Re: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF

On 2020-03-16, Andrii Nakryiko wrote:

On Mon, Mar 16, 2020 at 8:37 PM Fangrui Song [off-list ref] wrote:

quoted

On 2020-03-16, Andrii Nakryiko wrote:

quoted

On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song [off-list ref] wrote:

quoted

Simplify gen_btf logic to make it work with llvm-objcopy and
llvm-objdump.  We just need to retain one section .BTF. To do so, we can
use a simple objcopy --only-section=.BTF instead of jumping all the
hoops via an architecture-less binary file.

We use a dd comment to change the e_type field in the ELF header from
ET_EXEC to ET_REL so that .btf.vmlinux.bin.o will be accepted by lld.

Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Cc: Stanislav Fomichev <redacted>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: Nathan Chancellor <redacted>
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Fangrui Song <redacted>
---
 scripts/link-vmlinux.sh | 13 ++-----------
 1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..84be8d7c361d 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh

@@ -120,18 +120,9 @@ gen_btf()

        info "BTF" ${2}
        vmlinux_link ${1}
-       LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}

Is it really tested? Seems like you just dropped .BTF generation step
completely...

Sorry, dropped the whole line:/
I don't know how to test .BTF . I can only check readelf -S...

Attached the new patch.


 From 02afb9417d4f0f8d2175c94fc3797a94a95cc248 Mon Sep 17 00:00:00 2001
From: Fangrui Song <redacted>
Date: Mon, 16 Mar 2020 18:02:31 -0700
Subject: [PATCH bpf v2] bpf: Support llvm-objcopy and llvm-objdump for
  vmlinux BTF

Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump.
We use a dd comment to change the e_type field in the ELF header from
ET_EXEC to ET_REL so that .btf.vmlinux.bin.o can be accepted by lld.

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>
---
  scripts/link-vmlinux.sh | 14 +++-----------
  1 file changed, 3 insertions(+), 11 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..b23313944c89 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh

@@ -120,18 +120,10 @@ gen_btf()

        info "BTF" ${2}
        vmlinux_link ${1}
-       LLVM_OBJCOPY=${OBJCOPY} ${PAHOLE} -J ${1}
+       ${PAHOLE} -J ${1}

I'm not sure why you are touching this line at all. LLVM_OBJCOPY part
is necessary, pahole assumes llvm-objcopy by default, but that can
(and should for objcopy) be overridden with LLVM_OBJCOPY.

Why is LLVM_OBJCOPY assumed? What if llvm-objcopy is not available?
This is confusing that one tool assumes llvm-objcopy while the block
below immediately uses GNU objcopy (without this patch).

e83b9f55448afce3fe1abcd1d10db9584f8042a6 "kbuild: add ability to
generate BTF type info for vmlinux" does not say why LLVM_OBJCOPY is
set.

quoted

-       # 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}')
-       ${OBJCOPY} --change-section-address .BTF=0 \
-               --set-section-flags .BTF=alloc -O binary \
-               --only-section=.BTF ${1} .btf.vmlinux.bin
-       ${OBJCOPY} -I binary -O ${bin_format} -B ${bin_arch} \
-               --rename-section .data=.BTF .btf.vmlinux.bin ${2}
+       # Extract .BTF section, change e_type to ET_REL, to link with final vmlinux
+       ${OBJCOPY} --only-section=.BTF ${1} ${2} && printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16
  }

  # Create ${2} .o file with all symbols from the ${1} object file
--
2.25.1.481.gfbce0eb801-goog