Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> Cc: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Yeah, that's much better!
Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
