https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97713
Bug ID: 97713
Summary: [gsplit-dwarf] label generated for .debug_abbrev.dwo
offset, corresponding relocation ignored by objcopy
--extract-dwo
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: vries at gcc dot gnu.org
Target Milestone: ---
I.
Consider a simple hello.c, compiled with debug info:
...
$ gcc-11 -g ~/hello.c -dA -save-temps
...
In a-hello.s, in the CU header we have a label .Ldebug_abbrev0:
...
.section .debug_info,"",@progbits
.Ldebug_info0:
.long 0x87 # Length of Compilation Unit Info
.value 0x4 # DWARF version number
.long .Ldebug_abbrev0 # Offset Into Abbrev. Section
....
referring here:
...
.section .debug_abbrev,"",@progbits
.Ldebug_abbrev0:
.uleb128 0x1 # (abbrev code)
.uleb128 0x11 # (TAG: DW_TAG_compile_unit)
...
In a-hello.o, we find a zero at the abbrev offset:
...
Contents of the .debug_info section:
Compilation Unit @ offset 0x0:
Length: 0x87 (32-bit)
Version: 4
Abbrev Offset: 0x0
Pointer Size: 8
...
but there's a related relocation in place:
...
Relocation section '.rela.debug_info' at offset 0x4c8 contains 16 entries:
Offset Info Type Sym. Value Sym. Name +
Addend
000000000006 00070000000a R_X86_64_32 0000000000000000 .debug_abbrev + 0
...
and in the a.out, we end up with:
...
Compilation Unit @ offset 0xc7:
Length: 0x87 (32-bit)
Version: 4
Abbrev Offset: 0x64
...
II.
Now cp a-hello.s to hello.s and modify like this:
...
$ diff -u a-hello.s hello.s
--- a-hello.s 2020-11-04 13:04:03.325633204 +0100
+++ hello.s 2020-11-04 13:04:17.001509687 +0100
@@ -102,6 +102,7 @@
# DW_AT_GNU_all_tail_call_sites
.byte 0 # end of children of DIE 0xb
.section .debug_abbrev,"",@progbits
+ .uleb128 0x0 # (abbrev code)
.Ldebug_abbrev0:
.uleb128 0x1 # (abbrev code)
.uleb128 0x11 # (TAG: DW_TAG_compile_unit)
...
and recompile:
...
$ gcc-11 -g hello.s -save-temps
...
We can see that we've indeed moved the label by 1 byte:
...
$ llvm-dwarfdump -debug-abbrev a-hello.o
a-hello.o: file format ELF64-x86-64
.debug_abbrev contents:
Abbrev table for offset: 0x00000000
Abbrev table for offset: 0x00000001
[1] DW_TAG_compile_unit DW_CHILDREN_yes
...
and likewise, the relocation:
...
Relocation section '.rela.debug_info' at offset 0x4c8 contains 16 entries:
Offset Info Type Sym. Value Sym. Name +
Addend
000000000006 00070000000a R_X86_64_32 0000000000000000 .debug_abbrev + 1
...
and likewise, in the a.out:
...
Compilation Unit @ offset 0xc7:
Length: 0x87 (32-bit)
Version: 4
Abbrev Offset: 0x65
...
III.
Now, let's try the same with in .debug_abbrev.dwo. We compile with
-gsplit-dwarf:
...
$ gcc-11 ~/hello.c -g -gsplit-dwarf -dA -save-temps
...
and copy to hello.s and modify:
...
$ diff -u a-hello.s hello.s
--- a-hello.s 2020-11-04 13:12:57.188966796 +0100
+++ hello.s 2020-11-04 13:14:48.632059272 +0100
@@ -156,6 +156,7 @@
.byte 0
.byte 0 # end of skeleton .debug_abbrev
.section .debug_abbrev.dwo,"e",@progbits
+ .uleb128 0x0 # (abbrev code)
.Ldebug_abbrev0:
.uleb128 0x1 # (abbrev code)
.uleb128 0x11 # (TAG: DW_TAG_compile_unit)
...
We can see that we indeed added the entry to .debug_abbrev.dwo:
...
$ llvm-dwarfdump -debug-abbrev a-hello.dwo
a-hello.dwo: file format ELF64-x86-64
.debug_abbrev.dwo contents:
Abbrev table for offset: 0x00000000
Abbrev table for offset: 0x00000001
[1] DW_TAG_compile_unit DW_CHILDREN_yes
...
The abbrev offset is 0:
...
Contents of the .debug_info.dwo section:
Compilation Unit @ offset 0x0:
Length: 0x50 (32-bit)
Version: 4
Abbrev Offset: 0x0
...
but there's no related relocation so the CU is interpreted using the empty
abbrev table at offset 0, and we run into:
...
<0><b>: Abbrev Number: 1
readelf: Warning: DIE at offset 0xb refers to abbreviation number 1 which does
not exist
...
The split into .o/.dwo is done like this:
...
/usr/lib64/gcc/x86_64-suse-linux/11/../../../../x86_64-suse-linux/bin/as
--gdwarf2 -v --64 -o a-hello.o hello.s
objcopy --extract-dwo a-hello.o a-hello.dwo
objcopy --strip-dwo a-hello.o
...
so we can recreate the original a-hello.o, and find the relocation:
...
Relocation section '.rela.debug_info.dwo' at offset 0x710 contains 1 entry:
Offset Info Type Sym. Value Sym. Name +
Addend
000000000006 000a0000000a R_X86_64_32 0000000000000000 .debug_abbrev.dwo
+ 1
...
So, objcopy --extract-dwo (silently) drops the relocation rather than implement
it.
Now, should objcopy implement the relocation?
Note that llvm emits a '0' as abbrev offset instead of a label.
