[PATCH 0/2] function_graph: ftrace_graph_ret_addr(); there can be only one!

[PATCH 0/2] function_graph: ftrace_graph_ret_addr(); there can be only one!

[Steven Rostedt]

I noticed a slight bug in ftrace_graph_ret_addr() for when
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR was defined and fixed it up.
I then noticed it was buggy when not defined. Looking for an
architecture that did not have it defined, I couldn't find any.
So I removed it.

Steven Rostedt (Google) (2):
      function_graph: Fix up ftrace_graph_ret_addr()
      function_graph: Everyone uses HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, remove it

----
 Documentation/trace/ftrace-design.rst | 12 -------
 arch/arm64/include/asm/ftrace.h       | 11 -------
 arch/csky/include/asm/ftrace.h        |  2 --
 arch/loongarch/include/asm/ftrace.h   |  1 -
 arch/powerpc/include/asm/ftrace.h     |  2 --
 arch/riscv/include/asm/ftrace.h       |  1 -
 arch/s390/include/asm/ftrace.h        |  1 -
 arch/x86/include/asm/ftrace.h         |  2 --
 include/linux/ftrace.h                |  2 --
 kernel/trace/fgraph.c                 | 61 ++++++++++++-----------------------
 10 files changed, 20 insertions(+), 75 deletions(-)

[PATCH 1/2] function_graph: Fix up ftrace_graph_ret_addr()

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

Yang Li sent a patch to fix the kerneldoc of ftrace_graph_ret_addr().
While reviewing it, I realized that the comments in the entire function
header needed a rewrite. When doing that, I realized that @idx parameter
was being ignored. Every time this was called by the unwinder, it would
start the loop at the top of the shadow stack and look for the matching
stack pointer. When it found it, it would return it. When the unwinder
asked for the next function, it would search from the beginning again.

In reality, it should start from where it left off. That was the reason
for the @idx parameter in the first place. The first time the unwinder
calls this function, the @idx pointer would contain zero. That would mean
to start from the top of the stack. The function was supposed to update
the @idx with the index where it found the return address, so that the
next time the unwinder calls this function it doesn't have to search
through the previous addresses it found (making it O(n^2)!).

This speeds up the unwinder's use of ftrace_graph_ret_addr() by an order
of magnitude.

Link: https://lore.kernel.org/linux-trace-kernel/20240610181746.656e3759@gandalf.local.home/

Reported-by: Yang Li <yang.lee@linux.alibaba.com>
Fixes: 7aa1eaef9f428 ("function_graph: Allow multiple users to attach to function graph")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/trace/fgraph.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 63d0c2f84ce1..91f1eef256af 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -870,18 +870,24 @@ ftrace_graph_get_ret_stack(struct task_struct *task, int idx)
 }
 
 /**
- * ftrace_graph_ret_addr - convert a potentially modified stack return address
- *			   to its original value
+ * ftrace_graph_ret_addr - return the original value of the return address
+ * @task: The task the unwinder is being executed on
+ * @idx: An initialized pointer to the next stack index to use
+ * @ret: The current return address (likely pointing to return_handler)
+ * @retp: The address on the stack of the current return location
  *
  * This function can be called by stack unwinding code to convert a found stack
- * return address ('ret') to its original value, in case the function graph
+ * return address (@ret) to its original value, in case the function graph
  * tracer has modified it to be 'return_to_handler'.  If the address hasn't
- * been modified, the unchanged value of 'ret' is returned.
+ * been modified, the unchanged value of @ret is returned.
  *
- * 'idx' is a state variable which should be initialized by the caller to zero
- * before the first call.
+ * @idx holds the last index used to know where to start from. It should be
+ * initialized to zero for the first iteration as that will mean to start
+ * at the top of the shadow stack. If the location is found, this pointer
+ * will be assigned that location so that if called again, it will continue
+ * where it left off.
  *
- * 'retp' is a pointer to the return address on the stack.  It's ignored if
+ * @retp is a pointer to the return address on the stack.  It's ignored if
  * the arch doesn't have HAVE_FUNCTION_GRAPH_RET_ADDR_PTR defined.
  */
 #ifdef HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
@@ -895,6 +901,10 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 	if (ret != return_handler)
 		return ret;
 
+	if (!idx)
+		return ret;
+
+	i = *idx ? : task->curr_ret_stack;
 	while (i > 0) {
 		ret_stack = get_ret_stack(current, i, &i);
 		if (!ret_stack)
@@ -908,8 +918,10 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 		 * Thus we will continue to find real return address.
 		 */
 		if (ret_stack->retp == retp &&
-		    ret_stack->ret != return_handler)
+		    ret_stack->ret != return_handler) {
+			*idx = i;
 			return ret_stack->ret;
+		}
 	}
 
 	return ret;
-- 
2.43.0

[PATCH 2/2] function_graph: Everyone uses HAVE_FUNCTION_GRAPH_RET_ADDR_PTR, remove it

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@goodmis.org>

All architectures that implement function graph also implements
HAVE_FUNCTION_GRAPH_RET_ADDR_PTR. Remove it, as it is no longer a
differentiator.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 Documentation/trace/ftrace-design.rst | 12 ---------
 arch/arm64/include/asm/ftrace.h       | 11 ---------
 arch/csky/include/asm/ftrace.h        |  2 --
 arch/loongarch/include/asm/ftrace.h   |  1 -
 arch/powerpc/include/asm/ftrace.h     |  2 --
 arch/riscv/include/asm/ftrace.h       |  1 -
 arch/s390/include/asm/ftrace.h        |  1 -
 arch/x86/include/asm/ftrace.h         |  2 --
 include/linux/ftrace.h                |  2 --
 kernel/trace/fgraph.c                 | 35 +--------------------------
 10 files changed, 1 insertion(+), 68 deletions(-)

diff --git a/Documentation/trace/ftrace-design.rst b/Documentation/trace/ftrace-design.rst
index 6893399157f0..dc82d64b3a44 100644
--- a/Documentation/trace/ftrace-design.rst
+++ b/Documentation/trace/ftrace-design.rst
@@ -217,18 +217,6 @@ along to ftrace_push_return_trace() instead of a stub value of 0.
 
 Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
 
-HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
---------------------------------
-
-An arch may pass in a pointer to the return address on the stack.  This
-prevents potential stack unwinding issues where the unwinder gets out of
-sync with ret_stack and the wrong addresses are reported by
-ftrace_graph_ret_addr().
-
-Adding support for it is easy: just define the macro in asm/ftrace.h and
-pass the return address pointer as the 'retp' argument to
-ftrace_push_return_trace().
-
 HAVE_SYSCALL_TRACEPOINTS
 ------------------------
 
diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
index ab158196480c..dc9cf0bd2a4c 100644
--- a/arch/arm64/include/asm/ftrace.h
+++ b/arch/arm64/include/asm/ftrace.h
@@ -12,17 +12,6 @@
 
 #define HAVE_FUNCTION_GRAPH_FP_TEST
 
-/*
- * HAVE_FUNCTION_GRAPH_RET_ADDR_PTR means that the architecture can provide a
- * "return address pointer" which can be used to uniquely identify a return
- * address which has been overwritten.
- *
- * On arm64 we use the address of the caller's frame record, which remains the
- * same for the lifetime of the instrumented function, unlike the return
- * address in the LR.
- */
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
-
 #ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #else
diff --git a/arch/csky/include/asm/ftrace.h b/arch/csky/include/asm/ftrace.h
index fd215c38ef27..00f9f7647e3f 100644
--- a/arch/csky/include/asm/ftrace.h
+++ b/arch/csky/include/asm/ftrace.h
@@ -7,8 +7,6 @@
 
 #define HAVE_FUNCTION_GRAPH_FP_TEST
 
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
-
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 
 #define MCOUNT_ADDR	((unsigned long)_mcount)
diff --git a/arch/loongarch/include/asm/ftrace.h b/arch/loongarch/include/asm/ftrace.h
index de891c2c83d4..c0a682808e07 100644
--- a/arch/loongarch/include/asm/ftrace.h
+++ b/arch/loongarch/include/asm/ftrace.h
@@ -28,7 +28,6 @@ struct dyn_ftrace;
 struct dyn_arch_ftrace { };
 
 #define ARCH_SUPPORTS_FTRACE_OPS 1
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
 #define ftrace_init_nop ftrace_init_nop
 int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
diff --git a/arch/powerpc/include/asm/ftrace.h b/arch/powerpc/include/asm/ftrace.h
index 107fc5a48456..559560286e6d 100644
--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h
@@ -8,8 +8,6 @@
 #define MCOUNT_ADDR		((unsigned long)(_mcount))
 #define MCOUNT_INSN_SIZE	4 /* sizeof mcount call */
 
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
-
 /* Ignore unused weak functions which will have larger offsets */
 #if defined(CONFIG_MPROFILE_KERNEL) || defined(CONFIG_ARCH_USING_PATCHABLE_FUNCTION_ENTRY)
 #define FTRACE_MCOUNT_MAX_OFFSET	16
diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
index 9eb31a7ea0aa..2cddd79ff21b 100644
--- a/arch/riscv/include/asm/ftrace.h
+++ b/arch/riscv/include/asm/ftrace.h
@@ -11,7 +11,6 @@
 #if defined(CONFIG_FUNCTION_GRAPH_TRACER) && defined(CONFIG_FRAME_POINTER)
 #define HAVE_FUNCTION_GRAPH_FP_TEST
 #endif
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #ifndef __ASSEMBLY__
diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
index 77e479d44f1e..fbadca645af7 100644
--- a/arch/s390/include/asm/ftrace.h
+++ b/arch/s390/include/asm/ftrace.h
@@ -2,7 +2,6 @@
 #ifndef _ASM_S390_FTRACE_H
 #define _ASM_S390_FTRACE_H
 
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #define MCOUNT_INSN_SIZE	6
 
diff --git a/arch/x86/include/asm/ftrace.h b/arch/x86/include/asm/ftrace.h
index 897cf02c20b1..0152a81d9b4a 100644
--- a/arch/x86/include/asm/ftrace.h
+++ b/arch/x86/include/asm/ftrace.h
@@ -20,8 +20,6 @@
 #define ARCH_SUPPORTS_FTRACE_OPS 1
 #endif
 
-#define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
-
 #ifndef __ASSEMBLY__
 extern void __fentry__(void);
 
diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 4135dc171447..845c2ab0bc1c 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -1071,9 +1071,7 @@ struct ftrace_ret_stack {
 #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
 	unsigned long fp;
 #endif
-#ifdef HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 	unsigned long *retp;
-#endif
 };
 
 /*
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index 91f1eef256af..8317d1a7f43a 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -593,9 +593,7 @@ ftrace_push_return_trace(unsigned long ret, unsigned long func,
 #ifdef HAVE_FUNCTION_GRAPH_FP_TEST
 	ret_stack->fp = frame_pointer;
 #endif
-#ifdef HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 	ret_stack->retp = retp;
-#endif
 	return offset;
 }
 
@@ -887,10 +885,8 @@ ftrace_graph_get_ret_stack(struct task_struct *task, int idx)
  * will be assigned that location so that if called again, it will continue
  * where it left off.
  *
- * @retp is a pointer to the return address on the stack.  It's ignored if
- * the arch doesn't have HAVE_FUNCTION_GRAPH_RET_ADDR_PTR defined.
+ * @retp is a pointer to the return address on the stack.
  */
-#ifdef HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
 unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 				    unsigned long ret, unsigned long *retp)
 {
@@ -926,35 +922,6 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
 
 	return ret;
 }
-#else /* !HAVE_FUNCTION_GRAPH_RET_ADDR_PTR */
-unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
-				    unsigned long ret, unsigned long *retp)
-{
-	struct ftrace_ret_stack *ret_stack;
-	unsigned long return_handler = (unsigned long)dereference_kernel_function_descriptor(return_to_handler);
-	int offset = task->curr_ret_stack;
-	int i;
-
-	if (ret != return_handler)
-		return ret;
-
-	if (!idx)
-		return ret;
-
-	i = *idx;
-	do {
-		ret_stack = get_ret_stack(task, offset, &offset);
-		if (ret_stack && ret_stack->ret == return_handler)
-			continue;
-		i--;
-	} while (i >= 0 && ret_stack);
-
-	if (ret_stack)
-		return ret_stack->ret;
-
-	return ret;
-}
-#endif /* HAVE_FUNCTION_GRAPH_RET_ADDR_PTR */
 
 static struct ftrace_ops graph_ops = {
 	.func			= ftrace_graph_func,
-- 
2.43.0