On 2020-03-17, Stanislav Fomichev wrote:
On 03/16, Fangrui Song wrote:
On 2020-03-16, Andrii Nakryiko wrote:
> On Mon, Mar 16, 2020 at 10:21 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> >
> >
> > On 2020-03-16, Andrii Nakryiko wrote:
> > >On Mon, Mar 16, 2020 at 8:37 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> > >>
> > >> On 2020-03-16, Andrii Nakryiko wrote:
> > >> >On Mon, Mar 16, 2020 at 6:17 PM Fangrui Song <maskray@xxxxxxxxxx> wrote:
> > >> >>
> > >> >> 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 <sdf@xxxxxxxxxx>
> > >> >> Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > >> >> Tested-by: 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>
> > >> >> ---
> > >> >> 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 <maskray@xxxxxxxxxx>
> > >> 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 <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>
> > >> ---
> > >> 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 ${1}
> > >
> > >I'm not sure why you are touching this line at all. LLVM_OBJCOPY part
> > >is necessary, pahole assumes llvm-objcopy by default, but that can
> > >(and should for objcopy) be overridden with LLVM_OBJCOPY.
> >
> > Why is LLVM_OBJCOPY assumed? What if llvm-objcopy is not available?
>
> It's pahole assumption that we have to live with. pahole assumes
> llvm-objcopy internally, unless it is overriden with LLVM_OBJCOPY env
> var. So please revert this line otherwise you are breaking it for GCC
> objcopy case.
Acknowledged. Uploaded v3.
I added back 2>/dev/null which was removed by a previous change, to
suppress GNU objcopy warnings. The warnings could be annoying in V=1
output.
> > This is confusing that one tool assumes llvm-objcopy while the block
> > below immediately uses GNU objcopy (without this patch).
> >
> > e83b9f55448afce3fe1abcd1d10db9584f8042a6 "kbuild: add ability to
> > generate BTF type info for vmlinux" does not say why LLVM_OBJCOPY is
> > set.
> >
> > >>
> > >> - # dump .BTF section into raw binary file to link with final vmlinux
> > >> - bin_arch=$(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 <maskray@xxxxxxxxxx>
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 <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>
---
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
Attached v4.
* Added status=none to the dd command to suppress stderr output.
* `objcopy -I binary` synthesized symbols are only used in BTF, not
elsewhere. I think we can replace it with a more common trick,
__start_$sectionname __stop_$sectionname.
* GNU ld<2.23 can define __start_BTF and __stop_BTF as SHN_ABS.
I think it is totally fine for a SHN_ABS symbol to be referenced by an
R_X86_64_32S (absolute relocation), but arch/x86/tools/relocs.c
contains an unnecessarily rigid check that rejects this.
...
Invalid absolute R_X86_64_32S relocation: __start_BTF
make[3]: *** [arch/x86/boot/compressed/Makefile:123:
arch/x86/boot/compressed/vmlinux.relocs] Error 1
Since we are going to bump binutils version requirement to 2.23, which
will completely avoid the issue. I will not mention it again.
https://lore.kernel.org/lkml/202003161354.538479F16@keescook/
* I should mention that an orphan BTF (previously .BTF) could trigger
a --orphan-handling=warn warning. So eventually we might need to
add an output section description
BTF : { *(BTF) }
to the vmlinux linker script for every arch.
I'll not do that in this patch, though.
}
# Create ${2} .o file with all symbols from the ${1} object file
--
2.25.1.481.gfbce0eb801-goog
From 9b694d68fefe041464eccb948f6d246fab67942d Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@xxxxxxxxxx>
Date: Tue, 17 Mar 2020 13:51:04 -0700
Subject: [PATCH bpf-next v4] bpf: Support llvm-objcopy and
llvm-objdump for vmlinux BTF
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.
.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.
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 | 16 ++++++----------
3 files changed, 15 insertions(+), 21 deletions(-)
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 787140095e58..51fff49de561 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 dd484e92752e..c0d2ecf1bff7 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -122,16 +122,12 @@ 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, 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
}
# Create ${2} .o file with all symbols from the ${1} object file
--
2.25.1.481.gfbce0eb801-goog
