From 90ceddcb495008ac8ba7a3dce297841efcd7d584 Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Wed, 18 Mar 2020 15:27:46 -0700 Subject: bpf: Support llvm-objcopy for vmlinux BTF 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 Signed-off-by: Fangrui Song Signed-off-by: Daniel Borkmann Tested-by: Stanislav Fomichev Tested-by: Andrii Nakryiko Reviewed-by: Stanislav Fomichev Reviewed-by: Kees Cook Acked-by: Andrii Nakryiko Acked-by: Michael Ellerman (powerpc) Link: https://github.com/ClangBuiltLinux/linux/issues/871 Link: https://lore.kernel.org/bpf/20200318222746.173648-1-maskray@google.com --- kernel/bpf/sysfs_btf.c | 11 +++++------ 1 file changed, 5 insertions(+), 6 deletions(-) (limited to 'kernel/bpf/sysfs_btf.c') 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); } -- cgit v1.2.3