Currently, when DWARF5 is enabled in kernel, DEBUG_INFO_BTF needs to be
disabled. I hacked the kernel to enable DEBUG_INFO_BTF like:
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -286,7 +286,6 @@ config DEBUG_INFO_DWARF5
bool "Generate DWARF Version 5 debuginfo"
depends on GCC_VERSION >= 50000 || CC_IS_CLANG
depends on CC_IS_GCC || $(success,$(srctree)/scripts/test_dwarf5_support.sh $(CC) $(CLANG_FLAGS))
- depends on !DEBUG_INFO_BTF
help
and tried DWARF5 with latest trunk clang, thin-LTO and no LTO.
In both cases, I got a few additional failures like:
$ ./test_progs -n 55/2
...
libbpf: extern (var ksym) 'bpf_prog_active': failed to find BTF ID in kernel BTF(s).
libbpf: failed to load object 'kfunc_call_test_subprog'
libbpf: failed to load BPF skeleton 'kfunc_call_test_subprog': -22
test_subprog:FAIL:skel unexpected error: 0
#55/2 subprog:FAIL
Here, bpf_prog_active is a percpu global variable and pahole is supposed to
put into BTF, but it is not there.
Further analysis shows this is due to encoding difference between DWARF4
and DWARF5. In DWARF5, a new section .debug_addr and several new ops,
e.g. DW_OP_addrx, are introduced. DW_OP_addrx is actually an index into
.debug_addr section starting from an offset encoded with DW_AT_addr_base
in DW_TAG_compile_unit.
For the above 'bpf_prog_active' example, with DWARF4, we have
0x02281a96: DW_TAG_variable
DW_AT_name ("bpf_prog_active")
DW_AT_decl_file ("/home/yhs/work/bpf-next/include/linux/bpf.h")
DW_AT_decl_line (1170)
DW_AT_decl_column (0x01)
DW_AT_type (0x0226d171 "int")
DW_AT_external (true)
DW_AT_declaration (true)
0x02292f04: DW_TAG_variable
DW_AT_specification (0x02281a96 "bpf_prog_active")
DW_AT_decl_file ("/home/yhs/work/bpf-next/kernel/bpf/syscall.c")
DW_AT_decl_line (45)
DW_AT_location (DW_OP_addr 0x28940)
For DWARF5, we have
0x0138b0a1: DW_TAG_variable
DW_AT_name ("bpf_prog_active")
DW_AT_type (0x013760b9 "int")
DW_AT_external (true)
DW_AT_decl_file ("/home/yhs/work/bpf-next/kernel/bpf/syscall.c")
DW_AT_decl_line (45)
DW_AT_location (DW_OP_addrx 0x16)
This patch added support for DW_OP_addrx. With the patch, the above
failing bpf selftest and other similar failed selftests succeeded.
Signed-off-by: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Bill Wendling <morbo@google.com>
Cc: David Blaikie <dblaikie@gmail.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: bpf@vger.kernel.org
Cc: dwarves@vger.kernel.org
Cc: kernel-team@fb.com
Link: https://lore.kernel.org/r/20210403184158.2834387-1-yhs@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Build instructions:
1. install cmake
2. mkdir build
3. cd build
4. cmake -D__LIB=lib ..
5. make install
Default is to be installed on /usr/local, see rpm spec file for
installing on other places.
Known to work scenarios:
Mandriva Cooker:
cmake 2.4.5-1mdv2007.1
libelfutils1-devel 0.123-1mdv2007.1
Debian Unstable:
cmake 2.4.5-1
libdw-dev 0.123-2
Fedora Core 6:
cmake 2.4.5-2.fc6
elfutils-devel 0.126-1.fc6
ae0b7dde1f