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

Hi Nathan, Would you please submit this to Russell's patch tracking system: https://www.arm.linux.org.uk/developer/patches/ login -> add new patch.

On Mon, Jul 6, 2020 at 11:27 AM Nathan Huckleberry nhuck@google.com wrote:

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 miles.chen@mediatek.com Tested-by: Miles Chen miles.chen@mediatek.com Cc: stable@vger.kernel.org Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Nathan Huckleberry nhuck@google.com

---

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