[PATCH v2 8/9] ARM: call_with_stack: add unwind support

[Ard Biesheuvel]

Restructure the code and add the unwind annotations so that both the
frame pointer unwinder as well as the ELF unwind info based unwinder
will be able to follow the call stack through call_with_stack().

Note that the former cannot support GCC and Clang at the same time, as
they use a different idiom for the prologue/epilogue. So the code uses
the GCC idiom, adding full frame pointer based unwind support for GCC
while preserving the existing behavior of the Clang version, which
simply omits call_with_stack() from its call stack.

Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
---
 arch/arm/lib/call_with_stack.S | 44 +++++++++++++++++---
 1 file changed, 38 insertions(+), 6 deletions(-)

diff --git a/arch/arm/lib/call_with_stack.S b/arch/arm/lib/call_with_stack.S
index 28b0341ae786..133dffa2404a 100644
--- a/arch/arm/lib/call_with_stack.S
+++ b/arch/arm/lib/call_with_stack.S
@@ -8,25 +8,57 @@
 
 #include <linux/linkage.h>
 #include <asm/assembler.h>
+#include <asm/unwind.h>
 
 /*
  * void call_with_stack(void (*fn)(void *), void *arg, void *sp)
  *
  * Change the stack to that pointed at by sp, then invoke fn(arg) with
  * the new stack.
+ *
+ * The sequence below follows the APCS frame convention for frame pointer
+ * unwinding, and implements the unwinder annotations needed by the EABI
+ * unwinder.
+ */
+
+#if defined(CONFIG_THUMB2_KERNEL) || \
+    (defined(CONFIG_UNWINDER_FRAME_POINTER) && defined(CONFIG_CC_IS_CLANG))
+/*
+ * Thumb-2 builds must use R7 as the frame pointer due to the way our unwind
+ * info based unwinder is constructed.
+ *
+ * The code below uses the GCC idiom for managing the frame pointer in the
+ * function prologue and epilogue, which Clang does not support. So the best we
+ * can do here is not touch the frame pointer at all: this will simply omit
+ * this frame when unwinding the call stack. So use R7 in this case as well,
+ * and leave R11 unmodified.
  */
+	fpreg	.req	r7
+#else
+	fpreg	.req	fp
+#endif
+
 ENTRY(call_with_stack)
-	str	sp, [r2, #-4]!
-	str	lr, [r2, #-4]!
+UNWIND(	.fnstart		)
+UNWIND(	.movsp	ip		)
+	mov	ip, sp
+
+UNWIND(	.pad	#4		)
+UNWIND(	.save	{fpreg, ip, lr}	)
+THUMB(	sub	sp, #4		)
+	push	{fpreg, ip, lr ARM(, pc)}
+
+UNWIND(	.setfp	fpreg, ip, #-4	)
+	sub	fpreg, ip, #4
 
 	mov	sp, r2
 	mov	r2, r0
 	mov	r0, r1
 
-	badr	lr, 1f
-	ret	r2
+	bl_r	r2
 
-1:	ldr	lr, [sp]
-	ldr	sp, [sp, #4]
+	ldmdb	fpreg, {fpreg, ip, lr}
+	mov	sp, ip
 	ret	lr
+UNWIND(	.fnend			)
 ENDPROC(call_with_stack)
-- 
2.30.2