[PATCH v19 18/19] ftrace: Add ftrace_get_symaddr to convert fentry_ip to symaddr

Masami Hiramatsu (Google)
2024-11-10T15:52:26Z

This introduces ftrace_get_symaddr() which tries to convert fentry_ip
passed by ftrace or fgraph callback to symaddr without calling
kallsyms API. It returns the symbol address or 0 if it fails to
convert it.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
Changes in v19:
- Newly added.
---
arch/arm64/include/asm/ftrace.h | 2 +
arch/arm64/kernel/ftrace.c | 63 +++++++++++++++++++++++++++++++++++++++
arch/x86/include/asm/ftrace.h | 21 +++++++++++++
include/linux/ftrace.h | 13 ++++++++
4 files changed, 99 insertions(+)

diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index 876e88ad4119..f08e70bf09ea 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -52,6 +52,8 @@ extern unsigned long ftrace_graph_call;
extern void return_to_handler(void);

unsigned long ftrace_call_adjust(unsigned long addr);
+unsigned long arch_ftrace_call_adjust(unsigned long fentry_ip);
+#define ftrace_call_adjust(fentry_ip) arch_ftrace_call_adjust(fentry_ip)

#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
#define HAVE_ARCH_FTRACE_REGS
diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
index 606fd6994578..de1223669758 100644
--- a/arch/arm64/kernel/ftrace.c
+++ b/arch/arm64/kernel/ftrace.c
@@ -143,6 +143,69 @@ unsigned long ftrace_call_adjust(unsigned long addr)
return addr;
}

+/* Convert fentry_ip to the symbol address without kallsyms */
+unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
+{
+ u32 insn;
+
+ /*
+ * When using patchable-function-entry without pre-function NOPS, ftrace
+ * entry is the address of the first NOP after the function entry point.
+ *
+ * The compiler has either generated:
+ *
+ * func+00: func: NOP // To be patched to MOV X9, LR
+ * func+04: NOP // To be patched to BL <caller>
+ *
+ * Or:
+ *
+ * func-04: BTI C
+ * func+00: func: NOP // To be patched to MOV X9, LR
+ * func+04: NOP // To be patched to BL <caller>
+ *
+ * The fentry_ip is the address of `BL <caller>` which is at `func + 4`
+ * bytes in either case.
+ */
+ if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS))
+ return fentry_ip - AARCH64_INSN_SIZE;
+
+ /*
+ * When using patchable-function-entry with pre-function NOPs, BTI is
+ * a bit different.
+ *
+ * func+00: func: NOP // To be patched to MOV X9, LR
+ * func+04: NOP // To be patched to BL <caller>
+ *
+ * Or:
+ *
+ * func+00: func: BTI C
+ * func+04: NOP // To be patched to MOV X9, LR
+ * func+08: NOP // To be patched to BL <caller>
+ *
+ * The fentry_ip is the address of `BL <caller>` which is at either
+ * `func + 4` or `func + 8` depends on whether there is a BTI.
+ */
+
+ /* If there is no BTI, the func address should be one instruction before. */
+ if (!IS_ENABLED(CONFIG_ARM64_BTI_KERNEL))
+ return fentry_ip - AARCH64_INSN_SIZE;
+
+ /* We want to be extra safe in case entry ip is on the page edge,
+ * but otherwise we need to avoid get_kernel_nofault()'s overhead.
+ */
+ if ((fentry_ip & ~PAGE_MASK) < AARCH64_INSN_SIZE * 2) {
+ if (get_kernel_nofault(insn, (u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2)))
+ return 0;
+ } else {
+ insn = *(u32 *)(fentry_ip - AARCH64_INSN_SIZE * 2);
+ }
+
+ if (aarch64_insn_is_bti(le32_to_cpu((__le32)insn)))
+ return fentry_ip - AARCH64_INSN_SIZE * 2;
+
+ return fentry_ip - AARCH64_INSN_SIZE;
+}
+
/*
* Replace a single instruction, which may be a branch or NOP.
* If @validate == true, a replaced instruction is checked against 'old'.
diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index cc92c99ef276..f9cb4d07df58 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -34,6 +34,27 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
return addr;
}

+static inline unsigned long arch_ftrace_get_symaddr(unsigned long fentry_ip)
+{
+#ifdef CONFIG_X86_KERNEL_IBT
+ u32 instr;
+
+ /* We want to be extra safe in case entry ip is on the page edge,
+ * but otherwise we need to avoid get_kernel_nofault()'s overhead.
+ */
+ if ((fentry_ip & ~PAGE_MASK) < ENDBR_INSN_SIZE) {
+ if (get_kernel_nofault(instr, (u32 *)(fentry_ip - ENDBR_INSN_SIZE)))
+ return fentry_ip;
+ } else {
+ instr = *(u32 *)(fentry_ip - ENDBR_INSN_SIZE);
+ }
+ if (is_endbr(instr))
+ fentry_ip -= ENDBR_INSN_SIZE;
+#endif
+ return fentry_ip;
+}
+#define ftrace_get_symaddr(fentry_ip) arch_ftrace_get_symaddr(fentry_ip)
+
#ifdef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS

#include <linux/ftrace_regs.h>
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 4c553fe9c026..9659bb2cd76c 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -652,6 +652,19 @@ struct ftrace_ops *ftrace_ops_trampoline(unsigned long addr);

bool is_ftrace_trampoline(unsigned long addr);

+/* Arches can override ftrace_get_symaddr() to convert fentry_ip to symaddr. */
+#ifndef ftrace_get_symaddr
+/**
+ * ftrace_get_symaddr - return the symbol address from fentry_ip
+ * @fentry_ip: the address of ftrace location
+ *
+ * Get the symbol address from @fentry_ip (fast path). If there is no fast
+ * search path, this returns 0.
+ * User may need to use kallsyms API to find the symbol address.
+ */
+#define ftrace_get_symaddr(fentry_ip) (0)
+#endif
+
/*
* The dyn_ftrace record's flags field is split into two parts.
* the first part which is '0-FTRACE_REF_MAX' is a counter of