[kvm-unit-tests PATCH v3 4/6] arm64: stack: update trace stack on exception

Nadav Amit
2023-06-28T00:14:44Z

Using gdb for backtracing or dumping the stack following an exception is
not very helpful as the exact location of the exception is not saved.

Add an additional frame to save the location of the exception.

One delicate point is dealing with the pretty_print_stacks script. When
the stack is dumped, the script would not print the right address for
the exception address: for every return address it deducts "1" before
looking for the instruction location in the code (using addr2line). As a
somewhat hacky solution add "1" for the exception address when dumping
the stack.

Signed-off-by: Nadav Amit <namit@vmware.com>

---
v1->v2:
* .globl before label [Andrew]
* Some comments [Andrew]
---
arm/cstart64.S | 13 +++++++++++++
lib/arm64/asm-offsets.c | 6 +++++-
lib/arm64/stack.c | 18 ++++++++++++++++++
3 files changed, 36 insertions(+), 1 deletion(-)

diff --git a/arm/cstart64.S b/arm/cstart64.S
index cbd6b51..865a96d 100644
--- a/arm/cstart64.S
+++ b/arm/cstart64.S
@@ -314,6 +314,13 @@ exceptions_init:
mrs x2, spsr_el1
stp x1, x2, [sp, #S_PC]

+ /*
+ * Save a frame pointer using the link to allow unwinding of
+ * exceptions.
+ */
+ stp x29, x1, [sp, #S_FP]
+ add x29, sp, #S_FP
+
mov x0, \vec
mov x1, sp
mrs x2, esr_el1
@@ -349,6 +356,9 @@ exceptions_init:
eret
.endm

+.globl vector_stub_start
+vector_stub_start:
+
vector_stub el1t_sync, 0
vector_stub el1t_irq, 1
vector_stub el1t_fiq, 2
@@ -369,6 +379,9 @@ vector_stub el0_irq_32, 13
vector_stub el0_fiq_32, 14
vector_stub el0_error_32, 15

+.globl vector_stub_end
+vector_stub_end:
+
.section .text.ex

.macro ventry, label
diff --git a/lib/arm64/asm-offsets.c b/lib/arm64/asm-offsets.c
index 53a1277..80de023 100644
--- a/lib/arm64/asm-offsets.c
+++ b/lib/arm64/asm-offsets.c
@@ -25,6 +25,10 @@ int main(void)
OFFSET(S_PSTATE, pt_regs, pstate);
OFFSET(S_ORIG_X0, pt_regs, orig_x0);
OFFSET(S_SYSCALLNO, pt_regs, syscallno);
- DEFINE(S_FRAME_SIZE, sizeof(struct pt_regs));
+
+ /* FP and LR (16 bytes) go on the frame above pt_regs */
+ DEFINE(S_FP, sizeof(struct pt_regs));
+ DEFINE(S_FRAME_SIZE, (sizeof(struct pt_regs) + 16));
+
return 0;
}
diff --git a/lib/arm64/stack.c b/lib/arm64/stack.c
index a2024e8..82611f4 100644
--- a/lib/arm64/stack.c
+++ b/lib/arm64/stack.c
@@ -6,12 +6,16 @@
#include <stdbool.h>
#include <stack.h>

+extern char vector_stub_start, vector_stub_end;
+
int backtrace_frame(const void *frame, const void **return_addrs, int max_depth)
{
const void *fp = frame;
static bool walking;
void *lr;
int depth;
+ bool is_exception = false;
+ unsigned long addr;

if (walking) {
printf("RECURSIVE STACK WALK!!!\n");
@@ -31,6 +35,20 @@ int backtrace_frame(const void *frame, const void **return_addrs, int max_depth)
: );

return_addrs[depth] = lr;
+
+ /*
+ * If this is an exception, add 1 to the pointer so when the
+ * pretty_print_stacks script is run it would get the right
+ * address (it deducts 1 to find the call address, but we want
+ * the actual address).
+ */
+ if (is_exception)
+ return_addrs[depth] += 1;
+
+ /* Check if we are in the exception handlers for the next entry */
+ addr = (unsigned long)lr;
+ is_exception = (addr >= (unsigned long)&vector_stub_start &&
+ addr < (unsigned long)&vector_stub_end);
}

walking = false;
--
2.34.1