Thread (17 messages) read the whole thread 17 messages, 4 authors, 2020-03-18

Re: [PATCH bpf v3] bpf: Support llvm-objcopy and llvm-objdump for vmlinux BTF

From: Stanislav Fomichev <sdf@fomichev.me>
Date: 2020-03-17 16:24:11
Also in: bpf 

On 03/16, Fangrui Song wrote:
quoted hunk ↗ jump to hunk
On 2020-03-16, Andrii Nakryiko wrote:
quoted
On Mon, Mar 16, 2020 at 10:21 PM Fangrui Song [off-list ref] wrote:
quoted

On 2020-03-16, Andrii Nakryiko wrote:
quoted
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?
It's pahole assumption that we have to live with. pahole assumes
llvm-objcopy internally, unless it is overriden with LLVM_OBJCOPY env
var. So please revert this line otherwise you are breaking it for GCC
objcopy case.
Acknowledged. Uploaded v3.

I added back 2>/dev/null which was removed by a previous change, to
suppress GNU objcopy warnings. The warnings could be annoying in V=1
output.
quoted
quoted
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
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
From ca3597477542453e9f63185c27c162da081a4baf Mon Sep 17 00:00:00 2001
From: Fangrui Song <redacted>
Date: Mon, 16 Mar 2020 22:38:23 -0700
Subject: [PATCH bpf v3] bpf: Support llvm-objcopy and llvm-objdump for
 vmlinux BTF

Simplify gen_btf logic to make it work with llvm-objcopy and llvm-objdump.
Add 2>/dev/null to suppress GNU objcopy (but not llvm-objcopy) warnings
"empty loadable segment detected at vaddr=0xffffffff81000000, is this intentional?"
Our use of --only-section drops many SHF_ALLOC sections which will essentially nullify
program headers. When used as linker input, program headers are simply
ignored.

We use a dd command 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.
Accepting ET_EXEC as an input file is an extremely rare GNU ld feature
that lld does not intend to support, because this is very error-prone.

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 | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..c3e808a89d4a 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -122,16 +122,8 @@ gen_btf()
 	vmlinux_link ${1}
 	LLVM_OBJCOPY=${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}')
-	${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} ${2} 2> /dev/null && printf '\1' | dd of=${2} conv=notrunc bs=1 seek=16
No, it doesn't work unfortunately, I get "in-kernel BTF is malformed"
from the kernel.

I think that's because -O binary adds the following:
$ nm .btf.vmxlinux.bin
00000000002f7bc9 D _binary__btf_vmlinux_bin_end
00000000002f7bc9 A _binary__btf_vmlinux_bin_size
0000000000000000 D _binary__btf_vmlinux_bin_start

While non-binary mode doesn't:
$ nm .btf.vmxlinux.bin

We don't add them manually in the linker map and expect objcopy to add
them, see:
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/bpf/btf.c#n3480

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