[PATCH bpf-next v3 0/6] bpf trampoline support "jmp" mode

Menglong Dong
2025-11-18T12:36:50Z

For now, the bpf trampoline is called by the "call" instruction. However,
it break the RSB and introduce extra overhead in x86_64 arch.

For example, we hook the function "foo" with fexit, the call and return
logic will be like this:
call foo -> call trampoline -> call foo-body ->
return foo-body -> return foo

As we can see above, there are 3 call, but 2 return, which break the RSB
balance. We can pseudo a "return" here, but it's not the best choice,
as it will still cause once RSB miss:
call foo -> call trampoline -> call foo-body ->
return foo-body -> return dummy -> return foo

The "return dummy" doesn't pair the "call trampoline", which can also
cause the RSB miss.

Therefore, we introduce the "jmp" mode for bpf trampoline, as advised by
Alexei in [1]. And the logic will become this:
call foo -> jmp trampoline -> call foo-body ->
return foo-body -> return foo

As we can see above, the RSB is totally balanced after this series.

In this series, we introduce the FTRACE_OPS_FL_JMP for ftrace to make it
use the "jmp" instruction instead of "call".

And we also do some adjustment to bpf_arch_text_poke() to allow us specify
the old and new poke_type.

For the BPF_TRAMP_F_SHARE_IPMODIFY case, we will fallback to the "call"
mode, as it need to get the function address from the stack, which is not
supported in "jmp" mode.

Before this series, we have the following performance with the bpf
benchmark:

$ cd tools/testing/selftests/bpf
$ ./benchs/run_bench_trigger.sh
usermode-count : 890.171 ± 1.522M/s
kernel-count : 409.184 ± 0.330M/s
syscall-count : 26.792 ± 0.010M/s
fentry : 171.242 ± 0.322M/s
fexit : 80.544 ± 0.045M/s
fmodret : 78.301 ± 0.065M/s
rawtp : 192.906 ± 0.900M/s
tp : 81.883 ± 0.209M/s
kprobe : 52.029 ± 0.113M/s
kprobe-multi : 62.237 ± 0.060M/s
kprobe-multi-all: 4.761 ± 0.014M/s
kretprobe : 23.779 ± 0.046M/s
kretprobe-multi: 29.134 ± 0.012M/s
kretprobe-multi-all: 3.822 ± 0.003M/

And after this series, we have the following performance:

usermode-count : 890.443 ± 0.307M/s
kernel-count : 416.139 ± 0.055M/s
syscall-count : 31.037 ± 0.813M/s
fentry : 169.549 ± 0.519M/s
fexit : 136.540 ± 0.518M/s
fmodret : 159.248 ± 0.188M/s
rawtp : 194.475 ± 0.144M/s
tp : 84.505 ± 0.041M/s
kprobe : 59.951 ± 0.071M/s
kprobe-multi : 63.153 ± 0.177M/s
kprobe-multi-all: 4.699 ± 0.012M/s
kretprobe : 23.740 ± 0.015M/s
kretprobe-multi: 29.301 ± 0.022M/s
kretprobe-multi-all: 3.869 ± 0.005M/s

As we can see above, the performance of fexit increase from 80.544M/s to
136.540M/s, and the "fmodret" increase from 78.301M/s to 159.248M/s.

Link: https://lore.kernel.org/bpf/20251117034906.32036-1-dongml2@chinatelecom.cn/
Changes since v2:
* reject if the addr is already "jmp" in register_ftrace_direct() and
__modify_ftrace_direct() in the 1st patch.
* fix compile error in powerpc in the 5th patch.
* changes in the 6th patch:
- fix the compile error by wrapping the write to tr->fops->flags with
CONFIG_DYNAMIC_FTRACE_WITH_JMP
- reset BPF_TRAMP_F_SKIP_FRAME when the second try of modify_fentry in
bpf_trampoline_update()

Link: https://lore.kernel.org/bpf/20251114092450.172024-1-dongml2@chinatelecom.cn/
Changes since v1:
* change the bool parameter that we add to save_args() to "u32 flags"
* rename bpf_trampoline_need_jmp() to bpf_trampoline_use_jmp()
* add new function parameter to bpf_arch_text_poke instead of introduce
bpf_arch_text_poke_type()
* rename bpf_text_poke to bpf_trampoline_update_fentry
* remove the BPF_TRAMP_F_JMPED and check the current mode with the origin
flags instead.

Link: https://lore.kernel.org/bpf/CAADnVQLX54sVi1oaHrkSiLqjJaJdm3TQjoVrgU-LZimK6iDcSA@mail.gmail.com/[1]
Menglong Dong (6):
ftrace: introduce FTRACE_OPS_FL_JMP
x86/ftrace: implement DYNAMIC_FTRACE_WITH_JMP
bpf: fix the usage of BPF_TRAMP_F_SKIP_FRAME
bpf,x86: adjust the "jmp" mode for bpf trampoline
bpf: specify the old and new poke_type for bpf_arch_text_poke
bpf: implement "jmp" mode for trampoline

arch/arm64/net/bpf_jit_comp.c | 14 +++---
arch/loongarch/net/bpf_jit.c | 9 ++--
arch/powerpc/net/bpf_jit_comp.c | 10 +++--
arch/riscv/net/bpf_jit_comp64.c | 11 +++--
arch/s390/net/bpf_jit_comp.c | 7 +--
arch/x86/Kconfig | 1 +
arch/x86/kernel/ftrace.c | 7 ++-
arch/x86/kernel/ftrace_64.S | 12 +++++-
arch/x86/net/bpf_jit_comp.c | 55 ++++++++++++++----------
include/linux/bpf.h | 18 +++++++-
include/linux/ftrace.h | 33 ++++++++++++++
kernel/bpf/core.c | 5 ++-
kernel/bpf/trampoline.c | 76 ++++++++++++++++++++++++++-------
kernel/trace/Kconfig | 12 ++++++
kernel/trace/ftrace.c | 14 +++++-
15 files changed, 219 insertions(+), 65 deletions(-)

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2.51.2