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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>,
	dwarves@vger.kernel.org, bpf <bpf@vger.kernel.org>,
	Kernel Team <kernel-team@fb.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andriin@fb.com>
Subject: Re: [PATCH v2 dwarves 1/8] btf_loader: use libbpf to load BTF
Date: Fri, 9 Oct 2020 10:49:31 -0700
Message-ID: <CAEf4BzZCH7UTqqRqatBwOD9aCyR3r-JUb8v9_BwtddhSnZh37w@mail.gmail.com> (raw)
In-Reply-To: <20201009145343.GA322246@kernel.org>

On Fri, Oct 9, 2020 at 7:53 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> Em Thu, Oct 08, 2020 at 04:39:53PM -0700, Andrii Nakryiko escreveu:
> > From: Andrii Nakryiko <andriin@fb.com>
> >
> > Switch BTF loading to completely use libbpf's own struct btf and related APIs.
> > BTF encoding is still happening with pahole's own code, so these two code
> > paths are not sharing anything now. String fetching is happening based on
> > whether btfe->strings were set to non-NULL pointer by btf_encoder.
>
> While testing this one I noticed a problem, but it isn't caused by this
> particular patch:
>
> [acme@five pahole]$ cp ~/git/build/v5.9-rc6+/net/ipv4/tcp_ipv4.o .
> [acme@five pahole]$ readelf -SW tcp_ipv4.o | grep BTF
> [acme@five pahole]$ pahole -J tcp_ipv4.o
> [acme@five pahole]$ readelf -SW tcp_ipv4.o | grep BTF
>   [105] .BTF              PROGBITS        0000000000000000 0fbb3c 03f697 00      0   0  1
> [acme@five pahole]$ ./btfdiff tcp_ipv4.o
> --- /tmp/btfdiff.dwarf.BDAvGi   2020-10-09 11:41:45.161509391 -0300
> +++ /tmp/btfdiff.btf.p81icw     2020-10-09 11:41:45.177509720 -0300
> @@ -4056,7 +4056,7 @@ struct tcp_congestion_ops {
>         u32                        (*min_tso_segs)(struct sock *); /*    96     8 */
>         u32                        (*sndbuf_expand)(struct sock *); /*   104     8 */
>         void                       (*cong_control)(struct sock *, const struct rate_sample  *); /*   112     8 */
> -       size_t                     (*get_info)(struct sock *, u32, int *, union tcp_cc_info *); /*   120     8 */
> +       size_t                     (*get_info)(struct sock *, u32, int *, struct tcp_cc_info *); /*   120     8 */

It's a bug in btf_loader.c. When loading BTF_KIND_FWD, we always
assume it's struct forward reference. But I checked, generated BTF
does have the valid info (kind_flag), so it should be easy to fix,
I'll send a fix.

The reason we don't see it in vmlinux is because after dedup that
forward reference is resolved to a full type, which is clearly union.


[...]

  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-08 23:39 [PATCH v2 dwarves 0/8] Switch BTF loading and encoding to libbpf APIs Andrii Nakryiko
2020-10-08 23:39 ` [PATCH v2 dwarves 1/8] btf_loader: use libbpf to load BTF Andrii Nakryiko
2020-10-09 14:53   ` Arnaldo Carvalho de Melo
2020-10-09 17:49     ` Andrii Nakryiko [this message]
2020-10-08 23:39 ` [PATCH v2 dwarves 2/8] btf_encoder: use libbpf APIs to encode BTF type info Andrii Nakryiko
2020-10-08 23:39 ` [PATCH v2 dwarves 3/8] btf_encoder: fix emitting __ARRAY_SIZE_TYPE__ as index range type Andrii Nakryiko
2020-10-08 23:39 ` [PATCH v2 dwarves 4/8] btf_encoder: discard CUs after BTF encoding Andrii Nakryiko
2020-10-09 15:47   ` Arnaldo Carvalho de Melo
2020-10-08 23:39 ` [PATCH v2 dwarves 5/8] btf_encoder: revamp how per-CPU variables are encoded Andrii Nakryiko
2020-10-09 15:53   ` Arnaldo Carvalho de Melo
2020-10-08 23:39 ` [PATCH v2 dwarves 6/8] dwarf_loader: increase the size of lookup hash map Andrii Nakryiko
2020-10-08 23:39 ` [PATCH v2 dwarves 7/8] strings: use BTF's string APIs for strings management Andrii Nakryiko
2020-10-08 23:40 ` [PATCH v2 dwarves 8/8] btf_encoder: support cross-compiled ELF binaries with different endianness Andrii Nakryiko
2020-10-09 16:22 ` [PATCH v2 dwarves 0/8] Switch BTF loading and encoding to libbpf APIs Arnaldo Carvalho de Melo
2020-10-09 17:59   ` Andrii Nakryiko

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