Re: [PATCH v2] ARM: stacktrace: Fix unwind_frame for clang-built kernels

Since clang does not push pc and sp in function prologues, the current
implementation of unwind_frame does not work. By using the previous
frame's lr/fp instead of saved pc/sp we get valid unwinds on clang-built
kernels.

The bounds check on next frame pointer must be changed as well since
there are 8 less bytes between frames.

This fixes /proc/<pid>/stack.

Link: https://github.com/ClangBuiltLinux/linux/issues/912
Reported-by: Miles Chen <redacted>
Tested-by: Miles Chen <redacted>
Cc: stable@vger.kernel.org
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Huckleberry <redacted>
---
 arch/arm/kernel/stacktrace.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
index cc726afea023..76ea4178a55c 100644
--- a/arch/arm/kernel/stacktrace.c
+++ b/arch/arm/kernel/stacktrace.c

@@ -22,6 +22,19 @@
  * A simple function epilogue looks like this:
  *     ldm     sp, {fp, sp, pc}
  *
+ * When compiled with clang, pc and sp are not pushed. A simple function
+ * prologue looks like this when built with clang:
+ *
+ *     stmdb   {..., fp, lr}
+ *     add     fp, sp, #x
+ *     sub     sp, sp, #y
+ *
+ * A simple function epilogue looks like this when built with clang:
+ *
+ *     sub     sp, fp, #x
+ *     ldm     {..., fp, pc}
+ *
+ *
  * Note that with framepointer enabled, even the leaf functions have the same
  * prologue and epilogue, therefore we can ignore the LR value in this case.
  */

@@ -34,6 +47,16 @@ int notrace unwind_frame(struct stackframe *frame)
        low = frame->sp;
        high = ALIGN(low, THREAD_SIZE);

+#ifdef CONFIG_CC_IS_CLANG
+       /* check current frame pointer is within bounds */
+       if (fp < low + 4 || fp > high - 4)
+               return -EINVAL;
+
+       frame->sp = frame->fp;
+       frame->fp = *(unsigned long *)(fp);
+       frame->pc = frame->lr;
+       frame->lr = *(unsigned long *)(fp + 4);
+#else
        /* check current frame pointer is within bounds */
        if (fp < low + 12 || fp > high - 4)
                return -EINVAL;

@@ -42,6 +65,7 @@ int notrace unwind_frame(struct stackframe *frame)
        frame->fp = *(unsigned long *)(fp - 12);
        frame->sp = *(unsigned long *)(fp - 8);
        frame->pc = *(unsigned long *)(fp - 4);
+#endif

        return 0;
 }
--

2.27.0.212.ge8ba1cc988-goog