[PATCH v4 05/12] unwind_user: Enable archs that define CFA = SP_callsite + offset

[Jens Remus <jremus@linux.ibm.com>]

Most architectures define their CFA as the value of the stack pointer
(SP) at the call site in the previous frame, as suggested by the DWARF
standard.  Therefore the SP at call site can be unwound using an
implicitly assumed value offset from CFA rule with an offset of zero:

  .cfi_val_offset <SP>, 0

As a result the SP at call site computes as follows:

  SP = CFA

Enable unwinding of user space for architectures, such as s390, which
define their CFA as the value of the SP at the call site in the previous
frame with an offset.  Do so by enabling architectures to override the
default SP value offset from CFA of zero with an architecture-specific
one:

  .cfi_val_offset <SP>, offset

So that the SP at call site computes as follows:

  SP = CFA + offset

Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---
 arch/x86/include/asm/unwind_user.h        |  2 ++
 arch/x86/include/asm/unwind_user_sframe.h |  2 ++
 include/asm-generic/Kbuild                |  1 +
 include/asm-generic/unwind_user_sframe.h  | 12 ++++++++++++
 include/linux/unwind_user_types.h         |  1 +
 kernel/unwind/sframe.c                    |  1 +
 kernel/unwind/user.c                      | 11 ++++++-----
 7 files changed, 25 insertions(+), 5 deletions(-)
 create mode 100644 include/asm-generic/unwind_user_sframe.h

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index ae46906c3b39..0650bcfae461 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -66,6 +66,7 @@ static inline int unwind_user_get_reg(unsigned long *val, unsigned int regnum)
 		.rule		= UNWIND_USER_RULE_CFA_OFFSET_DEREF,\
 		.offset		= -2*(ws),		\
 			},				\
+	.sp_off		= 0,				\
 	.outermost	= false,
 
 #define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)		\
@@ -80,6 +81,7 @@ static inline int unwind_user_get_reg(unsigned long *val, unsigned int regnum)
 	.fp		= {				\
 		.rule		= UNWIND_USER_RULE_RETAIN,\
 			},				\
+	.sp_off		= 0,				\
 	.outermost	= false,
 
 static inline bool unwind_user_at_function_start(struct pt_regs *regs)
diff --git a/arch/x86/include/asm/unwind_user_sframe.h b/arch/x86/include/asm/unwind_user_sframe.h
index d828ae1a4aac..40b03b482d1a 100644
--- a/arch/x86/include/asm/unwind_user_sframe.h
+++ b/arch/x86/include/asm/unwind_user_sframe.h
@@ -9,4 +9,6 @@
 
 #endif
 
+#include <asm-generic/unwind_user_sframe.h>
+
 #endif /* _ASM_X86_UNWIND_USER_SFRAME_H */
diff --git a/include/asm-generic/Kbuild b/include/asm-generic/Kbuild
index 9aff61e7b8f2..91f86dd84677 100644
--- a/include/asm-generic/Kbuild
+++ b/include/asm-generic/Kbuild
@@ -61,6 +61,7 @@ mandatory-y += topology.h
 mandatory-y += trace_clock.h
 mandatory-y += uaccess.h
 mandatory-y += unwind_user.h
+mandatory-y += unwind_user_sframe.h
 mandatory-y += vermagic.h
 mandatory-y += vga.h
 mandatory-y += video.h
diff --git a/include/asm-generic/unwind_user_sframe.h b/include/asm-generic/unwind_user_sframe.h
new file mode 100644
index 000000000000..8c9ac47bc8bd
--- /dev/null
+++ b/include/asm-generic/unwind_user_sframe.h
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_UNWIND_USER_SFRAME_H
+#define _ASM_GENERIC_UNWIND_USER_SFRAME_H
+
+#include <linux/types.h>
+
+#ifndef SFRAME_SP_OFFSET
+/* Most archs/ABIs define CFA as SP at call site, so that SP = CFA + 0. */
+#define SFRAME_SP_OFFSET 0
+#endif
+
+#endif /* _ASM_GENERIC_UNWIND_USER_SFRAME_H */
diff --git a/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 059e5c76f2f3..89c71a4553b2 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -65,6 +65,7 @@ struct unwind_user_frame {
 	struct unwind_user_cfa_rule_data cfa;
 	struct unwind_user_rule_data ra;
 	struct unwind_user_rule_data fp;
+	s32 sp_off;
 	bool outermost;
 };
 
diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index f24997e84e05..6a6221ce6d12 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -496,6 +496,7 @@ static __always_inline int __find_fre(struct sframe_section *sec,
 		return -EINVAL;
 	sframe_init_rule_data(&frame->ra, fre->ra_ctl, fre->ra_off);
 	sframe_init_rule_data(&frame->fp, fre->fp_ctl, fre->fp_off);
+	frame->sp_off  = SFRAME_SP_OFFSET;
 	frame->outermost = SFRAME_V3_FRE_RA_UNDEFINED_P(fre->info);
 
 	return 0;
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index eb7d9489f671..e21d088f7543 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -30,7 +30,7 @@ get_user_word(unsigned long *word, unsigned long base, int off, unsigned int ws)
 static int unwind_user_next_common(struct unwind_user_state *state,
 				   const struct unwind_user_frame *frame)
 {
-	unsigned long cfa, fp, ra;
+	unsigned long cfa, sp, fp, ra;
 
 	/* Stop unwinding when reaching an outermost frame. */
 	if (frame->outermost) {
@@ -62,16 +62,17 @@ static int unwind_user_next_common(struct unwind_user_state *state,
 	    get_user_word(&cfa, cfa, 0, state->ws))
 		return -EINVAL;
 
+	/* Get the Stack Pointer (SP) */
+	sp = cfa + frame->sp_off;
 	/*
 	 * Make sure that stack is not going in wrong direction.  Allow SP
 	 * to be unchanged for the topmost frame, by subtracting topmost,
 	 * which is either 0 or 1.
 	 */
-	if (cfa <= state->sp - state->topmost)
+	if (sp <= state->sp - state->topmost)
 		return -EINVAL;
-
 	/* Make sure that the address is word aligned */
-	if (cfa & (state->ws - 1))
+	if (sp & (state->ws - 1))
 		return -EINVAL;
 
 	/* Get the Return Address (RA) */
@@ -122,7 +123,7 @@ static int unwind_user_next_common(struct unwind_user_state *state,
 		return -EINVAL;
 
 	state->ip = ra;
-	state->sp = cfa;
+	state->sp = sp;
 	state->fp = fp;
 	state->topmost = false;
 	return 0;
-- 
2.51.0