Re: [PATCH v10 01/36] tracing: Add a comment about ftrace_regs definition

On Thu, 23 May 2024 19:10:31 -0400
Steven Rostedt [off-list ref] wrote:

On Tue,  7 May 2024 23:08:12 +0900
"Masami Hiramatsu (Google)" [off-list ref] wrote:

quoted

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

To clarify what will be expected on ftrace_regs, add a comment to the
architecture independent definition of the ftrace_regs.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
---
 Changes in v8:
  - Update that the saved registers depends on the context.
 Changes in v3:
  - Add instruction pointer
 Changes in v2:
  - newly added.
---
 include/linux/ftrace.h |   26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 54d53f345d14..b81f1afa82a1 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h

@@ -118,6 +118,32 @@ extern int ftrace_enabled;
 
 #ifndef CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS
 
+/**
+ * ftrace_regs - ftrace partial/optimal register set
+ *
+ * ftrace_regs represents a group of registers which is used at the
+ * function entry and exit. There are three types of registers.
+ *
+ * - Registers for passing the parameters to callee, including the stack
+ *   pointer. (e.g. rcx, rdx, rdi, rsi, r8, r9 and rsp on x86_64)
+ * - Registers for passing the return values to caller.
+ *   (e.g. rax and rdx on x86_64)
+ * - Registers for hooking the function call and return including the
+ *   frame pointer (the frame pointer is architecture/config dependent)
+ *   (e.g. rip, rbp and rsp for x86_64)
+ *
+ * Also, architecture dependent fields can be used for internal process.
+ * (e.g. orig_ax on x86_64)
+ *
+ * On the function entry, those registers will be restored except for
+ * the stack pointer, so that user can change the function parameters
+ * and instruction pointer (e.g. live patching.)
+ * On the function exit, only registers which is used for return values
+ * are restored.

I wonder if we should also add a note about some architectures in some
circumstances may store all pt_regs in ftrace_regs. For example, if an
architecture supports FTRACE_WITH_REGS, it may pass the pt_regs within the
ftrace_regs. If that is the case, then ftrace_get_regs() called on it will
return a pointer to a valid pt_regs, or NULL if it is not supported or the
ftrace_regs does not have a all the registers.

Agreed. That case also should be noted. Thanks for pointing!

-- Steve

quoted

+ *
+ * NOTE: user *must not* access regs directly, only do it via APIs, because
+ * the member can be changed according to the architecture.
+ */
 struct ftrace_regs {
 	struct pt_regs		regs;
 };

-- 
Masami Hiramatsu (Google) [off-list ref]