[PATCH bpf-next v3 2/6] x86/ftrace: implement DYNAMIC_FTRACE_WITH_JMP

Menglong Dong
2025-11-18T12:37:02Z

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 <dongml2@chinatelecom.cn>
---
arch/x86/Kconfig | 1 +
arch/x86/kernel/ftrace.c | 7 ++++++-
arch/x86/kernel/ftrace_64.S | 12 +++++++++++-
3 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index fa3b616af03a..462250a20311 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -230,6 +230,7 @@ config X86
select HAVE_DYNAMIC_FTRACE_WITH_ARGS if X86_64
select HAVE_FTRACE_REGS_HAVING_PT_REGS if X86_64
select HAVE_DYNAMIC_FTRACE_WITH_DIRECT_CALLS
+ select HAVE_DYNAMIC_FTRACE_WITH_JMP if X86_64
select HAVE_SAMPLE_FTRACE_DIRECT if X86_64
select HAVE_SAMPLE_FTRACE_DIRECT_MULTI if X86_64
select HAVE_EBPF_JIT
diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 4450acec9390..0543b57f54ee 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -74,7 +74,12 @@ static const char *ftrace_call_replace(unsigned long ip, unsigned long addr)
* No need to translate into a callthunk. The trampoline does
* the depth accounting itself.
*/
- return text_gen_insn(CALL_INSN_OPCODE, (void *)ip, (void *)addr);
+ if (ftrace_is_jmp(addr)) {
+ addr = ftrace_jmp_get(addr);
+ return text_gen_insn(JMP32_INSN_OPCODE, (void *)ip, (void *)addr);
+ } else {
+ return text_gen_insn(CALL_INSN_OPCODE, (void *)ip, (void *)addr);
+ }
}

static int ftrace_verify_code(unsigned long ip, const char *old_code)
diff --git a/arch/x86/kernel/ftrace_64.S b/arch/x86/kernel/ftrace_64.S
index 823dbdd0eb41..a132608265f6 100644
--- a/arch/x86/kernel/ftrace_64.S
+++ b/arch/x86/kernel/ftrace_64.S
@@ -285,8 +285,18 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
ANNOTATE_NOENDBR
RET

+1:
+ testb $1, %al
+ jz 2f
+ andq $0xfffffffffffffffe, %rax
+ movq %rax, MCOUNT_REG_SIZE+8(%rsp)
+ restore_mcount_regs
+ /* Restore flags */
+ popfq
+ RET
+
/* Swap the flags with orig_rax */
-1: movq MCOUNT_REG_SIZE(%rsp), %rdi
+2: movq MCOUNT_REG_SIZE(%rsp), %rdi
movq %rdi, MCOUNT_REG_SIZE-8(%rsp)
movq %rax, MCOUNT_REG_SIZE(%rsp)

--
2.51.2