linux-kernel - [RFC PATCH v2 2/8] arm64: Implement frame types

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Message-Id: <20210315165800.5948-3-madvenka@linux.microsoft.com>
Date:   Mon, 15 Mar 2021 11:57:54 -0500
From:   madvenka@...ux.microsoft.com
To:     broonie@...nel.org, mark.rutland@....com, jpoimboe@...hat.com,
        jthierry@...hat.com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        madvenka@...ux.microsoft.com
Subject: [RFC PATCH v2 2/8] arm64: Implement frame types

From: "Madhavan T. Venkataraman" <madvenka@...ux.microsoft.com>

Apart from the task pt_regs, pt_regs is also created on the stack for other
other cases:

	- EL1 exception. A pt_regs is created on the stack to save register
	  state. In addition, pt_regs->stackframe is set up for the
	  interrupted kernel function so that the function shows up in the
	  EL1 exception stack trace.

	- When a traced function calls the ftrace infrastructure at the
	  beginning of the function, ftrace creates a pt_regs on the stack
	  at that point to save register state. In addition, it sets up
	  pt_regs->stackframe for the traced function so that the traced
	  function shows up in the stack trace taken from anywhere in the
	  ftrace code after that point. When the ftrace code returns to the
	  traced function, the pt_regs is removed from the stack.

To summarize, pt_regs->stackframe is used (or will be used) as a marker
frame in stack traces. To enable the unwinder to detect these frames, tag
each pt_regs->stackframe with a type. To record the type, use the unused2
field in struct pt_regs and rename it to frame_type. The types are:

TASK_FRAME
	Terminating frame for a normal stack trace.
EL0_FRAME
	Terminating frame for an EL0 exception.
EL1_FRAME
	EL1 exception frame.
FTRACE_FRAME
	FTRACE frame.

These frame types will be used by the unwinder later to validate frames.

Signed-off-by: Madhavan T. Venkataraman <madvenka@...ux.microsoft.com>
---
 arch/arm64/include/asm/ptrace.h | 15 +++++++++++++--
 arch/arm64/kernel/asm-offsets.c |  1 +
 arch/arm64/kernel/entry.S       |  4 ++++
 arch/arm64/kernel/head.S        |  2 ++
 arch/arm64/kernel/process.c     |  1 +
 5 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
index e58bca832dff..a75211ce009a 100644
--- a/arch/arm64/include/asm/ptrace.h
+++ b/arch/arm64/include/asm/ptrace.h
@@ -117,6 +117,17 @@
  */
 #define NO_SYSCALL (-1)
 
+/*
+ * pt_regs->stackframe is a marker frame that is used in different
+ * situations. These are the different types of frames. Use patterns
+ * for the frame types instead of (0, 1, 2, 3, ..) so that it is less
+ * likely to find them on the stack.
+ */
+#define TASK_FRAME	0xDEADBEE0	/* Task stack termination frame */
+#define EL0_FRAME	0xDEADBEE1	/* EL0 exception frame */
+#define EL1_FRAME	0xDEADBEE2	/* EL1 exception frame */
+#define FTRACE_FRAME	0xDEADBEE3	/* FTrace frame */
+
 #ifndef __ASSEMBLY__
 #include <linux/bug.h>
 #include <linux/types.h>
@@ -187,11 +198,11 @@ struct pt_regs {
 	};
 	u64 orig_x0;
 #ifdef __AARCH64EB__
-	u32 unused2;
+	u32 frame_type;
 	s32 syscallno;
 #else
 	s32 syscallno;
-	u32 unused2;
+	u32 frame_type;
 #endif
 	u64 sdei_ttbr1;
 	/* Only valid when ARM64_HAS_IRQ_PRIO_MASKING is enabled. */
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index a36e2fc330d4..43f97dbc7dfc 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -75,6 +75,7 @@ int main(void)
   DEFINE(S_SDEI_TTBR1,		offsetof(struct pt_regs, sdei_ttbr1));
   DEFINE(S_PMR_SAVE,		offsetof(struct pt_regs, pmr_save));
   DEFINE(S_STACKFRAME,		offsetof(struct pt_regs, stackframe));
+  DEFINE(S_FRAME_TYPE,		offsetof(struct pt_regs, frame_type));
   DEFINE(PT_REGS_SIZE,		sizeof(struct pt_regs));
   BLANK();
 #ifdef CONFIG_COMPAT
diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index e2dc2e998934..ecc3507d9cdd 100644
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -269,8 +269,12 @@ alternative_else_nop_endif
 	 */
 	.if \el == 0
 	stp	xzr, xzr, [sp, #S_STACKFRAME]
+	ldr	w17, =EL0_FRAME
+	str	w17, [sp, #S_FRAME_TYPE]
 	.else
 	stp	x29, x22, [sp, #S_STACKFRAME]
+	ldr	w17, =EL1_FRAME
+	str	w17, [sp, #S_FRAME_TYPE]
 	.endif
 	add	x29, sp, #S_STACKFRAME
 
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 2769b20934d4..d2ee78f8f97f 100644
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -410,6 +410,8 @@ SYM_FUNC_END(__create_page_tables)
 	 */
 	.macro setup_last_frame
 	sub	sp, sp, #PT_REGS_SIZE
+	ldr	w17, =TASK_FRAME
+	str	w17, [sp, #S_FRAME_TYPE]
 	stp	xzr, xzr, [sp, #S_STACKFRAME]
 	add	x29, sp, #S_STACKFRAME
 	ldr	x30, =ret_from_fork
diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 7ffa689e8b60..5c152fd60503 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -442,6 +442,7 @@ int copy_thread(unsigned long clone_flags, unsigned long stack_start,
 	 * as the last frame for the new task.
 	 */
 	p->thread.cpu_context.fp = (unsigned long)childregs->stackframe;
+	childregs->frame_type = TASK_FRAME;
 
 	ptrace_hw_copy_thread(p);
 
-- 
2.25.1