[RFC PATCH v1 06/25] 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>
---

Notes (jremus):
    Cherry-picked from "[PATCH v4 00/12] s390: SFrame user space unwinding"
    series and adjusted to .eh_frame:
    https://lore.kernel.org/all/20260127151926.2805123-6-jremus@linux.ibm.com/

 arch/x86/include/asm/unwind_user.h |  2 ++
 include/linux/unwind_user_types.h  |  1 +
 kernel/unwind/user.c               | 11 ++++++-----
 3 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/unwind_user.h b/arch/x86/include/asm/unwind_user.h
index f38f7c5ff1de..c96645c824d1 100644
--- a/arch/x86/include/asm/unwind_user.h
+++ b/arch/x86/include/asm/unwind_user.h
@@ -32,6 +32,7 @@ static inline int unwind_user_word_size(struct pt_regs *regs)
 		.rule		= UNWIND_USER_RULE_CFA_OFFSET_DEREF,\
 		.offset		= -2*(ws),		\
 			},				\
+	.sp_off		= 0,				\
 	.outermost	= false,
 
 #define ARCH_INIT_USER_FP_ENTRY_FRAME(ws)		\
@@ -46,6 +47,7 @@ static inline int unwind_user_word_size(struct pt_regs *regs)
 	.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/include/linux/unwind_user_types.h b/include/linux/unwind_user_types.h
index 670ac860ae76..4da058096259 100644
--- a/include/linux/unwind_user_types.h
+++ b/include/linux/unwind_user_types.h
@@ -67,6 +67,7 @@ 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/user.c b/kernel/unwind/user.c
index 9df5040df9b3..830c620fe453 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -43,7 +43,7 @@ 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) {
@@ -77,16 +77,17 @@ 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) */
@@ -149,7 +150,7 @@ 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;
-- 
2.53.0