diff mbox series

[bpf,v2] bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT

Message ID 20240624120533.873789-1-wutengda@huaweicloud.com (mailing list archive)
State Changes Requested
Delegated to: BPF
Headers show
Series [bpf,v2] bpf: Fix null pointer dereference in resolve_prog_type() for BPF_PROG_TYPE_EXT | expand

Checks

Context Check Description
netdev/series_format success Single patches do not need cover letters
netdev/tree_selection success Clearly marked for bpf
netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag present in non-next series
netdev/header_inline success No static functions without inline keyword in header files
netdev/build_32bit fail Errors and warnings before: 45 this patch: 45
netdev/build_tools success Errors and warnings before: 1 this patch: 1
netdev/cc_maintainers success CCed 13 of 13 maintainers
netdev/build_clang fail Errors and warnings before: 36 this patch: 36
netdev/verify_signedoff success Signed-off-by tag matches author and committer
netdev/deprecated_api success None detected
netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success Fixes tag looks correct
netdev/build_allmodconfig_warn fail Errors and warnings before: 45 this patch: 45
netdev/checkpatch warning WARNING: line length of 108 exceeds 80 columns WARNING: line length of 81 exceeds 80 columns
netdev/build_clang_rust success No Rust files in patch. Skipping build
netdev/kdoc success Errors and warnings before: 0 this patch: 0
netdev/source_inline success Was 0 now: 0
bpf/vmtest-bpf-VM_Test-0 success Logs for Lint
bpf/vmtest-bpf-VM_Test-11 success Logs for s390x-gcc / build / build for s390x with gcc
bpf/vmtest-bpf-VM_Test-12 success Logs for s390x-gcc / build-release
bpf/vmtest-bpf-VM_Test-10 success Logs for aarch64-gcc / veristat
bpf/vmtest-bpf-VM_Test-3 success Logs for Validate matrix.py
bpf/vmtest-bpf-VM_Test-1 success Logs for ShellCheck
bpf/vmtest-bpf-VM_Test-4 success Logs for aarch64-gcc / build / build for aarch64 with gcc
bpf/vmtest-bpf-VM_Test-20 success Logs for x86_64-gcc / build-release
bpf/vmtest-bpf-VM_Test-18 success Logs for set-matrix
bpf/vmtest-bpf-VM_Test-35 success Logs for x86_64-llvm-18 / build / build for x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-34 success Logs for x86_64-llvm-17 / veristat
bpf/vmtest-bpf-VM_Test-28 success Logs for x86_64-llvm-17 / build / build for x86_64 with llvm-17
bpf/vmtest-bpf-VM_Test-36 success Logs for x86_64-llvm-18 / build-release / build for x86_64 with llvm-18-O2
bpf/vmtest-bpf-VM_Test-2 success Logs for Unittests
bpf/vmtest-bpf-VM_Test-19 success Logs for x86_64-gcc / build / build for x86_64 with gcc
bpf/vmtest-bpf-VM_Test-5 success Logs for aarch64-gcc / build-release
bpf/vmtest-bpf-VM_Test-29 success Logs for x86_64-llvm-17 / build-release / build for x86_64 with llvm-17-O2
bpf/vmtest-bpf-VM_Test-42 success Logs for x86_64-llvm-18 / veristat
bpf/vmtest-bpf-VM_Test-17 success Logs for s390x-gcc / veristat
bpf/vmtest-bpf-VM_Test-9 success Logs for aarch64-gcc / test (test_verifier, false, 360) / test_verifier on aarch64 with gcc
bpf/vmtest-bpf-VM_Test-30 success Logs for x86_64-llvm-17 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-17
bpf/vmtest-bpf-VM_Test-26 success Logs for x86_64-gcc / test (test_verifier, false, 360) / test_verifier on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-27 fail Logs for x86_64-gcc / veristat / veristat on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-33 success Logs for x86_64-llvm-17 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-17
bpf/vmtest-bpf-VM_Test-6 success Logs for aarch64-gcc / test (test_maps, false, 360) / test_maps on aarch64 with gcc
bpf/vmtest-bpf-VM_Test-7 success Logs for aarch64-gcc / test (test_progs, false, 360) / test_progs on aarch64 with gcc
bpf/vmtest-bpf-VM_Test-8 success Logs for aarch64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on aarch64 with gcc
bpf/vmtest-bpf-VM_Test-41 success Logs for x86_64-llvm-18 / test (test_verifier, false, 360) / test_verifier on x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-37 success Logs for x86_64-llvm-18 / test (test_maps, false, 360) / test_maps on x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-31 success Logs for x86_64-llvm-17 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-17
bpf/vmtest-bpf-VM_Test-32 success Logs for x86_64-llvm-17 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-17
bpf/vmtest-bpf-VM_Test-40 success Logs for x86_64-llvm-18 / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-23 success Logs for x86_64-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-22 success Logs for x86_64-gcc / test (test_progs, false, 360) / test_progs on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-24 success Logs for x86_64-gcc / test (test_progs_no_alu32_parallel, true, 30) / test_progs_no_alu32_parallel on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-39 success Logs for x86_64-llvm-18 / test (test_progs_cpuv4, false, 360) / test_progs_cpuv4 on x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-25 success Logs for x86_64-gcc / test (test_progs_parallel, true, 30) / test_progs_parallel on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-13 success Logs for s390x-gcc / test (test_maps, false, 360) / test_maps on s390x with gcc
bpf/vmtest-bpf-VM_Test-21 success Logs for x86_64-gcc / test (test_maps, false, 360) / test_maps on x86_64 with gcc
bpf/vmtest-bpf-VM_Test-38 success Logs for x86_64-llvm-18 / test (test_progs, false, 360) / test_progs on x86_64 with llvm-18
bpf/vmtest-bpf-VM_Test-16 success Logs for s390x-gcc / test (test_verifier, false, 360) / test_verifier on s390x with gcc
bpf/vmtest-bpf-VM_Test-14 success Logs for s390x-gcc / test (test_progs, false, 360) / test_progs on s390x with gcc
bpf/vmtest-bpf-PR fail PR summary
bpf/vmtest-bpf-VM_Test-15 success Logs for s390x-gcc / test (test_progs_no_alu32, false, 360) / test_progs_no_alu32 on s390x with gcc

Commit Message

Tengda Wu June 24, 2024, 12:05 p.m. UTC 
When loading a EXT program without specifying `attr->attach_prog_fd`,
the `prog->aux->dst_prog` will be null. At this time, calling
resolve_prog_type() anywhere will result in a null pointer dereference.

Example stack trace:

[    8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
[    8.108262] Mem abort info:
[    8.108384]   ESR = 0x0000000096000004
[    8.108547]   EC = 0x25: DABT (current EL), IL = 32 bits
[    8.108722]   SET = 0, FnV = 0
[    8.108827]   EA = 0, S1PTW = 0
[    8.108939]   FSC = 0x04: level 0 translation fault
[    8.109102] Data abort info:
[    8.109203]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[    8.109399]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[    8.109614]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[    8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000
[    8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000
[    8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[    8.112783] Modules linked in:
[    8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1
[    8.113230] Hardware name: linux,dummy-virt (DT)
[    8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    8.113429] pc : may_access_direct_pkt_data+0x24/0xa0
[    8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8
[    8.113798] sp : ffff80008283b9f0
[    8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001
[    8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000
[    8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000
[    8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff
[    8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720
[    8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
[    8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4
[    8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f
[    8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c
[    8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000
[    8.114126] Call trace:
[    8.114159]  may_access_direct_pkt_data+0x24/0xa0
[    8.114202]  bpf_check+0x3bc/0x28c0
[    8.114214]  bpf_prog_load+0x658/0xa58
[    8.114227]  __sys_bpf+0xc50/0x2250
[    8.114240]  __arm64_sys_bpf+0x28/0x40
[    8.114254]  invoke_syscall.constprop.0+0x54/0xf0
[    8.114273]  do_el0_svc+0x4c/0xd8
[    8.114289]  el0_svc+0x3c/0x140
[    8.114305]  el0t_64_sync_handler+0x134/0x150
[    8.114331]  el0t_64_sync+0x168/0x170
[    8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403)
[    8.118672] ---[ end trace 0000000000000000 ]---

Fix this by adding dst_prog non-empty check in BPF_PROG_TYPE_EXT case
when calling bpf_prog_load().

Note the BPF_PROG_TYPE_EXT type detection in libbpf also needs to be
adapted by passing a valid attach_prog_fd, since an empty attach_prog_fd
is no longer allowed when loading EXT program.

Fixes: 4a9c7bbe2ed4 ("bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
Cc: stable@vger.kernel.org # v5.18+
---
v2: Fix libbpf_probe_prog_types test failure reported by CI by adapting
libbpf code. (thanks for jirka's reminder)

v1: https://lore.kernel.org/all/20240620060701.1465291-1-wutengda@huaweicloud.com/

 kernel/bpf/syscall.c          |  5 ++++-
 tools/lib/bpf/libbpf_probes.c | 13 ++++++++++++-
 2 files changed, 16 insertions(+), 2 deletions(-)

Comments

Daniel Borkmann July 1, 2024, 3:39 p.m. UTC | #1 
On 6/24/24 2:05 PM, Tengda Wu wrote:
> When loading a EXT program without specifying `attr->attach_prog_fd`,
> the `prog->aux->dst_prog` will be null. At this time, calling
> resolve_prog_type() anywhere will result in a null pointer dereference.
> 
> Example stack trace:
> 
> [    8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
> [    8.108262] Mem abort info:
> [    8.108384]   ESR = 0x0000000096000004
> [    8.108547]   EC = 0x25: DABT (current EL), IL = 32 bits
> [    8.108722]   SET = 0, FnV = 0
> [    8.108827]   EA = 0, S1PTW = 0
> [    8.108939]   FSC = 0x04: level 0 translation fault
> [    8.109102] Data abort info:
> [    8.109203]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
> [    8.109399]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
> [    8.109614]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
> [    8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000
> [    8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000
> [    8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
> [    8.112783] Modules linked in:
> [    8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1
> [    8.113230] Hardware name: linux,dummy-virt (DT)
> [    8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [    8.113429] pc : may_access_direct_pkt_data+0x24/0xa0
> [    8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8
> [    8.113798] sp : ffff80008283b9f0
> [    8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001
> [    8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000
> [    8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000
> [    8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff
> [    8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720
> [    8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
> [    8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4
> [    8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f
> [    8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c
> [    8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000
> [    8.114126] Call trace:
> [    8.114159]  may_access_direct_pkt_data+0x24/0xa0
> [    8.114202]  bpf_check+0x3bc/0x28c0
> [    8.114214]  bpf_prog_load+0x658/0xa58
> [    8.114227]  __sys_bpf+0xc50/0x2250
> [    8.114240]  __arm64_sys_bpf+0x28/0x40
> [    8.114254]  invoke_syscall.constprop.0+0x54/0xf0
> [    8.114273]  do_el0_svc+0x4c/0xd8
> [    8.114289]  el0_svc+0x3c/0x140
> [    8.114305]  el0t_64_sync_handler+0x134/0x150
> [    8.114331]  el0t_64_sync+0x168/0x170
> [    8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403)
> [    8.118672] ---[ end trace 0000000000000000 ]---
> 
> Fix this by adding dst_prog non-empty check in BPF_PROG_TYPE_EXT case
> when calling bpf_prog_load().
> 
> Note the BPF_PROG_TYPE_EXT type detection in libbpf also needs to be
> adapted by passing a valid attach_prog_fd, since an empty attach_prog_fd
> is no longer allowed when loading EXT program.
> 
> Fixes: 4a9c7bbe2ed4 ("bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT")
> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
> Cc: stable@vger.kernel.org # v5.18+
> ---
> v2: Fix libbpf_probe_prog_types test failure reported by CI by adapting
> libbpf code. (thanks for jirka's reminder)
> 
> v1: https://lore.kernel.org/all/20240620060701.1465291-1-wutengda@huaweicloud.com/
> 
>   kernel/bpf/syscall.c          |  5 ++++-
>   tools/lib/bpf/libbpf_probes.c | 13 ++++++++++++-
>   2 files changed, 16 insertions(+), 2 deletions(-)

Could you pls make this a 3-patch series against bpf?

First patch is the kernel-only fix, 2nd patch is the libbpf one, and 3rd patch adds
a small BPF selftest for the BPF CI.

Thanks a lot,
Daniel
Tengda Wu July 2, 2024, 1:23 a.m. UTC | #2 
On 2024/7/1 23:39, Daniel Borkmann wrote:
> On 6/24/24 2:05 PM, Tengda Wu wrote:
>> When loading a EXT program without specifying `attr->attach_prog_fd`,
>> the `prog->aux->dst_prog` will be null. At this time, calling
>> resolve_prog_type() anywhere will result in a null pointer dereference.
>>
>> Example stack trace:
>>
>> [    8.107863] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000004
>> [    8.108262] Mem abort info:
>> [    8.108384]   ESR = 0x0000000096000004
>> [    8.108547]   EC = 0x25: DABT (current EL), IL = 32 bits
>> [    8.108722]   SET = 0, FnV = 0
>> [    8.108827]   EA = 0, S1PTW = 0
>> [    8.108939]   FSC = 0x04: level 0 translation fault
>> [    8.109102] Data abort info:
>> [    8.109203]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
>> [    8.109399]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
>> [    8.109614]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>> [    8.109836] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101354000
>> [    8.110011] [0000000000000004] pgd=0000000000000000, p4d=0000000000000000
>> [    8.112624] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
>> [    8.112783] Modules linked in:
>> [    8.113120] CPU: 0 PID: 99 Comm: may_access_dire Not tainted 6.10.0-rc3-next-20240613-dirty #1
>> [    8.113230] Hardware name: linux,dummy-virt (DT)
>> [    8.113390] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>> [    8.113429] pc : may_access_direct_pkt_data+0x24/0xa0
>> [    8.113746] lr : add_subprog_and_kfunc+0x634/0x8e8
>> [    8.113798] sp : ffff80008283b9f0
>> [    8.113813] x29: ffff80008283b9f0 x28: ffff800082795048 x27: 0000000000000001
>> [    8.113881] x26: ffff0000c0bb2600 x25: 0000000000000000 x24: 0000000000000000
>> [    8.113897] x23: ffff0000c1134000 x22: 000000000001864f x21: ffff0000c1138000
>> [    8.113912] x20: 0000000000000001 x19: ffff0000c12b8000 x18: ffffffffffffffff
>> [    8.113929] x17: 0000000000000000 x16: 0000000000000000 x15: 0720072007200720
>> [    8.113944] x14: 0720072007200720 x13: 0720072007200720 x12: 0720072007200720
>> [    8.113958] x11: 0720072007200720 x10: 0000000000f9fca4 x9 : ffff80008021f4e4
>> [    8.113991] x8 : 0101010101010101 x7 : 746f72705f6d656d x6 : 000000001e0e0f5f
>> [    8.114006] x5 : 000000000001864f x4 : ffff0000c12b8000 x3 : 000000000000001c
>> [    8.114020] x2 : 0000000000000002 x1 : 0000000000000000 x0 : 0000000000000000
>> [    8.114126] Call trace:
>> [    8.114159]  may_access_direct_pkt_data+0x24/0xa0
>> [    8.114202]  bpf_check+0x3bc/0x28c0
>> [    8.114214]  bpf_prog_load+0x658/0xa58
>> [    8.114227]  __sys_bpf+0xc50/0x2250
>> [    8.114240]  __arm64_sys_bpf+0x28/0x40
>> [    8.114254]  invoke_syscall.constprop.0+0x54/0xf0
>> [    8.114273]  do_el0_svc+0x4c/0xd8
>> [    8.114289]  el0_svc+0x3c/0x140
>> [    8.114305]  el0t_64_sync_handler+0x134/0x150
>> [    8.114331]  el0t_64_sync+0x168/0x170
>> [    8.114477] Code: 7100707f 54000081 f9401c00 f9403800 (b9400403)
>> [    8.118672] ---[ end trace 0000000000000000 ]---
>>
>> Fix this by adding dst_prog non-empty check in BPF_PROG_TYPE_EXT case
>> when calling bpf_prog_load().
>>
>> Note the BPF_PROG_TYPE_EXT type detection in libbpf also needs to be
>> adapted by passing a valid attach_prog_fd, since an empty attach_prog_fd
>> is no longer allowed when loading EXT program.
>>
>> Fixes: 4a9c7bbe2ed4 ("bpf: Resolve to prog->aux->dst_prog->type only for BPF_PROG_TYPE_EXT")
>> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
>> Cc: stable@vger.kernel.org # v5.18+
>> ---
>> v2: Fix libbpf_probe_prog_types test failure reported by CI by adapting
>> libbpf code. (thanks for jirka's reminder)
>>
>> v1: https://lore.kernel.org/all/20240620060701.1465291-1-wutengda@huaweicloud.com/
>>
>>   kernel/bpf/syscall.c          |  5 ++++-
>>   tools/lib/bpf/libbpf_probes.c | 13 ++++++++++++-
>>   2 files changed, 16 insertions(+), 2 deletions(-)
> 
> Could you pls make this a 3-patch series against bpf?
> 
> First patch is the kernel-only fix, 2nd patch is the libbpf one, and 3rd patch adds
> a small BPF selftest for the BPF CI.
> 
> Thanks a lot,
> Daniel

Okay, I will resend soon.

Tengda
diff mbox series

Patch

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index f45ed6adc092..4490f8ccf006 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2632,9 +2632,12 @@  bpf_prog_load_check_attach(enum bpf_prog_type prog_type,
 			return 0;
 		return -EINVAL;
 	case BPF_PROG_TYPE_SYSCALL:
-	case BPF_PROG_TYPE_EXT:
 		if (expected_attach_type)
 			return -EINVAL;
+		return 0;
+	case BPF_PROG_TYPE_EXT:
+		if (expected_attach_type || !dst_prog)
+			return -EINVAL;
 		fallthrough;
 	default:
 		return 0;
diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 9dfbe7750f56..37f87c824933 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -112,6 +112,7 @@  static int probe_prog_load(enum bpf_prog_type prog_type,
 	int fd, err, exp_err = 0;
 	const char *exp_msg = NULL;
 	char buf[4096];
+	int attach_prog_fd = -1;
 
 	switch (prog_type) {
 	case BPF_PROG_TYPE_CGROUP_SOCK_ADDR:
@@ -148,9 +149,17 @@  static int probe_prog_load(enum bpf_prog_type prog_type,
 		opts.log_size = sizeof(buf);
 		opts.log_level = 1;
 		opts.attach_btf_id = 1;
+		/* choose socket_filter as the target program to attach, since it
+		 * is the earliest and most likely BPF program to be supported.
+		 */
+		attach_prog_fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, NULL,
+					       "GPL", insns, insns_cnt, NULL);
+		if (attach_prog_fd < 0)
+			return attach_prog_fd;
+		opts.attach_prog_fd = attach_prog_fd;
 
 		exp_err = -EINVAL;
-		exp_msg = "Cannot replace kernel functions";
+		exp_msg = "FENTRY/FEXIT program can only be attached to another program annotated with BTF";
 		break;
 	case BPF_PROG_TYPE_SYSCALL:
 		opts.prog_flags = BPF_F_SLEEPABLE;
@@ -192,6 +201,8 @@  static int probe_prog_load(enum bpf_prog_type prog_type,
 	err = -errno;
 	if (fd >= 0)
 		close(fd);
+	if (attach_prog_fd >= 0)
+		close(attach_prog_fd);
 	if (exp_err) {
 		if (fd >= 0 || err != exp_err)
 			return 0;