From ae0b7dde1fd50b12a41e513272b73666d976a81d Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Sat, 3 Apr 2021 11:41:58 -0700 Subject: [PATCH] dwarf_loader: Handle DWARF5 DW_OP_addrx properly 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 Cc: Alexei Starovoitov Cc: Bill Wendling Cc: David Blaikie Cc: Nick Desaulniers Cc: Sedat Dilek 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 --- dwarf_loader.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/dwarf_loader.c b/dwarf_loader.c index 82d7131..49336ac 100644 --- a/dwarf_loader.c +++ b/dwarf_loader.c @@ -401,8 +401,19 @@ static int attr_location(Dwarf_Die *die, Dwarf_Op **expr, size_t *exprlen) { Dwarf_Attribute attr; if (dwarf_attr(die, DW_AT_location, &attr) != NULL) { - if (dwarf_getlocation(&attr, expr, exprlen) == 0) + if (dwarf_getlocation(&attr, expr, exprlen) == 0) { + /* DW_OP_addrx needs additional lookup for real addr. */ + if (*exprlen != 0 && expr[0]->atom == DW_OP_addrx) { + Dwarf_Attribute addr_attr; + dwarf_getlocation_attr(&attr, expr[0], &addr_attr); + + Dwarf_Addr address; + dwarf_formaddr (&addr_attr, &address); + + expr[0]->number = address; + } return 0; + } } return 1; @@ -626,6 +637,7 @@ static enum vscope dwarf__location(Dwarf_Die *die, uint64_t *addr, struct locati Dwarf_Op *expr = location->expr; switch (expr->atom) { case DW_OP_addr: + case DW_OP_addrx: scope = VSCOPE_GLOBAL; *addr = expr[0].number; break;