bpf trampoline support "jmp" mode #10322
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added 5 commits
November 18, 2025 16:52
For now, the "nop" will be replaced with a "call" instruction when a function is hooked by the ftrace. However, sometimes the "call" can break the RSB and introduce extra overhead. Therefore, introduce the flag FTRACE_OPS_FL_JMP, which indicate that the ftrace_ops should be called with a "jmp" instead of "call". For now, it is only used by the direct call case. When a direct ftrace_ops is marked with FTRACE_OPS_FL_JMP, the last bit of the ops->direct_call will be set to 1. Therefore, we can tell if we should use "jmp" for the callback in ftrace_call_replace(). Signed-off-by: Menglong Dong <[email protected]> --- v3: - reject if the addr is already "jmp" in register_ftrace_direct() and __modify_ftrace_direct()
Implement the DYNAMIC_FTRACE_WITH_JMP for x86_64. In ftrace_call_replace,
we will use JMP32_INSN_OPCODE instead of CALL_INSN_OPCODE if the address
should use "jmp".
Meanwhile, adjust the direct call in the ftrace_regs_caller. The RSB is
balanced in the "jmp" mode. Take the function "foo" for example:
original_caller:
call foo -> foo:
call fentry -> fentry:
[do ftrace callbacks ]
move tramp_addr to stack
RET -> tramp_addr
tramp_addr:
[..]
call foo_body -> foo_body:
[..]
RET -> back to tramp_addr
[..]
RET -> back to original_caller
Signed-off-by: Menglong Dong <[email protected]>
Some places calculate the origin_call by checking if BPF_TRAMP_F_SKIP_FRAME is set. However, it should use BPF_TRAMP_F_ORIG_STACK for this propose. Just fix them. Signed-off-by: Menglong Dong <[email protected]> Acked-by: Alexei Starovoitov <[email protected]>
In the origin call case, if BPF_TRAMP_F_SKIP_FRAME is not set, it means that the trampoline is not called, but "jmp". Introduce the function bpf_trampoline_use_jmp() to check if the trampoline is in "jmp" mode. Do some adjustment on the "jmp" mode for the x86_64. The main adjustment that we make is for the stack parameter passing case, as the stack alignment logic changes in the "jmp" mode without the "rip". What's more, the location of the parameters on the stack also changes. Signed-off-by: Menglong Dong <[email protected]> --- v2: - rename bpf_trampoline_need_jmp() to bpf_trampoline_use_jmp()
In the origin logic, the bpf_arch_text_poke() assume that the old and new instructions have the same opcode. However, they can have different opcode if we want to replace a "call" insn with a "jmp" insn. Therefore, add the new function parameter "old_t" along with the "new_t", which are used to indicate the old and new poke type. Meanwhile, adjust the implement of bpf_arch_text_poke() for all the archs. "BPF_MOD_NOP" is added to make the code more readable. In bpf_arch_text_poke(), we still check if the new and old address is NULL to determine if nop insn should be used, which I think is more safe. Signed-off-by: Menglong Dong <[email protected]> --- v3: - fix compile error in powerpc v2: - add new function parameter to bpf_arch_text_poke instead of introduce bpf_arch_text_poke_type()
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Implement the "jmp" mode for the bpf trampoline. For the ftrace_managed case, we need only to set the FTRACE_OPS_FL_JMP on the tr->fops if "jmp" is needed. For the bpf poke case, we will check the origin poke type with the "origin_flags", and current poke type with "tr->flags". The function bpf_trampoline_update_fentry() is introduced to do the job. The "jmp" mode will only be enabled with CONFIG_DYNAMIC_FTRACE_WITH_JMP enabled and BPF_TRAMP_F_SHARE_IPMODIFY is not set. With BPF_TRAMP_F_SHARE_IPMODIFY, we need to get the origin call ip from the stack, so we can't use the "jmp" mode. Signed-off-by: Menglong Dong <[email protected]> --- v3: - wrap 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() v2: - 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.
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