[RFC PATCH v1 03/25] unwind_user: Enable archs that pass RA in a register

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

Not all architectures/ABIs pass the return address (RA) on the stack on
function entry, like x86-64 does due to its CALL instruction pushing
the RA onto the stack.  Architectures/ABIs, such as s390, also do not
require the RA to be saved on the stack in the function prologue.  In
particular, the RA may never be saved to the stack at all, such as in
leaf functions.  Unwinding must therefore not assume the presence of a
RA saved on stack for the topmost frame.

Treat a RA offset from CFA of zero as indication that the RA is not
saved (on the stack).  For the topmost frame treat it as indication that
the RA is in the link/RA register, such as on arm64 and s390, and obtain
it from there.  For non-topmost frames treat it as error, as the RA must
be saved.

Additionally allow the SP to be unchanged in the topmost frame, for
architectures where SP at function entry == SP at call site, such as
arm64 and s390.

Note that treating a RA offset from CFA of zero as indication that
the RA is not saved on the stack additionally allows for architectures,
such as s390, where the frame pointer (FP) may be saved without the RA
being saved as well.  Provided that such architectures represent this
in SFrame by encoding the "missing" RA offset using a padding RA offset
with a value of zero.

Reviewed-by: Indu Bhagat <ibhagatgnu@gmail.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Cherry-picked from "[PATCH v16 00/20] unwind_deferred: Implement sframe
    handling" series:
    https://lore.kernel.org/all/20260521142546.3908498-14-jremus@linux.ibm.com/

 include/linux/unwind_user.h | 10 ++++++++++
 kernel/unwind/user.c        | 20 ++++++++++++++++----
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/include/linux/unwind_user.h b/include/linux/unwind_user.h
index 64618618febd..941cef652435 100644
--- a/include/linux/unwind_user.h
+++ b/include/linux/unwind_user.h
@@ -23,6 +23,16 @@ static inline bool unwind_user_at_function_start(struct pt_regs *regs)
 #define unwind_user_at_function_start unwind_user_at_function_start
 #endif
 
+#ifndef unwind_user_get_ra_reg
+static inline int unwind_user_get_ra_reg(unsigned long *val)
+{
+	pr_debug("%s (%d): %s() not implemented\n",
+		 current->comm, current->pid, __func__);
+	return -EINVAL;
+}
+#define unwind_user_get_ra_reg unwind_user_get_ra_reg
+#endif
+
 int unwind_user(struct unwind_stacktrace *trace, unsigned int max_entries);
 
 #endif /* _LINUX_UNWIND_USER_H */
diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c
index 28091ece830c..f8a97d4167f0 100644
--- a/kernel/unwind/user.c
+++ b/kernel/unwind/user.c
@@ -2,6 +2,9 @@
 /*
 * Generic interfaces for unwinding user space
 */
+
+#define pr_fmt(fmt)	"unwind_user: " fmt
+
 #include <linux/kernel.h>
 #include <linux/sched.h>
 #include <linux/sched/task_stack.h>
@@ -47,8 +50,12 @@ static int unwind_user_next_common(struct unwind_user_state *state,
 	}
 	cfa += frame->cfa_off;
 
-	/* Make sure that stack is not going in wrong direction */
-	if (cfa <= state->sp)
+	/*
+	 * 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)
 		return -EINVAL;
 
 	/* Make sure that the address is word aligned */
@@ -56,8 +63,13 @@ static int unwind_user_next_common(struct unwind_user_state *state,
 		return -EINVAL;
 
 	/* Get the Return Address (RA) */
-	if (get_user_word(&ra, cfa, frame->ra_off, state->ws))
-		return -EINVAL;
+	if (frame->ra_off) {
+		if (get_user_word(&ra, cfa, frame->ra_off, state->ws))
+			return -EINVAL;
+	} else {
+		if (!state->topmost || unwind_user_get_ra_reg(&ra))
+			return -EINVAL;
+	}
 
 	/* Get the Frame Pointer (FP) */
 	if (frame->fp_off && get_user_word(&fp, cfa, frame->fp_off, state->ws))
-- 
2.53.0