sh: Handle the DWARF op, DW_CFA_undefined

Allow a DWARF register to have an undefined value. When applied to the
DWARF return address register this lets lets us label a function as
having no direct caller, e.g. kernel_thread_helper().

Signed-off-by: Matt Fleming <matt@console-pimps.org>

2 files changed, 14 insertions, 2 deletions

diff --git a/arch/sh/include/asm/dwarf.h b/arch/sh/include/asm/dwarf.h
index 8b0bcc087385..c367ed3373c5 100644
--- a/arch/sh/include/asm/dwarf.h
+++ b/arch/sh/include/asm/dwarf.h
@@ -297,6 +297,7 @@ struct dwarf_reg {
         unsigned long flags;
 #define DWARF_REG_OFFSET        (1 << 0)
 #define DWARF_VAL_OFFSET        (1 << 1)
+#define DWARF_UNDEFINED         (1 << 2)
 };
 
 /*
@@ -370,6 +371,7 @@ extern struct dwarf_frame *dwarf_unwind_stack(unsigned long,
 #define CFI_DEF_CFA     .cfi_def_cfa
 #define CFI_REGISTER    .cfi_register
 #define CFI_REL_OFFSET  .cfi_rel_offset
+#define CFI_UNDEFINED   .cfi_undefined
 
 #else
 
@@ -383,6 +385,7 @@ extern struct dwarf_frame *dwarf_unwind_stack(unsigned long,
 #define CFI_DEF_CFA     CFI_IGNORE
 #define CFI_REGISTER    CFI_IGNORE
 #define CFI_REL_OFFSET  CFI_IGNORE
+#define CFI_UNDEFINED   CFI_IGNORE
 
 #ifndef __ASSEMBLY__
 static inline void dwarf_unwinder_init(void)
diff --git a/arch/sh/kernel/dwarf.c b/arch/sh/kernel/dwarf.c
index e6f427cff5ba..577302f31e6a 100644
--- a/arch/sh/kernel/dwarf.c
+++ b/arch/sh/kernel/dwarf.c
@@ -452,6 +452,8 @@ static int dwarf_cfa_execute_insns(unsigned char *insn_start,
                 case DW_CFA_undefined:
                         count = dwarf_read_uleb128(current_insn, &reg);
                         current_insn += count;
+                        regp = dwarf_frame_alloc_reg(frame, reg);
+                        regp->flags |= DWARF_UNDEFINED;
                         break;
                 case DW_CFA_def_cfa:
                         count = dwarf_read_uleb128(current_insn,
@@ -629,9 +631,16 @@ struct dwarf_frame * dwarf_unwind_stack(unsigned long pc,
                 UNWINDER_BUG();
         }
 
-        /* If we haven't seen the return address reg, we're screwed. */
         reg = dwarf_frame_reg(frame, DWARF_ARCH_RA_REG);
-        UNWINDER_BUG_ON(!reg);
+
+        /*
+         * If we haven't seen the return address register or the return
+         * address column is undefined then we must assume that this is
+         * the end of the callstack.
+         */
+        if (!reg || reg->flags == DWARF_UNDEFINED)
+                goto bail;
+
         UNWINDER_BUG_ON(reg->flags != DWARF_REG_OFFSET);
 
         addr = frame->cfa + reg->addr;