Re: [PATCH dwarves] dwarf_loader: handle DWARF5 DW_OP_addrx properly

[David Blaikie <dblaikie@gmail.com>]

On Sat, Apr 3, 2021 at 11:42 AM Yonghong Song <yhs@fb.com> wrote:
>
> 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.
> ---
>  dwarf_loader.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> NOTE: with this patch, at least for clang trunk, all bpf selftests
>       are fine for DWARF5 w.r.t. compiler and pahole. Hopefully
>       after pahole 1.21 release, we can remove DWARF5 dependence
>       with !DEBUG_INFO_BTF.
>
> 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) {


^ this change is probably overly narrow. DW_OP_addrx could appear at
other locations in a DWARF expression. So whatever code is responsible
for doing general evaluation of DWARF expressions should probably be
the place to handle this so it can deal with the full generality of
expressions, not only this specific case of a DW_OP_addrx as being the
first operation in the expression?

>
> +                               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;
> --
> 2.30.2
>