Re: [PATCH bpf-next v5] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF
From: Stanislav Fomichev <sdf@fomichev.me>
Date: 2020-03-17 21:51:04
Also in:
bpf
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@google.com/raw (local) % 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?
quoted hunk ↗ jump to hunk
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux") Cc: Stanislav Fomichev <redacted> Cc: Nick Desaulniers <ndesaulniers@google.com> Reported-by: Nathan Chancellor <redacted> Link: https://github.com/ClangBuiltLinux/linux/issues/871 Signed-off-by: Fangrui Song <redacted> --- 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