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Mon, 15 Apr 2024 07:59:53 -0700 (PDT) X-Forwarded-Encrypted: i=1; AJvYcCX7iXDfS57YMMSI2bKUJa2ARJgsF710ysNHpBwdVLJaPBGYM7z6lG9UziXdU0aY5gp/Mxkn+8iLYB7ChamBagrmcVIkteS+zlb856nAxLgSkPmT5lRwZT0UGeX4JXctIha/eNFzdKWaywKjT25MYDzgfGq27iJEidf0DAOTajY3cDMWFCpuTuJy3ULr4Rwv0FrA7FFFrWhjkTPUMw== X-Gm-Message-State: AOJu0Yya4+ysGEoxWHuBFxw+pAqeVvOLhLxN7ePvzFHiUhgruFMVStIJ 1noEUVZIrpsPdsKvVtjbo/OsJpIsz1BnAnEgGxzKodNc+aME5OxPUTr0OOiz4mQbZ3f3NHr8J1L AcGLXjALKHXpl3du2vQI7hYxpfG8= X-Google-Smtp-Source: AGHT+IGdVjaPR0hPefaSFdbhKr9PxPG0R7xcDqMOIqw3PaoyBHxE4zMWgaT5BEiOnDHZhIO/HRp6NYvoe70H4+1xqUQ= X-Received: by 2002:a2e:9c4e:0:b0:2d8:4637:405f with SMTP id t14-20020a2e9c4e000000b002d84637405fmr3323763ljj.9.1713193191470; Mon, 15 Apr 2024 07:59:51 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20240415075837.2349766-5-ardb+git@google.com> <20240415075837.2349766-8-ardb+git@google.com> In-Reply-To: From: Ard Biesheuvel Date: Mon, 15 Apr 2024 16:59:40 +0200 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v3 3/3] btf: Avoid weak external references To: Masahiro Yamada Cc: Ard Biesheuvel , linux-kernel@vger.kernel.org, Arnd Bergmann , Martin KaFai Lau , linux-arch@vger.kernel.org, linux-kbuild@vger.kernel.org, bpf@vger.kernel.org, Andrii Nakryiko , Jiri Olsa Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Mon, 15 Apr 2024 at 16:55, Masahiro Yamada wrote: > > On Mon, Apr 15, 2024 at 4:58=E2=80=AFPM Ard Biesheuvel wrote: > > > > From: Ard Biesheuvel > > > > If the BTF code is enabled in the build configuration, the start/stop > > BTF markers are guaranteed to exist in the final link but not during th= e > > first linker pass. > > > > Avoid GOT based relocations to these markers in the final executable by > > providing preliminary definitions that will be used by the first linker > > pass, and superseded by the actual definitions in the subsequent ones. > > > > Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so > > that inadvertent references to this section will trigger a link failure > > if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF. > > > > Note that Clang will notice that taking the address of__start_BTF canno= t > > yield NULL any longer, so testing for that condition is no longer > > needed. > > > > Acked-by: Andrii Nakryiko > > Acked-by: Arnd Bergmann > > Signed-off-by: Ard Biesheuvel > > --- > > include/asm-generic/vmlinux.lds.h | 9 +++++++++ > > kernel/bpf/btf.c | 7 +++++-- > > kernel/bpf/sysfs_btf.c | 6 +++--- > > 3 files changed, 17 insertions(+), 5 deletions(-) > > > > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vm= linux.lds.h > > index e8449be62058..4cb3d88449e5 100644 > > --- a/include/asm-generic/vmlinux.lds.h > > +++ b/include/asm-generic/vmlinux.lds.h > > @@ -456,6 +456,7 @@ > > * independent code. > > */ > > #define PRELIMINARY_SYMBOL_DEFINITIONS = \ > > + PRELIMINARY_BTF_DEFINITIONS = \ > > PROVIDE(kallsyms_addresses =3D .); = \ > > PROVIDE(kallsyms_offsets =3D .); = \ > > PROVIDE(kallsyms_names =3D .); = \ > > @@ -466,6 +467,14 @@ > > PROVIDE(kallsyms_markers =3D .); = \ > > PROVIDE(kallsyms_seqs_of_names =3D .); > > > > +#ifdef CONFIG_DEBUG_INFO_BTF > > +#define PRELIMINARY_BTF_DEFINITIONS = \ > > + PROVIDE(__start_BTF =3D .); = \ > > + PROVIDE(__stop_BTF =3D .); > > +#else > > +#define PRELIMINARY_BTF_DEFINITIONS > > +#endif > > + > > > > Is this necessary? > I think so. This actually resulted in Jiri's build failure with v2, and the realization that there was dead code in btf_parse_vmlinux() that happily tried to load the contents of the BTF section if CONFIG_DEBUG_INFO_BTF was not enabled to begin with. So this is another pitfall with weak references: the symbol may unexpectedly be missing altogether rather than only during the first linker pass. > > > The following code (BOUNDED_SECTION_BY) > produces __start_BTF and __stop_BTF symbols > under the same condition. > Indeed. So if CONFIG_DEBUG_INFO_BTF=3Dn, code can still link to __start_BTF and __stop_BTF even though BTF is not enabled. > > > /* > * .BTF > */ > #ifdef CONFIG_DEBUG_INFO_BTF > #define BTF = \ > .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { = \ > BOUNDED_SECTION_BY(.BTF, _BTF) = \ > } = \ > . =3D ALIGN(4); = \ > .BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) { = \ > *(.BTF_ids) = \ > } > #else > #define BTF > #endif > > > > > > > > > > > > > /* > > * Read only Data > > */ > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c > > index 90c4a32d89ff..6d46cee47ae3 100644 > > --- a/kernel/bpf/btf.c > > +++ b/kernel/bpf/btf.c > > @@ -5642,8 +5642,8 @@ static struct btf *btf_parse(const union bpf_attr= *attr, bpfptr_t uattr, u32 uat > > return ERR_PTR(err); > > } > > > > -extern char __weak __start_BTF[]; > > -extern char __weak __stop_BTF[]; > > +extern char __start_BTF[]; > > +extern char __stop_BTF[]; > > extern struct btf *btf_vmlinux; > > > > #define BPF_MAP_TYPE(_id, _ops) > > @@ -5971,6 +5971,9 @@ struct btf *btf_parse_vmlinux(void) > > struct btf *btf =3D NULL; > > int err; > > > > + if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) > > + return ERR_PTR(-ENOENT); > > + > > env =3D kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN); > > if (!env) > > return ERR_PTR(-ENOMEM); > > diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c > > index ef6911aee3bb..fedb54c94cdb 100644 > > --- a/kernel/bpf/sysfs_btf.c > > +++ b/kernel/bpf/sysfs_btf.c > > @@ -9,8 +9,8 @@ > > #include > > > > /* See scripts/link-vmlinux.sh, gen_btf() func for details */ > > -extern char __weak __start_BTF[]; > > -extern char __weak __stop_BTF[]; > > +extern char __start_BTF[]; > > +extern char __stop_BTF[]; > > > > static ssize_t > > btf_vmlinux_read(struct file *file, struct kobject *kobj, > > @@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void) > > { > > bin_attr_btf_vmlinux.size =3D __stop_BTF - __start_BTF; > > > > - if (!__start_BTF || bin_attr_btf_vmlinux.size =3D=3D 0) > > + if (bin_attr_btf_vmlinux.size =3D=3D 0) > > return 0; > > > > btf_kobj =3D kobject_create_and_add("btf", kernel_kobj); > > -- > > 2.44.0.683.g7961c838ac-goog > > > > > -- > Best Regards > Masahiro Yamada