Re: [PATCH bpf-next v2 1/6] ftrace: introduce FTRACE_OPS_FL_JMP
From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Date: 2025-11-18 05:19:41
Also in:
bpf, lkml
On Mon, 17 Nov 2025 11:49:01 +0800 Menglong Dong [off-list ref] wrote:
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().
nit: Is it sure the last bit is always 0? At least register_ftrace_direct() needs to reject if @addr parameter has the last bit. Thanks,
quoted hunk ↗ jump to hunk
Signed-off-by: Menglong Dong <redacted> --- include/linux/ftrace.h | 33 +++++++++++++++++++++++++++++++++ kernel/trace/Kconfig | 12 ++++++++++++ kernel/trace/ftrace.c | 9 ++++++++- 3 files changed, 53 insertions(+), 1 deletion(-)diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h index 07f8c309e432..015dd1049bea 100644 --- a/include/linux/ftrace.h +++ b/include/linux/ftrace.h@@ -359,6 +359,7 @@ enum { FTRACE_OPS_FL_DIRECT = BIT(17), FTRACE_OPS_FL_SUBOP = BIT(18), FTRACE_OPS_FL_GRAPH = BIT(19), + FTRACE_OPS_FL_JMP = BIT(20), }; #ifndef CONFIG_DYNAMIC_FTRACE_WITH_ARGS@@ -577,6 +578,38 @@ static inline void arch_ftrace_set_direct_caller(struct ftrace_regs *fregs, unsigned long addr) { } #endif /* CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS */ +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_JMP +static inline bool ftrace_is_jmp(unsigned long addr) +{ + return addr & 1; +} + +static inline unsigned long ftrace_jmp_set(unsigned long addr) +{ + return addr | 1UL; +} + +static inline unsigned long ftrace_jmp_get(unsigned long addr) +{ + return addr & ~1UL; +} +#else +static inline bool ftrace_is_jmp(unsigned long addr) +{ + return false; +} + +static inline unsigned long ftrace_jmp_set(unsigned long addr) +{ + return addr; +} + +static inline unsigned long ftrace_jmp_get(unsigned long addr) +{ + return addr; +} +#endif /* CONFIG_DYNAMIC_FTRACE_WITH_JMP */ + #ifdef CONFIG_STACK_TRACER int stack_trace_sysctl(const struct ctl_table *table, int write, void *buffer,diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig index d2c79da81e4f..4661b9e606e0 100644 --- a/kernel/trace/Kconfig +++ b/kernel/trace/Kconfig@@ -80,6 +80,12 @@ config HAVE_DYNAMIC_FTRACE_NO_PATCHABLE If the architecture generates __patchable_function_entries sections but does not want them included in the ftrace locations. +config HAVE_DYNAMIC_FTRACE_WITH_JMP + bool + help + If the architecture supports to replace the __fentry__ with a + "jmp" instruction. + config HAVE_SYSCALL_TRACEPOINTS bool help@@ -330,6 +336,12 @@ config DYNAMIC_FTRACE_WITH_ARGS depends on DYNAMIC_FTRACE depends on HAVE_DYNAMIC_FTRACE_WITH_ARGS +config DYNAMIC_FTRACE_WITH_JMP + def_bool y + depends on DYNAMIC_FTRACE + depends on DYNAMIC_FTRACE_WITH_DIRECT_CALLS + depends on HAVE_DYNAMIC_FTRACE_WITH_JMP + config FPROBE bool "Kernel Function Probe (fprobe)" depends on HAVE_FUNCTION_GRAPH_FREGS && HAVE_FTRACE_GRAPH_FUNCdiff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c index 59cfacb8a5bb..a6c060a4f50b 100644 --- a/kernel/trace/ftrace.c +++ b/kernel/trace/ftrace.c@@ -5951,7 +5951,8 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long for (i = 0; i < size; i++) { hlist_for_each_entry(entry, &hash->buckets[i], hlist) { del = __ftrace_lookup_ip(direct_functions, entry->ip); - if (del && del->direct == addr) { + if (del && ftrace_jmp_get(del->direct) == + ftrace_jmp_get(addr)) { remove_hash_entry(direct_functions, del); kfree(del); }@@ -6018,6 +6019,9 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) mutex_lock(&direct_mutex); + if (ops->flags & FTRACE_OPS_FL_JMP) + addr = ftrace_jmp_set(addr); + /* Make sure requested entries are not already registered.. */ size = 1 << hash->size_bits; for (i = 0; i < size; i++) {@@ -6138,6 +6142,9 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr) lockdep_assert_held_once(&direct_mutex); + if (ops->flags & FTRACE_OPS_FL_JMP) + addr = ftrace_jmp_set(addr); + /* Enable the tmp_ops to have the same functions as the direct ops */ ftrace_ops_init(&tmp_ops); tmp_ops.func_hash = ops->func_hash;-- 2.51.2
-- Masami Hiramatsu (Google) [off-list ref]