samples don't build on v4.18

Hi,
When building BPF samples on v4.18, I get the following errors:

$ cd samples/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
Makefile:213: recipe for target 'elfdep' failed
-----------

I bissected it down to commit 5f9380572b4bb24f60cd492b1

Author: Jakub Kicinski [off-list ref]
Date:   Thu May 10 10:24:39 2018 -0700

    samples: bpf: compile and link against full libbpf
---------

Checking out a kernel before this commit makes the samples build. Also I do
have libelf on my system.

Any thoughts on this issue?

thank you,

- Joel

Forgot to add the patch author, doing so now. thanks

On Tue, Aug 14, 2018 at 6:20 PM, Joel Fernandes [off-list ref] wrote:

Hi,
When building BPF samples on v4.18, I get the following errors:

$ cd samples/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
Makefile:213: recipe for target 'elfdep' failed
-----------

I bissected it down to commit 5f9380572b4bb24f60cd492b1

Author: Jakub Kicinski [off-list ref]
Date:   Thu May 10 10:24:39 2018 -0700

    samples: bpf: compile and link against full libbpf
---------

Checking out a kernel before this commit makes the samples build. Also I do
have libelf on my system.

Any thoughts on this issue?

thank you,

- Joel

On Tue, Aug 14, 2018 at 06:22:21PM -0700, Joel Fernandes wrote:

Forgot to add the patch author, doing so now. thanks

On Tue, Aug 14, 2018 at 6:20 PM, Joel Fernandes [off-list ref] wrote:

quoted

Hi,
When building BPF samples on v4.18, I get the following errors:

$ cd samples/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
Makefile:213: recipe for target 'elfdep' failed
-----------

I bissected it down to commit 5f9380572b4bb24f60cd492b1

Author: Jakub Kicinski [off-list ref]
Date:   Thu May 10 10:24:39 2018 -0700

    samples: bpf: compile and link against full libbpf
---------

Checking out a kernel before this commit makes the samples build. Also I do
have libelf on my system.

Any thoughts on this issue?

There is some weirdness going on with my kernel tree. If I do a fresh clone
of v4.18 and build samples, everything works.

However if I take my existing checkout, do a:
git clean -f -d
make mrproper

Then I try to build the samples, I get the "No libelf found".

Obviously the existing checked out kernel tree is in some weird state that I
am not yet able to fix. But atleast if I blow the whole tree and clone again,
I'm able to build...

Is this related to the intermittent "No libelf found" issues that were
recently discussed?

- Joel

On Tue, 14 Aug 2018 20:01:32 -0700, Joel Fernandes wrote:

On Tue, Aug 14, 2018 at 06:22:21PM -0700, Joel Fernandes wrote:

quoted

Forgot to add the patch author, doing so now. thanks

On Tue, Aug 14, 2018 at 6:20 PM, Joel Fernandes [off-list ref] wrote:  

quoted

Hi,
When building BPF samples on v4.18, I get the following errors:

$ cd samples/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
Makefile:213: recipe for target 'elfdep' failed
-----------

I bissected it down to commit 5f9380572b4bb24f60cd492b1

Author: Jakub Kicinski [off-list ref]
Date:   Thu May 10 10:24:39 2018 -0700

    samples: bpf: compile and link against full libbpf
---------

Checking out a kernel before this commit makes the samples build. Also I do
have libelf on my system.

Any thoughts on this issue?  

There is some weirdness going on with my kernel tree. If I do a fresh clone
of v4.18 and build samples, everything works.

However if I take my existing checkout, do a:
git clean -f -d
make mrproper

Then I try to build the samples, I get the "No libelf found".

Obviously the existing checked out kernel tree is in some weird state that I
am not yet able to fix. But atleast if I blow the whole tree and clone again,
I'm able to build...

Is this related to the intermittent "No libelf found" issues that were
recently discussed?

Can't reproduce, could you provide all exact commands you run to see
this, including the initial clone?

12:32 linux$ git clean -f -d
12:32 linux$ make mrproper
  CLEAN   .
  CLEAN   arch/x86/tools
  CLEAN   .tmp_versions
  CLEAN   scripts/basic
  CLEAN   scripts/kconfig
  CLEAN   scripts/mod
  CLEAN   scripts
  CLEAN   include/config usr/include include/generated arch/x86/include/generated
  CLEAN   .config .config.old
12:32 linux$ make defconfig
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  YACC    scripts/kconfig/zconf.tab.c
  LEX     scripts/kconfig/zconf.lex.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
*** Default configuration is based on 'x86_64_defconfig'
#
# configuration written to .config
#
12:32 linux$ make headers_install
  UPD     include/generated/uapi/linux/version.h
  WRAP    arch/x86/include/generated/uapi/asm/bpf_perf_event.h
  WRAP    arch/x86/include/generated/uapi/asm/poll.h
  SYSTBL  arch/x86/include/generated/asm/syscalls_32.h
  SYSHDR  arch/x86/include/generated/uapi/asm/unistd_32.h
  SYSHDR  arch/x86/include/generated/uapi/asm/unistd_64.h
  SYSHDR  arch/x86/include/generated/uapi/asm/unistd_x32.h
  HOSTCC  arch/x86/tools/relocs_32.o
  HOSTCC  arch/x86/tools/relocs_64.o
  HOSTCC  arch/x86/tools/relocs_common.o
  HOSTLD  arch/x86/tools/relocs
  HOSTCC  scripts/unifdef
  INSTALL usr/include/asm-generic/ (37 files)
  INSTALL usr/include/drm/ (26 files)
  INSTALL usr/include/linux/ (497 files)
  INSTALL usr/include/linux/android/ (1 file)
  INSTALL usr/include/linux/byteorder/ (2 files)
  INSTALL usr/include/linux/caif/ (2 files)
  INSTALL usr/include/linux/can/ (6 files)
  INSTALL usr/include/linux/cifs/ (1 file)
  INSTALL usr/include/linux/dvb/ (8 files)
  INSTALL usr/include/linux/genwqe/ (1 file)
  INSTALL usr/include/linux/hdlc/ (1 file)
  INSTALL usr/include/linux/hsi/ (2 files)
  INSTALL usr/include/linux/iio/ (2 files)
  INSTALL usr/include/linux/isdn/ (1 file)
  INSTALL usr/include/linux/mmc/ (1 file)
  INSTALL usr/include/linux/netfilter/ (88 files)
  INSTALL usr/include/linux/netfilter/ipset/ (4 files)
  INSTALL usr/include/linux/netfilter_arp/ (2 files)
  INSTALL usr/include/linux/netfilter_bridge/ (17 files)
  INSTALL usr/include/linux/netfilter_ipv4/ (9 files)
  INSTALL usr/include/linux/netfilter_ipv6/ (13 files)
  INSTALL usr/include/linux/nfsd/ (5 files)
  INSTALL usr/include/linux/raid/ (2 files)
  INSTALL usr/include/linux/sched/ (1 file)
  INSTALL usr/include/linux/spi/ (1 file)
  INSTALL usr/include/linux/sunrpc/ (1 file)
  INSTALL usr/include/linux/tc_act/ (15 files)
  INSTALL usr/include/linux/tc_ematch/ (5 files)
  INSTALL usr/include/linux/usb/ (12 files)
  INSTALL usr/include/linux/wimax/ (1 file)
  INSTALL usr/include/misc/ (2 files)
  INSTALL usr/include/mtd/ (5 files)
  INSTALL usr/include/rdma/ (25 files)
  INSTALL usr/include/rdma/hfi/ (2 files)
  INSTALL usr/include/scsi/ (4 files)
  INSTALL usr/include/scsi/fc/ (4 files)
  INSTALL usr/include/sound/ (16 files)
  INSTALL usr/include/video/ (3 files)
  INSTALL usr/include/xen/ (4 files)
  INSTALL usr/include/asm/ (62 files)
12:32 linux$ cd samples/bpf/
12:32 bpf$ make  LLC=llc-6.0 LLVM_OBJCOPY=llvm-objcopy-6.0 CLANG=clang-6.0 
make -C ../../ /tmp/linux/samples/bpf/ BPF_SAMPLES_PATH=/tmp/linux/samples/bpf
make[1]: Entering directory '/tmp/linux'
scripts/kconfig/conf  --syncconfig Kconfig
  SYSHDR  arch/x86/include/generated/asm/unistd_32_ia32.h
  SYSHDR  arch/x86/include/generated/asm/unistd_64_x32.h
  SYSTBL  arch/x86/include/generated/asm/syscalls_64.h
  UPD     include/config/kernel.release
  WRAP    arch/x86/include/generated/asm/dma-contiguous.h
  WRAP    arch/x86/include/generated/asm/early_ioremap.h
  WRAP    arch/x86/include/generated/asm/mcs_spinlock.h
  WRAP    arch/x86/include/generated/asm/mm-arch-hooks.h
  UPD     include/generated/utsrelease.h
  CC      kernel/bounds.s
  UPD     include/generated/bounds.h
  UPD     include/generated/timeconst.h
  CC      arch/x86/kernel/asm-offsets.s
  UPD     include/generated/asm-offsets.h
  CALL    scripts/checksyscalls.sh
  DESCEND  objtool
  HOSTCC   /tmp/linux/tools/objtool/fixdep.o
  HOSTLD   /tmp/linux/tools/objtool/fixdep-in.o
  LINK     /tmp/linux/tools/objtool/fixdep
  CC       /tmp/linux/tools/objtool/exec-cmd.o
  CC       /tmp/linux/tools/objtool/help.o
  CC       /tmp/linux/tools/objtool/pager.o
  CC       /tmp/linux/tools/objtool/parse-options.o
  CC       /tmp/linux/tools/objtool/run-command.o
  CC       /tmp/linux/tools/objtool/sigchain.o
  CC       /tmp/linux/tools/objtool/subcmd-config.o
  LD       /tmp/linux/tools/objtool/libsubcmd-in.o
  AR       /tmp/linux/tools/objtool/libsubcmd.a
  CC       /tmp/linux/tools/objtool/arch/x86/decode.o
  LD       /tmp/linux/tools/objtool/arch/x86/objtool-in.o
  CC       /tmp/linux/tools/objtool/builtin-check.o
  CC       /tmp/linux/tools/objtool/builtin-orc.o
  CC       /tmp/linux/tools/objtool/check.o
  CC       /tmp/linux/tools/objtool/orc_gen.o
  CC       /tmp/linux/tools/objtool/orc_dump.o
  CC       /tmp/linux/tools/objtool/elf.o
  CC       /tmp/linux/tools/objtool/special.o
  CC       /tmp/linux/tools/objtool/objtool.o
  CC       /tmp/linux/tools/objtool/libstring.o
  CC       /tmp/linux/tools/objtool/str_error_r.o
  LD       /tmp/linux/tools/objtool/objtool-in.o
  LINK     /tmp/linux/tools/objtool/objtool
  CC      scripts/mod/empty.o
  HOSTCC  scripts/mod/mk_elfconfig
  MKELF   scripts/mod/elfconfig.h
  HOSTCC  scripts/mod/modpost.o
  CC      scripts/mod/devicetable-offsets.s
  UPD     scripts/mod/devicetable-offsets.h
  HOSTCC  scripts/mod/file2alias.o
  HOSTCC  scripts/mod/sumversion.o
  HOSTLD  scripts/mod/modpost
  HOSTCC  scripts/selinux/genheaders/genheaders
  HOSTCC  scripts/selinux/mdp/mdp
  HOSTCC  scripts/kallsyms
  HOSTCC  scripts/pnmtologo
  HOSTCC  scripts/conmakehash
  HOSTCC  scripts/sortextable
  HOSTCC  scripts/asn1_compiler
  HOSTCC  scripts/extract-cert
make -C /tmp/linux/samples/bpf/../../tools/lib/bpf/ RM='rm -rf' LDFLAGS= srctree=/tmp/linux/samples/bpf/../../ O=
  HOSTCC   fixdep.o
  HOSTLD   fixdep-in.o
  LINK     fixdep
  CC       libbpf.o
  CC       bpf.o
  CC       nlattr.o
  CC       btf.o
  LD       libbpf-in.o
  LINK     libbpf.a
  LINK     libbpf.so
  HOSTCC  /tmp/linux/samples/bpf/test_lru_dist
  HOSTCC  /tmp/linux/samples/bpf/sock_example
  HOSTCC  /tmp/linux/samples/bpf/bpf_load.o
  HOSTCC  /tmp/linux/samples/bpf/fds_example.o
  HOSTLD  /tmp/linux/samples/bpf/fds_example
  HOSTCC  /tmp/linux/samples/bpf/sockex1_user.o
  HOSTLD  /tmp/linux/samples/bpf/sockex1
  HOSTCC  /tmp/linux/samples/bpf/sockex2_user.o
  HOSTLD  /tmp/linux/samples/bpf/sockex2
  HOSTCC  /tmp/linux/samples/bpf/sockex3_user.o
  HOSTLD  /tmp/linux/samples/bpf/sockex3
  HOSTCC  /tmp/linux/samples/bpf/tracex1_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex1
  HOSTCC  /tmp/linux/samples/bpf/tracex2_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex2
  HOSTCC  /tmp/linux/samples/bpf/tracex3_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex3
  HOSTCC  /tmp/linux/samples/bpf/tracex4_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex4
  HOSTCC  /tmp/linux/samples/bpf/tracex5_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex5
  HOSTCC  /tmp/linux/samples/bpf/tracex6_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex6
  HOSTCC  /tmp/linux/samples/bpf/tracex7_user.o
  HOSTLD  /tmp/linux/samples/bpf/tracex7
  HOSTCC  /tmp/linux/samples/bpf/test_probe_write_user_user.o
  HOSTLD  /tmp/linux/samples/bpf/test_probe_write_user
  HOSTCC  /tmp/linux/samples/bpf/trace_output_user.o
  HOSTCC  /tmp/linux/samples/bpf/../../tools/testing/selftests/bpf/trace_helpers.o
  HOSTLD  /tmp/linux/samples/bpf/trace_output
  HOSTCC  /tmp/linux/samples/bpf/lathist_user.o
  HOSTLD  /tmp/linux/samples/bpf/lathist
  HOSTCC  /tmp/linux/samples/bpf/offwaketime_user.o
  HOSTLD  /tmp/linux/samples/bpf/offwaketime
  HOSTCC  /tmp/linux/samples/bpf/spintest_user.o
  HOSTLD  /tmp/linux/samples/bpf/spintest
  HOSTCC  /tmp/linux/samples/bpf/map_perf_test_user.o
  HOSTLD  /tmp/linux/samples/bpf/map_perf_test
  HOSTCC  /tmp/linux/samples/bpf/test_overhead_user.o
  HOSTLD  /tmp/linux/samples/bpf/test_overhead
  HOSTCC  /tmp/linux/samples/bpf/test_cgrp2_array_pin.o
  HOSTLD  /tmp/linux/samples/bpf/test_cgrp2_array_pin
  HOSTCC  /tmp/linux/samples/bpf/test_cgrp2_attach.o
  HOSTLD  /tmp/linux/samples/bpf/test_cgrp2_attach
  HOSTCC  /tmp/linux/samples/bpf/test_cgrp2_attach2.o
  HOSTCC  /tmp/linux/samples/bpf/../../tools/testing/selftests/bpf/cgroup_helpers.o
  HOSTLD  /tmp/linux/samples/bpf/test_cgrp2_attach2
  HOSTCC  /tmp/linux/samples/bpf/test_cgrp2_sock.o
  HOSTLD  /tmp/linux/samples/bpf/test_cgrp2_sock
  HOSTCC  /tmp/linux/samples/bpf/test_cgrp2_sock2.o
  HOSTLD  /tmp/linux/samples/bpf/test_cgrp2_sock2
  HOSTCC  /tmp/linux/samples/bpf/xdp1_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp1
  HOSTLD  /tmp/linux/samples/bpf/xdp2
  HOSTCC  /tmp/linux/samples/bpf/xdp_router_ipv4_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_router_ipv4
  HOSTCC  /tmp/linux/samples/bpf/test_current_task_under_cgroup_user.o
  HOSTLD  /tmp/linux/samples/bpf/test_current_task_under_cgroup
  HOSTCC  /tmp/linux/samples/bpf/trace_event_user.o
  HOSTLD  /tmp/linux/samples/bpf/trace_event
  HOSTCC  /tmp/linux/samples/bpf/sampleip_user.o
  HOSTLD  /tmp/linux/samples/bpf/sampleip
  HOSTCC  /tmp/linux/samples/bpf/tc_l2_redirect_user.o
  HOSTLD  /tmp/linux/samples/bpf/tc_l2_redirect
  HOSTCC  /tmp/linux/samples/bpf/lwt_len_hist_user.o
  HOSTLD  /tmp/linux/samples/bpf/lwt_len_hist
  HOSTCC  /tmp/linux/samples/bpf/xdp_tx_iptunnel_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_tx_iptunnel
  HOSTCC  /tmp/linux/samples/bpf/test_map_in_map_user.o
  HOSTLD  /tmp/linux/samples/bpf/test_map_in_map
  HOSTCC  /tmp/linux/samples/bpf/cookie_uid_helper_example.o
  HOSTLD  /tmp/linux/samples/bpf/per_socket_stats_example
  HOSTCC  /tmp/linux/samples/bpf/load_sock_ops.o
  HOSTLD  /tmp/linux/samples/bpf/load_sock_ops
  HOSTCC  /tmp/linux/samples/bpf/xdp_redirect_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_redirect
  HOSTCC  /tmp/linux/samples/bpf/xdp_redirect_map_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_redirect_map
  HOSTCC  /tmp/linux/samples/bpf/xdp_redirect_cpu_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_redirect_cpu
  HOSTCC  /tmp/linux/samples/bpf/xdp_monitor_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_monitor
  HOSTCC  /tmp/linux/samples/bpf/xdp_rxq_info_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_rxq_info
  HOSTCC  /tmp/linux/samples/bpf/syscall_tp_user.o
  HOSTLD  /tmp/linux/samples/bpf/syscall_tp
  HOSTCC  /tmp/linux/samples/bpf/cpustat_user.o
  HOSTLD  /tmp/linux/samples/bpf/cpustat
  HOSTCC  /tmp/linux/samples/bpf/xdp_adjust_tail_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_adjust_tail
  HOSTCC  /tmp/linux/samples/bpf/xdpsock_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdpsock
  HOSTCC  /tmp/linux/samples/bpf/xdp_fwd_user.o
  HOSTLD  /tmp/linux/samples/bpf/xdp_fwd
  HOSTCC  /tmp/linux/samples/bpf/task_fd_query_user.o
  HOSTLD  /tmp/linux/samples/bpf/task_fd_query
  CC      /tmp/linux/samples/bpf/syscall_nrs.s
  CLANG-bpf  /tmp/linux/samples/bpf/sockex1_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/sockex2_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/sockex3_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex1_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex2_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex3_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex4_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex5_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex6_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tracex7_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/sock_flags_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_probe_write_user_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/trace_output_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcbpf1_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tc_l2_redirect_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/lathist_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/offwaketime_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/spintest_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/map_perf_test_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_overhead_tp_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_overhead_raw_tp_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_overhead_kprobe_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/parse_varlen.o
  CLANG-bpf  /tmp/linux/samples/bpf/parse_simple.o
  CLANG-bpf  /tmp/linux/samples/bpf/parse_ldabs.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_cgrp2_tc_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp1_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp2_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_router_ipv4_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_current_task_under_cgroup_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/trace_event_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/sampleip_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/lwt_len_hist_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_tx_iptunnel_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/test_map_in_map_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_synrto_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_rwnd_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_bufs_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_cong_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_iw_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_clamp_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/tcp_basertt_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_redirect_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_redirect_map_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_redirect_cpu_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_monitor_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_rxq_info_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp2skb_meta_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/syscall_tp_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/cpustat_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_adjust_tail_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdpsock_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/xdp_fwd_kern.o
  CLANG-bpf  /tmp/linux/samples/bpf/task_fd_query_kern.o
make[1]: Leaving directory '/tmp/linux'

On Wed, Aug 15, 2018 at 12:34 PM, Jakub Kicinski
[off-list ref] wrote:

On Tue, 14 Aug 2018 20:01:32 -0700, Joel Fernandes wrote:

quoted

On Tue, Aug 14, 2018 at 06:22:21PM -0700, Joel Fernandes wrote:

quoted

Forgot to add the patch author, doing so now. thanks

On Tue, Aug 14, 2018 at 6:20 PM, Joel Fernandes [off-list ref] wrote:

quoted

Hi,
When building BPF samples on v4.18, I get the following errors:

$ cd samples/bpf/
$ make

Auto-detecting system features:
...                        libelf: [ OFF ]
...                           bpf: [ OFF ]

No libelf found
Makefile:213: recipe for target 'elfdep' failed
-----------

I bissected it down to commit 5f9380572b4bb24f60cd492b1

Author: Jakub Kicinski [off-list ref]
Date:   Thu May 10 10:24:39 2018 -0700

    samples: bpf: compile and link against full libbpf
---------

Checking out a kernel before this commit makes the samples build. Also I do
have libelf on my system.

Any thoughts on this issue?

There is some weirdness going on with my kernel tree. If I do a fresh clone
of v4.18 and build samples, everything works.

However if I take my existing checkout, do a:
git clean -f -d
make mrproper

Then I try to build the samples, I get the "No libelf found".

Obviously the existing checked out kernel tree is in some weird state that I
am not yet able to fix. But atleast if I blow the whole tree and clone again,
I'm able to build...

Is this related to the intermittent "No libelf found" issues that were
recently discussed?

Can't reproduce, could you provide all exact commands you run to see
this, including the initial clone?

Not sure if you saw that I replied to my own email. As I was saying,
doing a fresh clone and build of the kernel tree makes things work for
me. The problematic kernel tree which I cloned many months ago was the
one I was using when I reported the issue.

On the problematic tree, the steps I did to reproduce issue were:
git clean -f -d
make mrproper
make x86_64_defconfig
cd samples/bpf/
make

I have since moved onto using the fresh cloned tree since that's working for me.

I will let you know if I run into this again. Thanks for your time!

 - Joel