SFrame may represent an undefined return address (RA) as SFrame FRE
without any offsets as indication for an outermost frame.

Cc: Steven Rostedt <rostedt@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Indu Bhagat <indu.bhagat@oracle.com>
Cc: "Jose E. Marchesi" <jemarch@gnu.org>
Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: Jens Remus <jremus@linux.ibm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Sam James <sam@gentoo.org>
Cc: Kees Cook <kees@kernel.org>
Cc: "Carlos O'Donell" <codonell@redhat.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
---

Notes (jremus):
    Changes in v11:
    - New patch.

 kernel/unwind/sframe.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/unwind/sframe.c b/kernel/unwind/sframe.c
index d4ef825b1cbc..1e877c3e5417 100644
--- a/kernel/unwind/sframe.c
+++ b/kernel/unwind/sframe.c
@@ -33,6 +33,7 @@ struct sframe_fre_internal {
 	s32		ra_off;
 	s32		fp_off;
 	u8		info;
+	bool		ra_undefined;
 };
 
 DEFINE_STATIC_SRCU(sframe_srcu);
@@ -187,6 +188,7 @@ static __always_inline int __read_fre(struct sframe_section *sec,
 	unsigned char offset_count, offset_size;
 	s32 cfa_off, ra_off, fp_off;
 	unsigned long cur = fre_addr;
+	bool ra_undefined = false;
 	unsigned char addr_size;
 	u32 ip_off;
 	u8 info;
@@ -205,7 +207,7 @@ static __always_inline int __read_fre(struct sframe_section *sec,
 	UNSAFE_GET_USER_INC(info, cur, 1, Efault);
 	offset_count = SFRAME_FRE_OFFSET_COUNT(info);
 	offset_size  = offset_size_enum_to_size(SFRAME_FRE_OFFSET_SIZE(info));
-	if (!offset_count || !offset_size)
+	if (!offset_size)
 		return -EFAULT;
 
 	if (cur + (offset_count * offset_size) > sec->fres_end)
@@ -213,6 +215,14 @@ static __always_inline int __read_fre(struct sframe_section *sec,
 
 	fre->size = addr_size + 1 + (offset_count * offset_size);
 
+	if (!offset_count) {
+		cfa_off		= 0;
+		ra_off		= 0;
+		fp_off		= 0;
+		ra_undefined	= true;
+		goto done;
+	}
+
 	UNSAFE_GET_USER_INC(cfa_off, cur, offset_size, Efault);
 	offset_count--;
 
@@ -233,11 +243,13 @@ static __always_inline int __read_fre(struct sframe_section *sec,
 	if (offset_count)
 		return -EFAULT;
 
+done:
 	fre->ip_off		= ip_off;
 	fre->cfa_off		= cfa_off;
 	fre->ra_off		= ra_off;
 	fre->fp_off		= fp_off;
 	fre->info		= info;
+	fre->ra_undefined	= ra_undefined;
 
 	return 0;
 
@@ -298,6 +310,7 @@ static __always_inline int __find_fre(struct sframe_section *sec,
 	frame->ra_off  = fre->ra_off;
 	frame->fp_off  = fre->fp_off;
 	frame->use_fp  = SFRAME_FRE_CFA_BASE_REG_ID(fre->info) == SFRAME_BASE_REG_FP;
+	frame->outermost = fre->ra_undefined;
 
 	return 0;
 }
-- 
2.48.1