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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id g2sm3507364pgi.20.2020.03.17.14.55.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 17 Mar 2020 14:55:31 -0700 (PDT) Date: Tue, 17 Mar 2020 14:55:30 -0700 From: Kees Cook To: Fangrui Song Cc: bpf , Networking , Andrii Nakryiko , Daniel Borkmann , Nick Desaulniers , Nathan Chancellor , clang-built-linux@googlegroups.com, Stanislav Fomichev , "David S. Miller" , Alexei Starovoitov Subject: Re: [PATCH bpf-next v5] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF Message-ID: <202003171451.6B11E25636@keescook> References: <20200317211649.o4fzaxrzy6qxvz4f@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200317211649.o4fzaxrzy6qxvz4f@google.com> Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Tue, Mar 17, 2020 at 02:16:49PM -0700, Fangrui Song wrote: > Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump. > The existing 'file format' and 'architecture' parsing logic is brittle > and does not work with llvm-objcopy/llvm-objdump. > > .BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag and > rename .BTF to BTF so that C code can reference the section via linker > synthesized __start_BTF and __stop_BTF. This fixes a small problem that > previous .BTF had the SHF_WRITE flag. Additionally, `objcopy -I binary` > synthesized symbols _binary__btf_vmlinux_bin_start and > _binary__btf_vmlinux_bin_start (not used elsewhere) are replaced with > more common __start_BTF and __stop_BTF. I'm glad to see the name change benefit here. Just reducing the number of execs in this path is only worth this change, IMO. Going from 2 objdump and 2 objcopy calls to a single objcopy is very nice. :) > > Add 2>/dev/null because GNU objcopy (but not llvm-objcopy) warns > "empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?" > > We use a dd command to change the e_type field in the ELF header from > ET_EXEC to ET_REL so that lld will accept .btf.vmlinux.bin.o. Accepting > ET_EXEC as an input file is an extremely rare GNU ld feature that lld > does not intend to support, because this is error-prone. Nice ELF trick. :) > > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Cc: Stanislav Fomichev > Cc: Nick Desaulniers > Reported-by: Nathan Chancellor > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Fangrui Song Reviewed-by: Kees Cook -Kees > --- > kernel/bpf/btf.c | 9 ++++----- > kernel/bpf/sysfs_btf.c | 11 +++++------ > scripts/link-vmlinux.sh | 17 ++++++----------- > 3 files changed, 15 insertions(+), 22 deletions(-) > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c > index 50080add2ab9..6f397c4da05e 100644 > --- a/kernel/bpf/btf.c > +++ b/kernel/bpf/btf.c > @@ -3477,8 +3477,8 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size, > return ERR_PTR(err); > } > -extern char __weak _binary__btf_vmlinux_bin_start[]; > -extern char __weak _binary__btf_vmlinux_bin_end[]; > +extern char __weak __start_BTF[]; > +extern char __weak __stop_BTF[]; > extern struct btf *btf_vmlinux; > #define BPF_MAP_TYPE(_id, _ops) > @@ -3605,9 +3605,8 @@ struct btf *btf_parse_vmlinux(void) > } > env->btf = btf; > - btf->data = _binary__btf_vmlinux_bin_start; > - btf->data_size = _binary__btf_vmlinux_bin_end - > - _binary__btf_vmlinux_bin_start; > + btf->data = __start_BTF; > + btf->data_size = __stop_BTF - __start_BTF; > err = btf_parse_hdr(env); > if (err) > diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c > index 7ae5dddd1fe6..3b495773de5a 100644 > --- a/kernel/bpf/sysfs_btf.c > +++ b/kernel/bpf/sysfs_btf.c > @@ -9,15 +9,15 @@ > #include > /* See scripts/link-vmlinux.sh, gen_btf() func for details */ > -extern char __weak _binary__btf_vmlinux_bin_start[]; > -extern char __weak _binary__btf_vmlinux_bin_end[]; > +extern char __weak __start_BTF[]; > +extern char __weak __stop_BTF[]; > static ssize_t > btf_vmlinux_read(struct file *file, struct kobject *kobj, > struct bin_attribute *bin_attr, > char *buf, loff_t off, size_t len) > { > - memcpy(buf, _binary__btf_vmlinux_bin_start + off, len); > + memcpy(buf, __start_BTF + off, len); > return len; > } > @@ -30,15 +30,14 @@ static struct kobject *btf_kobj; > static int __init btf_vmlinux_init(void) > { > - if (!_binary__btf_vmlinux_bin_start) > + if (!__start_BTF) > return 0; > btf_kobj = kobject_create_and_add("btf", kernel_kobj); > if (!btf_kobj) > return -ENOMEM; > - bin_attr_btf_vmlinux.size = _binary__btf_vmlinux_bin_end - > - _binary__btf_vmlinux_bin_start; > + bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF; > return sysfs_create_bin_file(btf_kobj, &bin_attr_btf_vmlinux); > } > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh > index ac569e197bfa..ae2048625f1e 100755 > --- a/scripts/link-vmlinux.sh > +++ b/scripts/link-vmlinux.sh > @@ -133,17 +133,12 @@ gen_btf() > info "BTF" ${2} > LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1} > - # dump .BTF section into raw binary file to link with final vmlinux > - bin_arch=$(LANG=C ${OBJDUMP} -f ${1} | grep architecture | \ > - cut -d, -f1 | cut -d' ' -f2) > - bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \ > - awk '{print $4}') > - bin_file=.btf.vmlinux.bin > - ${OBJCOPY} --change-section-address .BTF=0 \ > - --set-section-flags .BTF=alloc -O binary \ > - --only-section=.BTF ${1} $bin_file > - ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \ > - --rename-section .data=.BTF $bin_file ${2} > + # Extract .BTF, add SHF_ALLOC, rename to BTF so that we can reference > + # it via linker synthesized __start_BTF and __stop_BTF. Change e_type > + # to ET_REL so that it can be used to link final vmlinux. > + ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \ > + --rename-section .BTF=BTF ${1} ${2} 2>/dev/null && \ > + printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none > } > # Create ${2} .o file with all symbols from the ${1} object file > -- > 2.25.1.481.gfbce0eb801-goog -- Kees Cook