selftests/bpf: add a test for subprogram extables

In certain situations a program with subprograms may have a NULL
extable entry. This should not happen, and when it does, it turns a
single trap into multiple. Add a test case for further debugging and to
prevent regressions.

The test-case contains three essentially identical versions of the same
test because just one program may not be sufficient to trigger the oops.
This is due to the fact that the items are stored in a binary tree and
have identical values so it's possible to sometimes find the ksym with
the extable. With 3 copies, this has been reliable on this author's
test systems.

When triggered out of this test case, the oops looks like this:

BUG: kernel NULL pointer dereference, address: 000000000000000c
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
PGD 0 P4D 0
Oops: 0000 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 1132 Comm: test_progs Tainted: G OE 6.4.0-rc3+ #2
RIP: 0010:cmp_ex_search+0xb/0x30
Code: cc cc cc cc e8 36 cb 03 00 66 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 f3 0f 1e fa 55 48 89 e5 48 8b 07 <48> 63 0e 48 01 f1 31 d2 48 39 c8 19 d2 48 39 c8 b8 01 00 00 00 0f
RSP: 0018:ffffb30c4291f998 EFLAGS: 00010006
RAX: ffffffffc00b49da RBX: 0000000000000002 RCX: 000000000000000c
RDX: 0000000000000002 RSI: 000000000000000c RDI: ffffb30c4291f9e8
RBP: ffffb30c4291f998 R08: ffffffffab1a42d0 R09: 0000000000000001
R10: 0000000000000000 R11: ffffffffab1a42d0 R12: ffffb30c4291f9e8
R13: 000000000000000c R14: 000000000000000c R15: 0000000000000000
FS: 00007fb5d9e044c0(0000) GS:ffff92e95ee00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000000000000c CR3: 000000010c3a2005 CR4: 00000000007706f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<TASK>
bsearch+0x41/0x90
? __pfx_cmp_ex_search+0x10/0x10
? bpf_prog_45a7907e7114d0ff_handle_fexit_ret_subprogs3+0x2a/0x6c
search_extable+0x3b/0x60
? bpf_prog_45a7907e7114d0ff_handle_fexit_ret_subprogs3+0x2a/0x6c
search_bpf_extables+0x10d/0x190
? bpf_prog_45a7907e7114d0ff_handle_fexit_ret_subprogs3+0x2a/0x6c
search_exception_tables+0x5d/0x70
fixup_exception+0x3f/0x5b0
? look_up_lock_class+0x61/0x110
? __lock_acquire+0x6b8/0x3560
? __lock_acquire+0x6b8/0x3560
? __lock_acquire+0x6b8/0x3560
kernelmode_fixup_or_oops+0x46/0x110
__bad_area_nosemaphore+0x68/0x2b0
? __lock_acquire+0x6b8/0x3560
bad_area_nosemaphore+0x16/0x20
do_kern_addr_fault+0x81/0xa0
exc_page_fault+0xd6/0x210
asm_exc_page_fault+0x2b/0x30
RIP: 0010:bpf_prog_45a7907e7114d0ff_handle_fexit_ret_subprogs3+0x2a/0x6c
Code: f3 0f 1e fa 0f 1f 44 00 00 66 90 55 48 89 e5 f3 0f 1e fa 48 8b 7f 08 49 bb 00 00 00 00 00 80 00 00 4c 39 df 73 04 31 f6 eb 04 <48> 8b 77 00 49 bb 00 00 00 00 00 80 00 00 48 81 c7 7c 00 00 00 4c
RSP: 0018:ffffb30c4291fcb8 EFLAGS: 00010282
RAX: 0000000000000001 RBX: 0000000000000001 RCX: 0000000000000000
RDX: 00000000cddf1af1 RSI: 000000005315a00d RDI: ffffffffffffffea
RBP: ffffb30c4291fcb8 R08: ffff92e644bf38a8 R09: 0000000000000000
R10: 0000000000000000 R11: 0000800000000000 R12: ffff92e663652690
R13: 00000000000001c8 R14: 00000000000001c8 R15: 0000000000000003
bpf_trampoline_251255721842_2+0x63/0x1000
bpf_testmod_return_ptr+0x9/0xb0 [bpf_testmod]
? bpf_testmod_test_read+0x43/0x2d0 [bpf_testmod]
sysfs_kf_bin_read+0x60/0x90
kernfs_fop_read_iter+0x143/0x250
vfs_read+0x240/0x2a0
ksys_read+0x70/0xe0
__x64_sys_read+0x1f/0x30
do_syscall_64+0x68/0xa0
? syscall_exit_to_user_mode+0x77/0x1f0
? do_syscall_64+0x77/0xa0
? irqentry_exit+0x35/0xa0
? sysvec_apic_timer_interrupt+0x4d/0x90
entry_SYSCALL_64_after_hwframe+0x72/0xdc
RIP: 0033:0x7fb5da00a392
Code: ac 00 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb be 0f 1f 80 00 00 00 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 0f 05 <48> 3d 00 f0 ff ff 77 56 c3 0f 1f 44 00 00 48 83 ec 28 48 89 54 24
RSP: 002b:00007ffc5b3cab68 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 000055bee7b8b100 RCX: 00007fb5da00a392
RDX: 00000000000001c8 RSI: 0000000000000000 RDI: 0000000000000009
RBP: 00007ffc5b3caba0 R08: 0000000000000000 R09: 0000000000000037
R10: 000055bee7b8c2a7 R11: 0000000000000246 R12: 000055bee78f1f60
R13: 00007ffc5b3cae90 R14: 0000000000000000 R15: 0000000000000000
</TASK>
Modules linked in: bpf_testmod(OE) nls_iso8859_1 dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua intel_rapl_msr intel_rapl_common intel_uncore_frequency_common ppdev nfit crct10dif_pclmul crc32_pclmul psmouse ghash_clmulni_intel sha512_ssse3 aesni_intel parport_pc crypto_simd cryptd input_leds parport rapl ena i2c_piix4 mac_hid serio_raw ramoops reed_solomon pstore_blk drm pstore_zone efi_pstore autofs4 [last unloaded: bpf_testmod(OE)]
CR2: 000000000000000c

Though there may be some variation, depending on which suprogram
triggers the bug.

Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/4ebf95ec857cd785b81db69f3e408c039ad8408b.1686616663.git.kjlx@templeofstupid.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>