linux/arch/x86
Tiezhu Yang ebf7f6f0a6 bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33
In the current code, the actual max tail call count is 33 which is greater
than MAX_TAIL_CALL_CNT (defined as 32). The actual limit is not consistent
with the meaning of MAX_TAIL_CALL_CNT and thus confusing at first glance.
We can see the historical evolution from commit 04fd61ab36 ("bpf: allow
bpf programs to tail-call other bpf programs") and commit f9dabe016b
("bpf: Undo off-by-one in interpreter tail call count limit"). In order
to avoid changing existing behavior, the actual limit is 33 now, this is
reasonable.

After commit 874be05f52 ("bpf, tests: Add tail call test suite"), we can
see there exists failed testcase.

On all archs when CONFIG_BPF_JIT_ALWAYS_ON is not set:
 # echo 0 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf
 # dmesg | grep -w FAIL
 Tail call error path, max count reached jited:0 ret 34 != 33 FAIL

On some archs:
 # echo 1 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf
 # dmesg | grep -w FAIL
 Tail call error path, max count reached jited:1 ret 34 != 33 FAIL

Although the above failed testcase has been fixed in commit 18935a72eb
("bpf/tests: Fix error in tail call limit tests"), it would still be good
to change the value of MAX_TAIL_CALL_CNT from 32 to 33 to make the code
more readable.

The 32-bit x86 JIT was using a limit of 32, just fix the wrong comments and
limit to 33 tail calls as the constant MAX_TAIL_CALL_CNT updated. For the
mips64 JIT, use "ori" instead of "addiu" as suggested by Johan Almbladh.
For the riscv JIT, use RV_REG_TCC directly to save one register move as
suggested by Björn Töpel. For the other implementations, no function changes,
it does not change the current limit 33, the new value of MAX_TAIL_CALL_CNT
can reflect the actual max tail call count, the related tail call testcases
in test_bpf module and selftests can work well for the interpreter and the
JIT.

Here are the test results on x86_64:

 # uname -m
 x86_64
 # echo 0 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf test_suite=test_tail_calls
 # dmesg | tail -1
 test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [0/8 JIT'ed]
 # rmmod test_bpf
 # echo 1 > /proc/sys/net/core/bpf_jit_enable
 # modprobe test_bpf test_suite=test_tail_calls
 # dmesg | tail -1
 test_bpf: test_tail_calls: Summary: 8 PASSED, 0 FAILED, [8/8 JIT'ed]
 # rmmod test_bpf
 # ./test_progs -t tailcalls
 #142 tailcalls:OK
 Summary: 1/11 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Tested-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Acked-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/bpf/1636075800-3264-1-git-send-email-yangtiezhu@loongson.cn
2021-11-16 14:03:15 +01:00
..
boot - Do not #GP on userspace use of CLI/STI but pretend it was a NOP to 2021-11-02 07:56:47 -07:00
configs
crypto Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6 2021-11-01 21:24:02 -07:00
entry Merge branch 'exit-cleanups-for-v5.16' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2021-11-10 16:15:54 -08:00
events pci-v5.16-changes 2021-11-06 14:36:12 -07:00
hyperv x86/hyperv: Protect set_hv_tscchange_cb() against getting preempted 2021-10-28 11:59:13 +00:00
ia32 audit/stable-5.16 PR 20211101 2021-11-01 21:17:39 -07:00
include asm-generic: asm/syscall.h cleanup 2021-11-10 11:22:03 -08:00
kernel Merge branch 'exit-cleanups-for-v5.16' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2021-11-10 16:15:54 -08:00
kvm ARM: 2021-11-02 11:24:14 -07:00
lib - Do not #GP on userspace use of CLI/STI but pretend it was a NOP to 2021-11-02 07:56:47 -07:00
math-emu x86/math-emu: Convert to fpstate 2021-10-20 23:57:54 +02:00
mm Merge branch 'akpm' (patches from Andrew) 2021-11-09 10:11:53 -08:00
net bpf: Change value of MAX_TAIL_CALL_CNT from 32 to 33 2021-11-16 14:03:15 +01:00
pci xen: branch for v5.16-rc1 2021-11-10 11:14:21 -08:00
platform Add an interface called cc_platform_has() which is supposed to be used 2021-11-01 15:16:52 -07:00
power x86/fpu: Replace the includes of fpu/internal.h 2021-10-20 15:27:29 +02:00
purgatory
ras
realmode x86/sev: Replace occurrences of sev_es_active() with cc_platform_has() 2021-10-04 11:47:09 +02:00
tools Driver core changes for 5.16-rc1 2021-11-04 08:32:38 -07:00
um
video
xen xen: branch for v5.16-rc1 2021-11-10 11:14:21 -08:00
.gitignore
Kbuild kbuild: use more subdir- for visiting subdirectories while cleaning 2021-10-24 13:49:46 +09:00
Kconfig s390 updates for the 5.16 merge window 2021-11-06 14:48:06 -07:00
Kconfig.assembler
Kconfig.cpu x86/CPU: Add support for Vortex CPUs 2021-10-21 15:49:07 +02:00
Kconfig.debug
Makefile Kbuild updates for v5.16 2021-11-08 09:15:45 -08:00
Makefile_32.cpu
Makefile.um