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From: Yonghong Song <yhs@fb.com>
To: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	<dwarves@vger.kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Bill Wendling <morbo@google.com>, <bpf@vger.kernel.org>,
	David Blaikie <dblaikie@gmail.com>, <kernel-team@fb.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Sedat Dilek <sedat.dilek@gmail.com>
Subject: Re: [PATCH dwarves] dwarf_loader: handle DWARF5 DW_OP_addrx properly
Date: Sun, 4 Apr 2021 09:45:42 -0700	[thread overview]
Message-ID: <d4786dab-b35c-c8f4-a848-3fc9f93228a0@fb.com> (raw)
In-Reply-To: <20210403184158.2834387-1-yhs@fb.com>



On 4/3/21 11:41 AM, Yonghong Song 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.

Arnaldo, sorry, I just found that I forgot signed-off. Here it is,

Signed-off-by: Yonghong Song <yhs@fb.com>

If no further change is needed for this patch, maybe you can help add 
it? Otherwise, I can add it in v2. Thanks!

> ---
>   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) {
> +				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;
> 

  parent reply	other threads:[~2021-04-04 16:46 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-03 18:41 [PATCH dwarves] dwarf_loader: handle DWARF5 DW_OP_addrx properly Yonghong Song
2021-04-03 18:52 ` David Blaikie
2021-04-03 20:20   ` Yonghong Song
2021-04-03 23:31     ` David Blaikie
     [not found] ` <CA+icZUWLf4W_1u_p4-Rx1OD7h_ydP4Xzv12tMA2HZqj9CCOH0Q@mail.gmail.com>
2021-04-03 23:13   ` Yonghong Song
2021-04-03 23:27   ` Yonghong Song
     [not found]     ` <CA+icZUV4fw5GNXFnyOjvajkVFdPhkOrhr3rn5OrAKGujpSrmgQ@mail.gmail.com>
2021-04-04 12:46       ` Sedat Dilek
2021-04-04 16:39         ` Yonghong Song
2021-04-04 17:25           ` Sedat Dilek
2021-04-05  2:24             ` Yonghong Song
     [not found]               ` <CA+icZUVcQ+vQjc0VavetA3s6jzNhC20dU4Sa9ApBLNXbY=w5wA@mail.gmail.com>
2021-04-05 11:04                 ` Sedat Dilek
2021-04-05 11:46                   ` Sedat Dilek
2021-04-05 16:17                 ` Yonghong Song
2021-04-05 18:32                   ` Sedat Dilek
2021-04-05 18:56                     ` Yonghong Song
2021-04-05 20:42                       ` Sedat Dilek
     [not found]             ` <CA+icZUVp3UTPUS-ZjCOnHbNXxaA7DN=4x_08jc8BExFe4Nf2ZQ@mail.gmail.com>
2021-04-05  2:30               ` Yonghong Song
     [not found]                 ` <CA+icZUVtzXNxuVtEUwfULa7nivV0VFfJznsRnSZtEh+V=C=RPg@mail.gmail.com>
2021-04-05  6:56                   ` Sedat Dilek
2021-04-04 14:59 ` Usage of CXX in tools directory Sedat Dilek
2021-04-04 15:19   ` Sedat Dilek
2021-04-06 18:39   ` Nick Desaulniers
2021-04-04 16:45 ` Yonghong Song [this message]
2021-04-04 17:29   ` [PATCH dwarves] dwarf_loader: handle DWARF5 DW_OP_addrx properly Sedat Dilek

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