BTF without CONFIG_DEBUG_INFO_BTF=y

BTF without CONFIG_DEBUG_INFO_BTF=y

[Kevin Sheldrake]

Hello

I've seen mention a few times that BTF information can be made available from a kernel that wasn't configured with CONFIG_DEBUG_INFO_BTF.  Please can someone tell me if this is true and, if so, how I could go about accessing and using it in kernels 4.15 to 5.8?

I have built the dwarves package from the github latest and run pahole with '-J' against my kernel image to no avail - it actually seg faults:

~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
btf_elf__new: cannot get elf header.
ctf__new: cannot get elf header.
~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
btf_elf__new: cannot get elf header.
ctf__new: cannot get elf header.
Segmentation fault
~/dwarves/build $ sudo ./pahole --version
v1.17

Judging by the output, I'm guessing that my kernel image isn't the right kind of file.  Can someone point me in the right direction?

Additionally, if this is possible, is there any documentation on the API so I could incorporate the functionality into my own programs directly?

Thanks

Kev

Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Toke Høiland-Jørgensen]

Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:

> Hello
>
> I've seen mention a few times that BTF information can be made
> available from a kernel that wasn't configured with
> CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true and,
> if so, how I could go about accessing and using it in kernels 4.15 to
> 5.8?
>
> I have built the dwarves package from the github latest and run pahole
> with '-J' against my kernel image to no avail - it actually seg
> faults:
>
> ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
> btf_elf__new: cannot get elf header.
> ctf__new: cannot get elf header.
> ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
> btf_elf__new: cannot get elf header.
> ctf__new: cannot get elf header.
> Segmentation fault
> ~/dwarves/build $ sudo ./pahole --version
> v1.17
>
> Judging by the output, I'm guessing that my kernel image isn't the
> right kind of file. Can someone point me in the right direction?

vmlinuz is a compressed image. There's a script in the kernel source
tree (scripts/extract-vmlinux), however the kernel image in /boot/
probably also has debug information stripped from it, so that likely
won't help you. You'll need to get hold of a kernel image with debug
information still intact somehow...

(Either way, pahole shouldn't be segfaulting, so hopefully someone can
take a look at that).

-Toke

Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Arnaldo Carvalho de Melo]

Em Thu, Oct 01, 2020 at 12:33:18PM +0200, Toke Høiland-Jørgensen escreveu:
> Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:
> 
> > Hello
> >
> > I've seen mention a few times that BTF information can be made
> > available from a kernel that wasn't configured with
> > CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true and,
> > if so, how I could go about accessing and using it in kernels 4.15 to
> > 5.8?
> >
> > I have built the dwarves package from the github latest and run pahole
> > with '-J' against my kernel image to no avail - it actually seg
> > faults:
> >
> > ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
> > btf_elf__new: cannot get elf header.
> > ctf__new: cannot get elf header.
> > ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
> > btf_elf__new: cannot get elf header.
> > ctf__new: cannot get elf header.
> > Segmentation fault
> > ~/dwarves/build $ sudo ./pahole --version
> > v1.17
> >
> > Judging by the output, I'm guessing that my kernel image isn't the
> > right kind of file. Can someone point me in the right direction?
> 
> vmlinuz is a compressed image. There's a script in the kernel source
> tree (scripts/extract-vmlinux), however the kernel image in /boot/
> probably also has debug information stripped from it, so that likely
> won't help you. You'll need to get hold of a kernel image with debug
> information still intact somehow...
> 
> (Either way, pahole shouldn't be segfaulting, so hopefully someone can
> take a look at that).

Reproduced:

[acme@five pahole]$ cp /boot/vmlinuz-5.9.0-rc6+ .
[acme@five pahole]$ pahole -J vmlinuz-5.9.0-rc6+
btf_elf__new: cannot get elf header.
ctf__new: cannot get elf header.
tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
pahole: type 'vmlinuz-5.9.0-rc6+' not found
libbpf: Unsupported BTF_KIND:0
btf_elf__encode: btf__new failed!
free(): double free detected in tcache 2
Aborted (core dumped)
[acme@five pahole]$

Working on a fix. Thanks for the report!

- Arnaldo

Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Arnaldo Carvalho de Melo]

Em Thu, Oct 01, 2020 at 09:50:29AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Thu, Oct 01, 2020 at 12:33:18PM +0200, Toke Høiland-Jørgensen escreveu:
> > Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:
> > > I've seen mention a few times that BTF information can be made
> > > available from a kernel that wasn't configured with
> > > CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true and,
> > > if so, how I could go about accessing and using it in kernels 4.15 to
> > > 5.8?

> > > I have built the dwarves package from the github latest and run pahole
> > > with '-J' against my kernel image to no avail - it actually seg
> > > faults:

> > > ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
> > > btf_elf__new: cannot get elf header.
> > > ctf__new: cannot get elf header.
> > > ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
> > > btf_elf__new: cannot get elf header.
> > > ctf__new: cannot get elf header.
> > > Segmentation fault
> > > ~/dwarves/build $ sudo ./pahole --version
> > > v1.17

> > > Judging by the output, I'm guessing that my kernel image isn't the
> > > right kind of file. Can someone point me in the right direction?

> > vmlinuz is a compressed image. There's a script in the kernel source
> > tree (scripts/extract-vmlinux), however the kernel image in /boot/
> > probably also has debug information stripped from it, so that likely
> > won't help you. You'll need to get hold of a kernel image with debug
> > information still intact somehow...

> > (Either way, pahole shouldn't be segfaulting, so hopefully someone can
> > take a look at that).

> Reproduced:

> [acme@five pahole]$ cp /boot/vmlinuz-5.9.0-rc6+ .
> [acme@five pahole]$ pahole -J vmlinuz-5.9.0-rc6+
> btf_elf__new: cannot get elf header.
> ctf__new: cannot get elf header.
> tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
> pahole: type 'vmlinuz-5.9.0-rc6+' not found
> libbpf: Unsupported BTF_KIND:0
> btf_elf__encode: btf__new failed!
> free(): double free detected in tcache 2
> Aborted (core dumped)
> [acme@five pahole]$
 
> Working on a fix. Thanks for the report!

commit 4e55425d9eaac78689fbd296283e1557bb6ca725
Author: Arnaldo Carvalho de Melo <acme@redhat.com>
Date:   Thu Oct 1 10:10:50 2020 -0300

    pahole: Only try using a single file name as a type name if not encoding BTF or CTF
    
    Otherwise we end up trying to encode without any debug info and this
    causes a segfault:
    
    Before:
    
      $ pahole -J vmlinuz-5.9.0-rc6+
      tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
      pahole: type 'vmlinuz-5.9.0-rc6+' not found
      libbpf: Unsupported BTF_KIND:0
      btf_elf__encode: btf__new failed!
      free(): double free detected in tcache 2
      Aborted (core dumped)
      $
    
    The vmlinuz file doesn't contain any debugging info, fixing it we get:
    
      $ pahole -J vmlinuz-5.9.0-rc6+
      pahole: vmlinuz-5.9.0-rc6+: No debugging information found
      $
    
    If debugging info is available, it all works as before:
    
    Using /sys/kernel/btf/vmlinux
    
    $ ls -la /sys/kernel/btf/vmlinux
    -r--r--r--. 1 root root 3393761 Oct  1 09:50 /sys/kernel/btf/vmlinux
    
      $ pahole -E fw_cache_entry
      struct fw_cache_entry {
            struct list_head {
                    struct list_head * next;          /*     0     8 */
                    struct list_head * prev;          /*     8     8 */
            } list; /*     0    16 */
            const char  *              name;          /*    16     8 */
    
            /* size: 24, cachelines: 1, members: 2 */
            /* last cacheline: 24 bytes */
      };
      $
    
    Or explicitely asking for DWARF, where it will find the appropriate
    vmlinux according to its buildid in /sys/kernel/notes:
    
      $ pahole -F dwarf pm_clock_entry
      struct pm_clock_entry {
            struct list_head           node;          /*     0    16 */
            char *                     con_id;        /*    16     8 */
            struct clk *               clk;           /*    24     8 */
            enum pce_status            status;        /*    32     4 */
    
            /* size: 40, cachelines: 1, members: 4 */
            /* padding: 4 */
            /* last cacheline: 40 bytes */
      };
      $ pahole -F dwarf --expand_types pm_clock_entry
      struct pm_clock_entry {
            struct list_head {
                    struct list_head * next;          /*     0     8 */
                    struct list_head * prev;          /*     8     8 */
            } node; /*     0    16 */
            char *                     con_id;        /*    16     8 */
            struct clk *               clk;           /*    24     8 */
            enum pce_status            status;        /*    32     4 */
    
            /* size: 40, cachelines: 1, members: 4 */
            /* padding: 4 */
            /* last cacheline: 40 bytes */
      };
      $
    
    Reported-by: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>
    Cc: Toke Høiland-Jørgensen <toke@redhat.com>
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/pahole.c b/pahole.c
index ee5f2f7f76b0f1d6..dac7a492a266e7f1 100644
--- a/pahole.c
+++ b/pahole.c
@@ -2686,7 +2686,7 @@ try_sole_arg_as_class_names:
 
 	err = cus__load_files(cus, &conf_load, argv + remaining);
 	if (err != 0) {
-		if (class_name == NULL) {
+		if (class_name == NULL && !btf_encode && !ctf_encode) {
 			class_name = argv[remaining];
 			remaining = argc;
 			goto try_sole_arg_as_class_names;

Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Toke Høiland-Jørgensen]

Arnaldo Carvalho de Melo <acme@kernel.org> writes:

> Em Thu, Oct 01, 2020 at 09:50:29AM -0300, Arnaldo Carvalho de Melo escreveu:
>> Em Thu, Oct 01, 2020 at 12:33:18PM +0200, Toke Høiland-Jørgensen escreveu:
>> > Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:
>> > > I've seen mention a few times that BTF information can be made
>> > > available from a kernel that wasn't configured with
>> > > CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true and,
>> > > if so, how I could go about accessing and using it in kernels 4.15 to
>> > > 5.8?
>
>> > > I have built the dwarves package from the github latest and run pahole
>> > > with '-J' against my kernel image to no avail - it actually seg
>> > > faults:
>
>> > > ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
>> > > btf_elf__new: cannot get elf header.
>> > > ctf__new: cannot get elf header.
>> > > ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
>> > > btf_elf__new: cannot get elf header.
>> > > ctf__new: cannot get elf header.
>> > > Segmentation fault
>> > > ~/dwarves/build $ sudo ./pahole --version
>> > > v1.17
>
>> > > Judging by the output, I'm guessing that my kernel image isn't the
>> > > right kind of file. Can someone point me in the right direction?
>
>> > vmlinuz is a compressed image. There's a script in the kernel source
>> > tree (scripts/extract-vmlinux), however the kernel image in /boot/
>> > probably also has debug information stripped from it, so that likely
>> > won't help you. You'll need to get hold of a kernel image with debug
>> > information still intact somehow...
>
>> > (Either way, pahole shouldn't be segfaulting, so hopefully someone can
>> > take a look at that).
>
>> Reproduced:
>
>> [acme@five pahole]$ cp /boot/vmlinuz-5.9.0-rc6+ .
>> [acme@five pahole]$ pahole -J vmlinuz-5.9.0-rc6+
>> btf_elf__new: cannot get elf header.
>> ctf__new: cannot get elf header.
>> tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
>> pahole: type 'vmlinuz-5.9.0-rc6+' not found
>> libbpf: Unsupported BTF_KIND:0
>> btf_elf__encode: btf__new failed!
>> free(): double free detected in tcache 2
>> Aborted (core dumped)
>> [acme@five pahole]$
>  
>> Working on a fix. Thanks for the report!
>
> commit 4e55425d9eaac78689fbd296283e1557bb6ca725
> Author: Arnaldo Carvalho de Melo <acme@redhat.com>
> Date:   Thu Oct 1 10:10:50 2020 -0300
>
>     pahole: Only try using a single file name as a type name if not encoding BTF or CTF
>     
>     Otherwise we end up trying to encode without any debug info and this
>     causes a segfault:
>     
>     Before:
>     
>       $ pahole -J vmlinuz-5.9.0-rc6+
>       tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
>       pahole: type 'vmlinuz-5.9.0-rc6+' not found
>       libbpf: Unsupported BTF_KIND:0
>       btf_elf__encode: btf__new failed!
>       free(): double free detected in tcache 2
>       Aborted (core dumped)
>       $
>     
>     The vmlinuz file doesn't contain any debugging info, fixing it we get:
>     
>       $ pahole -J vmlinuz-5.9.0-rc6+
>       pahole: vmlinuz-5.9.0-rc6+: No debugging information found
>       $
>     
>     If debugging info is available, it all works as before:
>     
>     Using /sys/kernel/btf/vmlinux
>     
>     $ ls -la /sys/kernel/btf/vmlinux
>     -r--r--r--. 1 root root 3393761 Oct  1 09:50 /sys/kernel/btf/vmlinux
>     
>       $ pahole -E fw_cache_entry
>       struct fw_cache_entry {
>             struct list_head {
>                     struct list_head * next;          /*     0     8 */
>                     struct list_head * prev;          /*     8     8 */
>             } list; /*     0    16 */
>             const char  *              name;          /*    16     8 */
>     
>             /* size: 24, cachelines: 1, members: 2 */
>             /* last cacheline: 24 bytes */
>       };
>       $
>     
>     Or explicitely asking for DWARF, where it will find the appropriate
>     vmlinux according to its buildid in /sys/kernel/notes:
>     
>       $ pahole -F dwarf pm_clock_entry
>       struct pm_clock_entry {
>             struct list_head           node;          /*     0    16 */
>             char *                     con_id;        /*    16     8 */
>             struct clk *               clk;           /*    24     8 */
>             enum pce_status            status;        /*    32     4 */
>     
>             /* size: 40, cachelines: 1, members: 4 */
>             /* padding: 4 */
>             /* last cacheline: 40 bytes */
>       };
>       $ pahole -F dwarf --expand_types pm_clock_entry
>       struct pm_clock_entry {
>             struct list_head {
>                     struct list_head * next;          /*     0     8 */
>                     struct list_head * prev;          /*     8     8 */
>             } node; /*     0    16 */
>             char *                     con_id;        /*    16     8 */
>             struct clk *               clk;           /*    24     8 */
>             enum pce_status            status;        /*    32     4 */
>     
>             /* size: 40, cachelines: 1, members: 4 */
>             /* padding: 4 */
>             /* last cacheline: 40 bytes */
>       };
>       $
>     
>     Reported-by: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>
>     Cc: Toke Høiland-Jørgensen <toke@redhat.com>
>     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Yeah, that's much better!

Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

RE: [EXTERNAL] Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Kevin Sheldrake]

> -----Original Message-----
> From: Toke Høiland-Jørgensen <toke@redhat.com>
> Sent: 01 October 2020 14:37
> To: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>;
> bpf@vger.kernel.org
> Subject: [EXTERNAL] Re: BTF without CONFIG_DEBUG_INFO_BTF=y
> 
> Arnaldo Carvalho de Melo <acme@kernel.org> writes:
> 
> > Em Thu, Oct 01, 2020 at 09:50:29AM -0300, Arnaldo Carvalho de Melo
> escreveu:
> >> Em Thu, Oct 01, 2020 at 12:33:18PM +0200, Toke Høiland-Jørgensen
> escreveu:
> >> > Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:
> >> > > I've seen mention a few times that BTF information can be made
> >> > > available from a kernel that wasn't configured with
> >> > > CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true
> >> > > and, if so, how I could go about accessing and using it in
> >> > > kernels 4.15 to 5.8?
> >
> >> > > I have built the dwarves package from the github latest and run
> >> > > pahole with '-J' against my kernel image to no avail - it
> >> > > actually seg
> >> > > faults:
> >
> >> > > ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
> >> > > btf_elf__new: cannot get elf header.
> >> > > ctf__new: cannot get elf header.
> >> > > ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
> >> > > btf_elf__new: cannot get elf header.
> >> > > ctf__new: cannot get elf header.
> >> > > Segmentation fault
> >> > > ~/dwarves/build $ sudo ./pahole --version
> >> > > v1.17
> >
> >> > > Judging by the output, I'm guessing that my kernel image isn't
> >> > > the right kind of file. Can someone point me in the right direction?
> >
> >> > vmlinuz is a compressed image. There's a script in the kernel
> >> > source tree (scripts/extract-vmlinux), however the kernel image in
> >> > /boot/ probably also has debug information stripped from it, so
> >> > that likely won't help you. You'll need to get hold of a kernel
> >> > image with debug information still intact somehow...
> >

SNIP
> >     Reported-by: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>
> >     Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> >     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> 
> Yeah, that's much better!
> 
> Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

Thank you to Arnaldo for identifying and fixing the seg fault.

Am I right in thinking that pahole can't work on a standard out-of-the-box distro such as Ubuntu 16.04 (v4.15) or 18.04 (v5.3/5.4) as debugging information isn't available in the kernel, and equally isn't available anywhere else, without recompiling it?

Many thanks

Kev

Re: [EXTERNAL] Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Arnaldo Carvalho de Melo]

Em Thu, Oct 01, 2020 at 01:45:25PM +0000, Kevin Sheldrake escreveu:
> > Arnaldo Carvalho de Melo <acme@kernel.org> writes:
> SNIP
> > >     Reported-by: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>
> > >     Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> > >     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

> > Yeah, that's much better!

> > Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

> Thank you to Arnaldo for identifying and fixing the seg fault.
 
> Am I right in thinking that pahole can't work on a standard
> out-of-the-box distro such as Ubuntu 16.04 (v4.15) or 18.04 (v5.3/5.4)
> as debugging information isn't available in the kernel, and equally
> isn't available anywhere else, without recompiling it?

You mean recompiling the kernel with debug info? 

You just have to install de debuginfo packages, that in Ubuntu is
different than in Fedora but in the end is just the same principle: you
have matching separate debug info files for each kernel released by the
distro, please take a look at:

https://wiki.ubuntu.com/Debug%20Symbol%20Packages

There is also a page for Systemtap where it states the steps
specifically for the kernel debug symbol packages:

https://wiki.ubuntu.com/Kernel/Systemtap

This part:

codename=$(lsb_release -c | awk  '{print $2}')
sudo tee /etc/apt/sources.list.d/ddebs.list << EOF
deb http://ddebs.ubuntu.com/ ${codename}      main restricted universe multiverse
deb http://ddebs.ubuntu.com/ ${codename}-security main restricted universe multiverse
deb http://ddebs.ubuntu.com/ ${codename}-updates  main restricted universe multiverse
deb http://ddebs.ubuntu.com/ ${codename}-proposed main restricted universe multiverse
EOF

sudo apt-get update
sudo apt-get install linux-image-$(uname -r)-dbgsym

Also make sure you have the dwarves package installed, if you haven't
built it from sources as Toke suggested, as below, but then,
ubuntu:18.04 has a way too old version of pahole/dwarves, 1.10, so build
it from sources to have the BTF support and many other goodies.

I just checked and ubuntu:19.10 has dwarves 1.15, that is not enough,
1.16 is what the kernel expects to generate BTF:

[acme@five perf]$ grep "pahole version" scripts/*.sh
scripts/link-vmlinux.sh:		echo >&2 "BTF: ${1}: pahole version $(${PAHOLE} --version) is too old, need at least v1.16"
[acme@five perf]$

- Arnaldo

[perfbuilder@five ~]$ podman run --rm -ti ubuntu:18.04
root@f9beca14daaf:/# bash
root@f9beca14daaf:/# apt-get update
Get:1 http://archive.ubuntu.com/ubuntu bionic InRelease [242 kB]
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
Get:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
Get:4 http://security.ubuntu.com/ubuntu bionic-security/multiverse amd64 Packages [14.6 kB]
Get:5 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]                     
Get:6 http://security.ubuntu.com/ubuntu bionic-security/universe amd64 Packages [1337 kB]
Get:7 http://archive.ubuntu.com/ubuntu bionic/multiverse amd64 Packages [186 kB]  
Get:8 http://archive.ubuntu.com/ubuntu bionic/restricted amd64 Packages [13.5 kB]
Get:9 http://archive.ubuntu.com/ubuntu bionic/universe amd64 Packages [11.3 MB]          
Get:10 http://security.ubuntu.com/ubuntu bionic-security/restricted amd64 Packages [193 kB]
Get:11 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages [1693 kB]     
Get:12 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages [1344 kB]                 
Get:13 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages [2110 kB]
Get:14 http://archive.ubuntu.com/ubuntu bionic-updates/universe amd64 Packages [2099 kB]
Get:15 http://archive.ubuntu.com/ubuntu bionic-updates/restricted amd64 Packages [220 kB]
Get:16 http://archive.ubuntu.com/ubuntu bionic-updates/multiverse amd64 Packages [44.6 kB]
Get:17 http://archive.ubuntu.com/ubuntu bionic-backports/universe amd64 Packages [11.4 kB]
Get:18 http://archive.ubuntu.com/ubuntu bionic-backports/main amd64 Packages [11.3 kB]
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root@f9beca14daaf:/#

- Arnaldo

Re: BTF without CONFIG_DEBUG_INFO_BTF=y

[Arnaldo Carvalho de Melo]

Em Thu, Oct 01, 2020 at 03:37:22PM +0200, Toke Høiland-Jørgensen escreveu:
> Arnaldo Carvalho de Melo <acme@kernel.org> writes:
> 
> > Em Thu, Oct 01, 2020 at 09:50:29AM -0300, Arnaldo Carvalho de Melo escreveu:
> >> Em Thu, Oct 01, 2020 at 12:33:18PM +0200, Toke Høiland-Jørgensen escreveu:
> >> > Kevin Sheldrake <Kevin.Sheldrake@microsoft.com> writes:
> >> > > I've seen mention a few times that BTF information can be made
> >> > > available from a kernel that wasn't configured with
> >> > > CONFIG_DEBUG_INFO_BTF. Please can someone tell me if this is true and,
> >> > > if so, how I could go about accessing and using it in kernels 4.15 to
> >> > > 5.8?
> >
> >> > > I have built the dwarves package from the github latest and run pahole
> >> > > with '-J' against my kernel image to no avail - it actually seg
> >> > > faults:
> >
> >> > > ~/dwarves/build $ sudo ./pahole /boot/vmlinuz-5.3.0-1022-azure
> >> > > btf_elf__new: cannot get elf header.
> >> > > ctf__new: cannot get elf header.
> >> > > ~/dwarves/build $ sudo ./pahole -J /boot/vmlinuz-5.3.0-1022-azure
> >> > > btf_elf__new: cannot get elf header.
> >> > > ctf__new: cannot get elf header.
> >> > > Segmentation fault
> >> > > ~/dwarves/build $ sudo ./pahole --version
> >> > > v1.17
> >
> >> > > Judging by the output, I'm guessing that my kernel image isn't the
> >> > > right kind of file. Can someone point me in the right direction?
> >
> >> > vmlinuz is a compressed image. There's a script in the kernel source
> >> > tree (scripts/extract-vmlinux), however the kernel image in /boot/
> >> > probably also has debug information stripped from it, so that likely
> >> > won't help you. You'll need to get hold of a kernel image with debug
> >> > information still intact somehow...
> >
> >> > (Either way, pahole shouldn't be segfaulting, so hopefully someone can
> >> > take a look at that).
> >
> >> Reproduced:
> >
> >> [acme@five pahole]$ cp /boot/vmlinuz-5.9.0-rc6+ .
> >> [acme@five pahole]$ pahole -J vmlinuz-5.9.0-rc6+
> >> btf_elf__new: cannot get elf header.
> >> ctf__new: cannot get elf header.
> >> tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
> >> pahole: type 'vmlinuz-5.9.0-rc6+' not found
> >> libbpf: Unsupported BTF_KIND:0
> >> btf_elf__encode: btf__new failed!
> >> free(): double free detected in tcache 2
> >> Aborted (core dumped)
> >> [acme@five pahole]$
> >  
> >> Working on a fix. Thanks for the report!
> >
> > commit 4e55425d9eaac78689fbd296283e1557bb6ca725
> > Author: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Date:   Thu Oct 1 10:10:50 2020 -0300
> >
> >     pahole: Only try using a single file name as a type name if not encoding BTF or CTF
> >     
> >     Otherwise we end up trying to encode without any debug info and this
> >     causes a segfault:
> >     
> >     Before:
> >     
> >       $ pahole -J vmlinuz-5.9.0-rc6+
> >       tag__check_id_drift: subroutine_type id drift, core_id: 1145, btf_type_id: 1143, type_id_off: 0
> >       pahole: type 'vmlinuz-5.9.0-rc6+' not found
> >       libbpf: Unsupported BTF_KIND:0
> >       btf_elf__encode: btf__new failed!
> >       free(): double free detected in tcache 2
> >       Aborted (core dumped)
> >       $
> >     
> >     The vmlinuz file doesn't contain any debugging info, fixing it we get:
> >     
> >       $ pahole -J vmlinuz-5.9.0-rc6+
> >       pahole: vmlinuz-5.9.0-rc6+: No debugging information found
> >       $
> >     
> >     If debugging info is available, it all works as before:
> >     
> >     Using /sys/kernel/btf/vmlinux
> >     
> >     $ ls -la /sys/kernel/btf/vmlinux
> >     -r--r--r--. 1 root root 3393761 Oct  1 09:50 /sys/kernel/btf/vmlinux
> >     
> >       $ pahole -E fw_cache_entry
> >       struct fw_cache_entry {
> >             struct list_head {
> >                     struct list_head * next;          /*     0     8 */
> >                     struct list_head * prev;          /*     8     8 */
> >             } list; /*     0    16 */
> >             const char  *              name;          /*    16     8 */
> >     
> >             /* size: 24, cachelines: 1, members: 2 */
> >             /* last cacheline: 24 bytes */
> >       };
> >       $
> >     
> >     Or explicitely asking for DWARF, where it will find the appropriate
> >     vmlinux according to its buildid in /sys/kernel/notes:
> >     
> >       $ pahole -F dwarf pm_clock_entry
> >       struct pm_clock_entry {
> >             struct list_head           node;          /*     0    16 */
> >             char *                     con_id;        /*    16     8 */
> >             struct clk *               clk;           /*    24     8 */
> >             enum pce_status            status;        /*    32     4 */
> >     
> >             /* size: 40, cachelines: 1, members: 4 */
> >             /* padding: 4 */
> >             /* last cacheline: 40 bytes */
> >       };
> >       $ pahole -F dwarf --expand_types pm_clock_entry
> >       struct pm_clock_entry {
> >             struct list_head {
> >                     struct list_head * next;          /*     0     8 */
> >                     struct list_head * prev;          /*     8     8 */
> >             } node; /*     0    16 */
> >             char *                     con_id;        /*    16     8 */
> >             struct clk *               clk;           /*    24     8 */
> >             enum pce_status            status;        /*    32     4 */
> >     
> >             /* size: 40, cachelines: 1, members: 4 */
> >             /* padding: 4 */
> >             /* last cacheline: 40 bytes */
> >       };
> >       $
> >     
> >     Reported-by: Kevin Sheldrake <Kevin.Sheldrake@microsoft.com>
> >     Cc: Toke Høiland-Jørgensen <toke@redhat.com>
> >     Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
> 
> Yeah, that's much better!
> 
> Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>

Thanks, adding your Acked-by and pushing it to the public repos at
github and kernel.org