* [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
@ 2020-03-16 22:25 Stanislav Fomichev
2020-03-16 23:15 ` Fangrui Song
2020-03-16 23:57 ` Nathan Chancellor
0 siblings, 2 replies; 7+ messages in thread
From: Stanislav Fomichev @ 2020-03-16 22:25 UTC (permalink / raw)
To: netdev, bpf
Cc: davem, ast, daniel, Stanislav Fomichev, Nick Desaulniers,
Nathan Chancellor
Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
BTF") switched from --dump-section to
--only-section/--change-section-address for BTF export assuming
those ("legacy") options should cover all objcopy versions.
Turns out llvm-objcopy doesn't implement --change-section-address [1],
but it does support --dump-section. Let's partially roll back and
try to use --dump-section first and fall back to
--only-section/--change-section-address for the older binutils.
1. https://bugs.llvm.org/show_bug.cgi?id=45217
Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/871
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
scripts/link-vmlinux.sh | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index dd484e92752e..8ddf57cbc439 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -127,6 +127,16 @@ gen_btf()
cut -d, -f1 | cut -d' ' -f2)
bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
awk '{print $4}')
+
+ # Compatibility issues:
+ # - pre-2.25 binutils objcopy doesn't support --dump-section
+ # - llvm-objcopy doesn't support --change-section-address, but
+ # does support --dump-section
+ #
+ # Try to use --dump-section which should cover both recent
+ # binutils and llvm-objcopy and fall back to --only-section
+ # for pre-2.25 binutils.
+ ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
${OBJCOPY} --change-section-address .BTF=0 \
--set-section-flags .BTF=alloc -O binary \
--only-section=.BTF ${1} .btf.vmlinux.bin
--
2.25.1.481.gfbce0eb801-goog
^ permalink raw reply related [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-16 22:25 [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF Stanislav Fomichev
@ 2020-03-16 23:15 ` Fangrui Song
2020-03-16 23:56 ` Stanislav Fomichev
2020-03-16 23:57 ` Nathan Chancellor
1 sibling, 1 reply; 7+ messages in thread
From: Fangrui Song @ 2020-03-16 23:15 UTC (permalink / raw)
To: Stanislav Fomichev
Cc: netdev, bpf, davem, ast, daniel, Nick Desaulniers,
Nathan Chancellor, clang-built-linux
On 2020-03-16, Stanislav Fomichev wrote:
>Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
>BTF") switched from --dump-section to
>--only-section/--change-section-address for BTF export assuming
>those ("legacy") options should cover all objcopy versions.
>
>Turns out llvm-objcopy doesn't implement --change-section-address [1],
>but it does support --dump-section. Let's partially roll back and
>try to use --dump-section first and fall back to
>--only-section/--change-section-address for the older binutils.
>
>1. https://bugs.llvm.org/show_bug.cgi?id=45217
>
>Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
>Tested-by: Nick Desaulniers <ndesaulniers@google.com>
>Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>Link: https://github.com/ClangBuiltLinux/linux/issues/871
>Signed-off-by: Stanislav Fomichev <sdf@google.com>
>---
> scripts/link-vmlinux.sh | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
>diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>index dd484e92752e..8ddf57cbc439 100755
>--- a/scripts/link-vmlinux.sh
>+++ b/scripts/link-vmlinux.sh
>@@ -127,6 +127,16 @@ gen_btf()
> cut -d, -f1 | cut -d' ' -f2)
> bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> awk '{print $4}')
>+
>+ # Compatibility issues:
>+ # - pre-2.25 binutils objcopy doesn't support --dump-section
>+ # - llvm-objcopy doesn't support --change-section-address, but
>+ # does support --dump-section
>+ #
>+ # Try to use --dump-section which should cover both recent
>+ # binutils and llvm-objcopy and fall back to --only-section
>+ # for pre-2.25 binutils.
>+ ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> ${OBJCOPY} --change-section-address .BTF=0 \
> --set-section-flags .BTF=alloc -O binary \
> --only-section=.BTF ${1} .btf.vmlinux.bin
>--
>2.25.1.481.gfbce0eb801-goog
So let me take advantage of this email to ask some questions about
commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").
Does .BTF have the SHF_ALLOC flag?
Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?
Regarding
>Turns out llvm-objcopy doesn't implement --change-section-address [1],
This option will be difficult to implement in llvm-objcopy if we intend
it to have a GNU objcopy compatible behavior.
Without --only-section, it is not very clear how
--change-section-{address,vma,lma} will affect program headers.
There will be a debate even if we decide to implement them in llvm-objcopy.
Some PT_LOAD rewriting examples:
objcopy --change-section-address .plt=0 a b
objcopy --change-section-address .text=0 a b
There is another bug related to -B
(https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):
+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
-O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
+ objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
objcopy: architecture x86_64 unknown
+ echo 'Failed to generate BTF for vmlinux'
It should be i386:x86_64.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-16 23:15 ` Fangrui Song
@ 2020-03-16 23:56 ` Stanislav Fomichev
2020-03-17 0:56 ` Fangrui Song
0 siblings, 1 reply; 7+ messages in thread
From: Stanislav Fomichev @ 2020-03-16 23:56 UTC (permalink / raw)
To: Fangrui Song
Cc: Stanislav Fomichev, netdev, bpf, davem, ast, daniel,
Nick Desaulniers, Nathan Chancellor, clang-built-linux
On 03/16, Fangrui Song wrote:
> On 2020-03-16, Stanislav Fomichev wrote:
> > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
> > BTF") switched from --dump-section to
> > --only-section/--change-section-address for BTF export assuming
> > those ("legacy") options should cover all objcopy versions.
> >
> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
> > but it does support --dump-section. Let's partially roll back and
> > try to use --dump-section first and fall back to
> > --only-section/--change-section-address for the older binutils.
> >
> > 1. https://bugs.llvm.org/show_bug.cgi?id=45217
> >
> > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/871
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> > scripts/link-vmlinux.sh | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index dd484e92752e..8ddf57cbc439 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -127,6 +127,16 @@ gen_btf()
> > cut -d, -f1 | cut -d' ' -f2)
> > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> > awk '{print $4}')
> > +
> > + # Compatibility issues:
> > + # - pre-2.25 binutils objcopy doesn't support --dump-section
> > + # - llvm-objcopy doesn't support --change-section-address, but
> > + # does support --dump-section
> > + #
> > + # Try to use --dump-section which should cover both recent
> > + # binutils and llvm-objcopy and fall back to --only-section
> > + # for pre-2.25 binutils.
> > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> > ${OBJCOPY} --change-section-address .BTF=0 \
> > --set-section-flags .BTF=alloc -O binary \
> > --only-section=.BTF ${1} .btf.vmlinux.bin
> > --
> > 2.25.1.481.gfbce0eb801-goog
>
> So let me take advantage of this email to ask some questions about
> commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").
>
> Does .BTF have the SHF_ALLOC flag?
No, that's why we manually do '--set-section-flags .BTF=alloc' to
make --only-section work.
> Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
> Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?
I think that '--set-section-flags .BTF=alloc' causes objcopy to put
some non-zero (valid) sh_addr, that's why we need to reset it to 0.
(it's not clear if it's a feature or a bug and man isn't helpful)
> Regarding
>
> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
>
> This option will be difficult to implement in llvm-objcopy if we intend
> it to have a GNU objcopy compatible behavior.
> Without --only-section, it is not very clear how
> --change-section-{address,vma,lma} will affect program headers.
> There will be a debate even if we decide to implement them in llvm-objcopy.
>
> Some PT_LOAD rewriting examples:
>
> objcopy --change-section-address .plt=0 a b
> objcopy --change-section-address .text=0 a b
>
> There is another bug related to -B
> (https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):
>
> + objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
> -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
> + objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
> objcopy: architecture x86_64 unknown
> + echo 'Failed to generate BTF for vmlinux'
>
> It should be i386:x86_64.
Here is what I get:
+ bin_arch=i386:x86-64
+ bin_format=elf64-x86-64
+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
+ objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.
Can you try to see where your x86_64 is coming from?
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-16 22:25 [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF Stanislav Fomichev
2020-03-16 23:15 ` Fangrui Song
@ 2020-03-16 23:57 ` Nathan Chancellor
2020-03-17 0:01 ` Stanislav Fomichev
1 sibling, 1 reply; 7+ messages in thread
From: Nathan Chancellor @ 2020-03-16 23:57 UTC (permalink / raw)
To: Stanislav Fomichev; +Cc: netdev, bpf, davem, ast, daniel, Nick Desaulniers
On Mon, Mar 16, 2020 at 03:25:18PM -0700, Stanislav Fomichev wrote:
> Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
> BTF") switched from --dump-section to
> --only-section/--change-section-address for BTF export assuming
> those ("legacy") options should cover all objcopy versions.
>
> Turns out llvm-objcopy doesn't implement --change-section-address [1],
> but it does support --dump-section. Let's partially roll back and
> try to use --dump-section first and fall back to
> --only-section/--change-section-address for the older binutils.
>
> 1. https://bugs.llvm.org/show_bug.cgi?id=45217
>
> Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/871
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
> scripts/link-vmlinux.sh | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index dd484e92752e..8ddf57cbc439 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -127,6 +127,16 @@ gen_btf()
> cut -d, -f1 | cut -d' ' -f2)
> bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> awk '{print $4}')
> +
> + # Compatibility issues:
> + # - pre-2.25 binutils objcopy doesn't support --dump-section
> + # - llvm-objcopy doesn't support --change-section-address, but
> + # does support --dump-section
> + #
> + # Try to use --dump-section which should cover both recent
> + # binutils and llvm-objcopy and fall back to --only-section
> + # for pre-2.25 binutils.
> + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> ${OBJCOPY} --change-section-address .BTF=0 \
> --set-section-flags .BTF=alloc -O binary \
> --only-section=.BTF ${1} .btf.vmlinux.bin
> --
> 2.25.1.481.gfbce0eb801-goog
>
Hi Stanislav,
Thank you for the patch! This is targeting the bpf tree but it uses the
bin_file variable that comes from commit af73d78bd384 ("kbuild: Remove
debug info from kallsyms linking") in the bpf-next tree. In this form,
when applied to mainline, the first command fails because $bin_file
doesn't exist and falls back to the second command, which results in no
net change for llvm-objcopy.
When manually applied to next-20200316 or applied to mainline with
$bin_file replaced with .btf.vmlinux.bin, x86_64 and aarch64 successfully
generate BTF with tip of tree llvm-objcopy.
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-16 23:57 ` Nathan Chancellor
@ 2020-03-17 0:01 ` Stanislav Fomichev
0 siblings, 0 replies; 7+ messages in thread
From: Stanislav Fomichev @ 2020-03-17 0:01 UTC (permalink / raw)
To: Nathan Chancellor
Cc: Stanislav Fomichev, netdev, bpf, davem, ast, daniel, Nick Desaulniers
On 03/16, Nathan Chancellor wrote:
> On Mon, Mar 16, 2020 at 03:25:18PM -0700, Stanislav Fomichev wrote:
> > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
> > BTF") switched from --dump-section to
> > --only-section/--change-section-address for BTF export assuming
> > those ("legacy") options should cover all objcopy versions.
> >
> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
> > but it does support --dump-section. Let's partially roll back and
> > try to use --dump-section first and fall back to
> > --only-section/--change-section-address for the older binutils.
> >
> > 1. https://bugs.llvm.org/show_bug.cgi?id=45217
> >
> > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/871
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> > scripts/link-vmlinux.sh | 10 ++++++++++
> > 1 file changed, 10 insertions(+)
> >
> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> > index dd484e92752e..8ddf57cbc439 100755
> > --- a/scripts/link-vmlinux.sh
> > +++ b/scripts/link-vmlinux.sh
> > @@ -127,6 +127,16 @@ gen_btf()
> > cut -d, -f1 | cut -d' ' -f2)
> > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> > awk '{print $4}')
> > +
> > + # Compatibility issues:
> > + # - pre-2.25 binutils objcopy doesn't support --dump-section
> > + # - llvm-objcopy doesn't support --change-section-address, but
> > + # does support --dump-section
> > + #
> > + # Try to use --dump-section which should cover both recent
> > + # binutils and llvm-objcopy and fall back to --only-section
> > + # for pre-2.25 binutils.
> > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> > ${OBJCOPY} --change-section-address .BTF=0 \
> > --set-section-flags .BTF=alloc -O binary \
> > --only-section=.BTF ${1} .btf.vmlinux.bin
> > --
> > 2.25.1.481.gfbce0eb801-goog
> >
>
> Hi Stanislav,
>
> Thank you for the patch! This is targeting the bpf tree but it uses the
> bin_file variable that comes from commit af73d78bd384 ("kbuild: Remove
> debug info from kallsyms linking") in the bpf-next tree. In this form,
> when applied to mainline, the first command fails because $bin_file
> doesn't exist and falls back to the second command, which results in no
> net change for llvm-objcopy.
>
> When manually applied to next-20200316 or applied to mainline with
> $bin_file replaced with .btf.vmlinux.bin, x86_64 and aarch64 successfully
> generate BTF with tip of tree llvm-objcopy.
>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Oops, sorry, I did it on top of bpf-next, but then blindly cherry-picked
on top of bpf tree. In this case, should we put in only in bpf-next?
Do we need a stable backport for the older versions?
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-16 23:56 ` Stanislav Fomichev
@ 2020-03-17 0:56 ` Fangrui Song
2020-03-17 3:14 ` Andrii Nakryiko
0 siblings, 1 reply; 7+ messages in thread
From: Fangrui Song @ 2020-03-17 0:56 UTC (permalink / raw)
To: Stanislav Fomichev
Cc: Stanislav Fomichev, netdev, bpf, davem, ast, daniel,
Nick Desaulniers, Nathan Chancellor, clang-built-linux
On 2020-03-16, Stanislav Fomichev wrote:
>On 03/16, Fangrui Song wrote:
>> On 2020-03-16, Stanislav Fomichev wrote:
>> > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
>> > BTF") switched from --dump-section to
>> > --only-section/--change-section-address for BTF export assuming
>> > those ("legacy") options should cover all objcopy versions.
>> >
>> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
>> > but it does support --dump-section. Let's partially roll back and
>> > try to use --dump-section first and fall back to
>> > --only-section/--change-section-address for the older binutils.
>> >
>> > 1. https://bugs.llvm.org/show_bug.cgi?id=45217
>> >
>> > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
>> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
>> > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
>> > Link: https://github.com/ClangBuiltLinux/linux/issues/871
>> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
>> > ---
>> > scripts/link-vmlinux.sh | 10 ++++++++++
>> > 1 file changed, 10 insertions(+)
>> >
>> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
>> > index dd484e92752e..8ddf57cbc439 100755
>> > --- a/scripts/link-vmlinux.sh
>> > +++ b/scripts/link-vmlinux.sh
>> > @@ -127,6 +127,16 @@ gen_btf()
>> > cut -d, -f1 | cut -d' ' -f2)
>> > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
>> > awk '{print $4}')
>> > +
>> > + # Compatibility issues:
>> > + # - pre-2.25 binutils objcopy doesn't support --dump-section
>> > + # - llvm-objcopy doesn't support --change-section-address, but
>> > + # does support --dump-section
>> > + #
>> > + # Try to use --dump-section which should cover both recent
>> > + # binutils and llvm-objcopy and fall back to --only-section
>> > + # for pre-2.25 binutils.
>> > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
>> > ${OBJCOPY} --change-section-address .BTF=0 \
>> > --set-section-flags .BTF=alloc -O binary \
>> > --only-section=.BTF ${1} .btf.vmlinux.bin
>> > --
>> > 2.25.1.481.gfbce0eb801-goog
>>
>> So let me take advantage of this email to ask some questions about
>> commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").
>>
>> Does .BTF have the SHF_ALLOC flag?
>No, that's why we manually do '--set-section-flags .BTF=alloc' to
>make --only-section work.
>
>> Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
>> Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?
>I think that '--set-section-flags .BTF=alloc' causes objcopy to put
>some non-zero (valid) sh_addr, that's why we need to reset it to 0.
>
>(it's not clear if it's a feature or a bug and man isn't helpful)
>
>> Regarding
>>
>> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
>>
>> This option will be difficult to implement in llvm-objcopy if we intend
>> it to have a GNU objcopy compatible behavior.
>> Without --only-section, it is not very clear how
>> --change-section-{address,vma,lma} will affect program headers.
>> There will be a debate even if we decide to implement them in llvm-objcopy.
>>
>> Some PT_LOAD rewriting examples:
>>
>> objcopy --change-section-address .plt=0 a b
>> objcopy --change-section-address .text=0 a b
>>
>> There is another bug related to -B
>> (https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):
>>
>> + objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
>> -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
>> + objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
>> objcopy: architecture x86_64 unknown
>> + echo 'Failed to generate BTF for vmlinux'
>>
>> It should be i386:x86_64.
>Here is what I get:
>
>+ bin_arch=i386:x86-64
>+ bin_format=elf64-x86-64
>+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
>+ objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.
>
>Can you try to see where your x86_64 is coming from?
llvm-objdump -f does not print bfdarch (ARCH= in binutils-gdb/ld/emulparams/*.sh).
% objdump -f .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: file format elf64-x86-64
architecture: i386:x86-64, flags 0x00000010:
HAS_SYMS
start address 0x0000000000000000
% llvm-objdump -f .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: file format elf64-x86-64
architecture: x86_64
start address: 0x0000000000000000
% objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
# happy
% objcopy -I binary -O elf64-x86-64 -B x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
objcopy: architecture x86-64 unknown
As a non-x86 example, elf64-powerpcle / powerpc:common64:
% powerpc64le-linux-gnu-objdump -f meow.btf.vmlinux.bin.o
meow.btf.vmlinux.bin.o: file format elf64-powerpcle
architecture: powerpc:common64, flags 0x00000010:
Unfortunately, GNU objcopy<2.34 (before I complained about the redundant -B https://sourceware.org/bugzilla/show_bug.cgi?id=24968)
could not infer -B from -O elf* .
% objcopy -I binary -O elf64-x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o #<2.34
% file .btf.vmlinux.bin.o
.btf.vmlinux.bin.o: ELF 64-bit LSB relocatable, no machine, version 1 (SYSV), not stripped
objcopy: architecture x86-64 unknown
GNU ld and lld will error for e_machine==0.
I will be a bit nervous to make llvm-objdump behave more BFD like.
Adding i386:x86-64, powerpc:common64, etc does not look particularly clean.
Fortunately, looking at the code, it seems that we only want to retain .BTF
The following scheme may be simpler:
objcopy --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin.o && printf '\1' | dd of=.btf.vmlinux.bin.o conv=notrunc bs=1 seek=16
The command after && is to change e_type from ET_EXEC to ET_REL. GNU ld has an extremely rare feature that allows ET_EXEC to be linked,
but lld is more rigid and will reject such an input file.
https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF
2020-03-17 0:56 ` Fangrui Song
@ 2020-03-17 3:14 ` Andrii Nakryiko
0 siblings, 0 replies; 7+ messages in thread
From: Andrii Nakryiko @ 2020-03-17 3:14 UTC (permalink / raw)
To: Fangrui Song
Cc: Stanislav Fomichev, Stanislav Fomichev, Networking, bpf,
David S. Miller, Alexei Starovoitov, Daniel Borkmann,
Nick Desaulniers, Nathan Chancellor, clang-built-linux
On Mon, Mar 16, 2020 at 6:09 PM Fangrui Song <maskray@google.com> wrote:
>
> On 2020-03-16, Stanislav Fomichev wrote:
> >On 03/16, Fangrui Song wrote:
> >> On 2020-03-16, Stanislav Fomichev wrote:
> >> > Commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux
> >> > BTF") switched from --dump-section to
> >> > --only-section/--change-section-address for BTF export assuming
> >> > those ("legacy") options should cover all objcopy versions.
> >> >
> >> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
> >> > but it does support --dump-section. Let's partially roll back and
> >> > try to use --dump-section first and fall back to
> >> > --only-section/--change-section-address for the older binutils.
> >> >
> >> > 1. https://bugs.llvm.org/show_bug.cgi?id=45217
> >> >
> >> > Fixes: df786c9b9476 ("bpf: Force .BTF section start to zero when dumping from vmlinux")
> >> > Tested-by: Nick Desaulniers <ndesaulniers@google.com>
> >> > Reported-by: Nathan Chancellor <natechancellor@gmail.com>
> >> > Link: https://github.com/ClangBuiltLinux/linux/issues/871
> >> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> >> > ---
> >> > scripts/link-vmlinux.sh | 10 ++++++++++
> >> > 1 file changed, 10 insertions(+)
> >> >
> >> > diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> >> > index dd484e92752e..8ddf57cbc439 100755
> >> > --- a/scripts/link-vmlinux.sh
> >> > +++ b/scripts/link-vmlinux.sh
> >> > @@ -127,6 +127,16 @@ gen_btf()
> >> > cut -d, -f1 | cut -d' ' -f2)
> >> > bin_format=$(LANG=C ${OBJDUMP} -f ${1} | grep 'file format' | \
> >> > awk '{print $4}')
> >> > +
> >> > + # Compatibility issues:
> >> > + # - pre-2.25 binutils objcopy doesn't support --dump-section
> >> > + # - llvm-objcopy doesn't support --change-section-address, but
> >> > + # does support --dump-section
> >> > + #
> >> > + # Try to use --dump-section which should cover both recent
> >> > + # binutils and llvm-objcopy and fall back to --only-section
> >> > + # for pre-2.25 binutils.
> >> > + ${OBJCOPY} --dump-section .BTF=$bin_file ${1} 2>/dev/null || \
> >> > ${OBJCOPY} --change-section-address .BTF=0 \
> >> > --set-section-flags .BTF=alloc -O binary \
> >> > --only-section=.BTF ${1} .btf.vmlinux.bin
> >> > --
> >> > 2.25.1.481.gfbce0eb801-goog
> >>
> >> So let me take advantage of this email to ask some questions about
> >> commit da5fb18225b4 ("bpf: Support pre-2.25-binutils objcopy for vmlinux BTF").
> >>
> >> Does .BTF have the SHF_ALLOC flag?
> >No, that's why we manually do '--set-section-flags .BTF=alloc' to
> >make --only-section work.
> >
> >> Is it a GNU objcopy<2.25 bug that objcopy --set-section-flags .BTF=alloc -O binary --only-section=.BTF does not skip the content?
> >> Non-SHF_ALLOC sections usually have 0 sh_addr. Why do they need --change-section-address .BTF=0 at all?
> >I think that '--set-section-flags .BTF=alloc' causes objcopy to put
> >some non-zero (valid) sh_addr, that's why we need to reset it to 0.
> >
> >(it's not clear if it's a feature or a bug and man isn't helpful)
> >
> >> Regarding
> >>
> >> > Turns out llvm-objcopy doesn't implement --change-section-address [1],
> >>
> >> This option will be difficult to implement in llvm-objcopy if we intend
> >> it to have a GNU objcopy compatible behavior.
> >> Without --only-section, it is not very clear how
> >> --change-section-{address,vma,lma} will affect program headers.
> >> There will be a debate even if we decide to implement them in llvm-objcopy.
> >>
> >> Some PT_LOAD rewriting examples:
> >>
> >> objcopy --change-section-address .plt=0 a b
> >> objcopy --change-section-address .text=0 a b
> >>
> >> There is another bug related to -B
> >> (https://github.com/ClangBuiltLinux/linux/issues/871#issuecomment-599790909):
> >>
> >> + objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc
> >> -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
> >> + objcopy -I binary -O elf64-x86-64 -B x86_64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o
> >> objcopy: architecture x86_64 unknown
> >> + echo 'Failed to generate BTF for vmlinux'
> >>
> >> It should be i386:x86_64.
> >Here is what I get:
> >
> >+ bin_arch=i386:x86-64
> >+ bin_format=elf64-x86-64
> >+ objcopy --change-section-address .BTF=0 --set-section-flags .BTF=alloc -O binary --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin
> >+ objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.
> >
> >Can you try to see where your x86_64 is coming from?
>
> llvm-objdump -f does not print bfdarch (ARCH= in binutils-gdb/ld/emulparams/*.sh).
>
> % objdump -f .btf.vmlinux.bin.o
>
> .btf.vmlinux.bin.o: file format elf64-x86-64
> architecture: i386:x86-64, flags 0x00000010:
> HAS_SYMS
> start address 0x0000000000000000
>
> % llvm-objdump -f .btf.vmlinux.bin.o
>
> .btf.vmlinux.bin.o: file format elf64-x86-64
>
> architecture: x86_64
> start address: 0x0000000000000000
>
> % objcopy -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
> # happy
> % objcopy -I binary -O elf64-x86-64 -B x86-64 --rename-section .data=.BTF .btf.vmlinux.bin meow.btf.vmlinux.bin.o
> objcopy: architecture x86-64 unknown
>
>
> As a non-x86 example, elf64-powerpcle / powerpc:common64:
>
> % powerpc64le-linux-gnu-objdump -f meow.btf.vmlinux.bin.o
>
> meow.btf.vmlinux.bin.o: file format elf64-powerpcle
> architecture: powerpc:common64, flags 0x00000010:
>
>
> Unfortunately, GNU objcopy<2.34 (before I complained about the redundant -B https://sourceware.org/bugzilla/show_bug.cgi?id=24968)
> could not infer -B from -O elf* .
> % objcopy -I binary -O elf64-x86-64 --rename-section .data=.BTF .btf.vmlinux.bin .btf.vmlinux.bin.o #<2.34
> % file .btf.vmlinux.bin.o
> .btf.vmlinux.bin.o: ELF 64-bit LSB relocatable, no machine, version 1 (SYSV), not stripped
> objcopy: architecture x86-64 unknown
>
> GNU ld and lld will error for e_machine==0.
>
>
>
> I will be a bit nervous to make llvm-objdump behave more BFD like.
> Adding i386:x86-64, powerpc:common64, etc does not look particularly clean.
> Fortunately, looking at the code, it seems that we only want to retain .BTF
> The following scheme may be simpler:
>
> objcopy --only-section=.BTF .tmp_vmlinux.btf .btf.vmlinux.bin.o && printf '\1' | dd of=.btf.vmlinux.bin.o conv=notrunc bs=1 seek=16
>
This part looks pretty crazy. Would it be simpler to detect whether
$(OBJCOPY) is llvm-objcopy and handle that in a bit less hacky way?
> The command after && is to change e_type from ET_EXEC to ET_REL. GNU ld has an extremely rare feature that allows ET_EXEC to be linked,
> but lld is more rigid and will reject such an input file.
> https://mail.coreboot.org/hyperkitty/list/seabios@seabios.org/thread/HHIUPUXRIZ3KLTK4TPLG2V4PFP32HRBE/
^ permalink raw reply [flat|nested] 7+ messages in thread
end of thread, other threads:[~2020-03-17 3:14 UTC | newest]
Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-03-16 22:25 [PATCH bpf] bpf: Support llvm-objcopy for vmlinux BTF Stanislav Fomichev
2020-03-16 23:15 ` Fangrui Song
2020-03-16 23:56 ` Stanislav Fomichev
2020-03-17 0:56 ` Fangrui Song
2020-03-17 3:14 ` Andrii Nakryiko
2020-03-16 23:57 ` Nathan Chancellor
2020-03-17 0:01 ` Stanislav Fomichev
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