From: Fangrui Song maskray@google.com
commit 90ceddcb495008ac8ba7a3dce297841efcd7d584 upstream.
Simplify gen_btf logic to make it work with llvm-objcopy. The existing 'file format' and 'architecture' parsing logic is brittle and does not work with llvm-objcopy/llvm-objdump.
'file format' output of llvm-objdump>=11 will match GNU objdump, but 'architecture' (bfdarch) may not.
.BTF in .tmp_vmlinux.btf is non-SHF_ALLOC. Add the SHF_ALLOC flag because it is part of vmlinux image used for introspection. C code can reference the section via linker script defined __start_BTF and __stop_BTF. This fixes a small problem that previous .BTF had the SHF_WRITE flag (objcopy -I binary -O elf* synthesized .data).
Additionally, `objcopy -I binary` synthesized symbols _binary__btf_vmlinux_bin_start and _binary__btf_vmlinux_bin_stop (not used elsewhere) are replaced with more commonplace __start_BTF and __stop_BTF.
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.
The output section description .BTF in include/asm-generic/vmlinux.lds.h avoids potential subtle orphan section placement issues and suppresses --orphan-handling=warn warnings.
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Fixes: cb0cc635c7a9 ("powerpc: Include .BTF section") Reported-by: Nathan Chancellor natechancellor@gmail.com Signed-off-by: Fangrui Song maskray@google.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Tested-by: Stanislav Fomichev sdf@google.com Tested-by: Andrii Nakryiko andriin@fb.com Reviewed-by: Stanislav Fomichev sdf@google.com Reviewed-by: Kees Cook keescook@chromium.org Acked-by: Andrii Nakryiko andriin@fb.com Acked-by: Michael Ellerman mpe@ellerman.id.au (powerpc) Link: https://github.com/ClangBuiltLinux/linux/issues/871 Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com Signed-off-by: Maria Teguiani teguiani@google.com Tested-by: Matthias Maennich maennich@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/kernel/vmlinux.lds.S | 6 ------ include/asm-generic/vmlinux.lds.h | 22 +++++++++++++++++++--- kernel/bpf/sysfs_btf.c | 11 +++++------ scripts/link-vmlinux.sh | 24 ++++++++++-------------- 4 files changed, 34 insertions(+), 29 deletions(-)
--- a/arch/powerpc/kernel/vmlinux.lds.S +++ b/arch/powerpc/kernel/vmlinux.lds.S @@ -326,12 +326,6 @@ SECTIONS *(.branch_lt) }
-#ifdef CONFIG_DEBUG_INFO_BTF - .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { - *(.BTF) - } -#endif - .opd : AT(ADDR(.opd) - LOAD_OFFSET) { __start_opd = .; KEEP(*(.opd)) --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -496,10 +496,12 @@ __start___modver = .; \ KEEP(*(__modver)) \ __stop___modver = .; \ - . = ALIGN((align)); \ - __end_rodata = .; \ } \ - . = ALIGN((align)); + \ + BTF \ + \ + . = ALIGN((align)); \ + __end_rodata = .;
/* RODATA & RO_DATA provided for backward compatibility. * All archs are supposed to use RO_DATA() */ @@ -589,6 +591,20 @@ }
/* + * .BTF + */ +#ifdef CONFIG_DEBUG_INFO_BTF +#define BTF \ + .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) { \ + __start_BTF = .; \ + *(.BTF) \ + __stop_BTF = .; \ + } +#else +#define BTF +#endif + +/* * Init task */ #define INIT_TASK_DATA_SECTION(align) \ --- a/kernel/bpf/sysfs_btf.c +++ b/kernel/bpf/sysfs_btf.c @@ -9,15 +9,15 @@ #include <linux/sysfs.h>
/* 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); } --- a/scripts/link-vmlinux.sh +++ b/scripts/link-vmlinux.sh @@ -113,9 +113,6 @@ vmlinux_link() gen_btf() { local pahole_ver - local bin_arch - local bin_format - local bin_file
if ! [ -x "$(command -v ${PAHOLE})" ]; then echo >&2 "BTF: ${1}: pahole (${PAHOLE}) is not available" @@ -133,17 +130,16 @@ 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} + # Create ${2} which contains just .BTF section but no symbols. Add + # SHF_ALLOC because .BTF will be part of the vmlinux image. --strip-all + # deletes all symbols including __start_BTF and __stop_BTF, which will + # be redefined in the linker script. Add 2>/dev/null to suppress GNU + # objcopy warnings: "empty loadable segment detected at ..." + ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \ + --strip-all ${1} ${2} 2>/dev/null + # Change e_type to ET_REL so that it can be used to link final vmlinux. + # Unlike GNU ld, lld does not allow an ET_EXEC input. + printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none }
# Create ${2} .o file with all symbols from the ${1} object file