and target boards
> cc-with-gdb-index, cc-with-dwz and cc-with-dwz-m.
> 

I've updated the test-case to trigger the error condition in a native
testrun (before it only triggered with target board cc-with-gdb-index).

Committed.

Thanks,
- Tom


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