On 03/17, 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. Thanks, it all makes sense and looks much better/portable (too much dependence on binutils :-). I've left a bunch of questions/nits below. > > .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. > > 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. Please keep small changelog here, for example: v5: * rebased on top of bpfnext Btw, I tried to pull and test it and failed: $ curl -LO https://lore.kernel.org/bpf/20200317211649.o4fzaxrzy6qxvz4f@xxxxxxxxxx/raw % Total % Received % Xferd Average Speed Time Time Time Current Dload Upload Total Spent Left Speed 100 9627 100 9627 0 0 47191 0 --:--:-- --:--:-- --:--:-- 47191 $ git am raw warning: Patch sent with format=flowed; space at the end of lines might be lost. Applying: bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF error: corrupt patch at line 20 Patch failed at 0001 bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF hint: Use 'git am --show-current-patch' to see the failed patch When you have resolved this problem, run "git am --continue". If you prefer to skip this patch, run "git am --skip" instead. To restore the original branch and stop patching, run "git am --abort". $ git describe v5.6-rc5-1621-g230021539e8c Are you sure it's on top of the bpf-next? Or am I doing something wrong? > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") > Cc: Stanislav Fomichev <sdf@xxxxxxxxxx> > Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > Link: https://github.com/ClangBuiltLinux/linux/issues/871 > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> > --- > 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 <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); > } > 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 We still have the following in gen_btf that you need to remove: local bin_arch local bin_format local bin_file > - ${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 We have 'set -e' so && can be omitted. Maybe it will be a bit more clear: # Extract .BTF, add SHF_ALLOC, rename to BTF so that we can reference # it via linker synthesized __start_BTF and __stop_BTF. ${OBJCOPY} --only-section=.BTF --set-section-flags .BTF=alloc,readonly \ --rename-section .BTF=BTF ${1} ${2} 2>/dev/null # Change e_type to ET_REL (0x01) so that it can be used to link final # vmlinux. printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16 status=none ^^^ maybe also point out that this is required for llvm's ldd. I know you point it out in the commit message, but still nice to have as a comment here. > } > # Create ${2} .o file with all symbols from the ${1} object file > -- > 2.25.1.481.gfbce0eb801-goog
