Patch "s390/ftrace: Use unwinder instead of __builtin_return_address()" has been added to the 6.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    s390/ftrace: Use unwinder instead of __builtin_return_address()

to the 6.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     s390-ftrace-use-unwinder-instead-of-__builtin_return.patch
and it can be found in the queue-6.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f95e54a8b0b12fb71bacb0554c1d005684c87ccd
Author: Sven Schnelle <svens@xxxxxxxxxxxxx>
Date:   Fri Apr 26 12:02:15 2024 +0200

    s390/ftrace: Use unwinder instead of __builtin_return_address()
    
    [ Upstream commit cae74ba8c295bc41bda749ef27a8f2b3ee957a41 ]
    
    Using __builtin_return_address(n) might return undefined values
    when used with values of n outside of the stack. This was noticed
    when __builtin_return_address() was called in ftrace on top level
    functions like the interrupt handlers.
    
    As this behaviour cannot be fixed, use the s390 stack unwinder and
    remove the ftrace compilation flags for unwind_bc.c and stacktrace.c
    to prevent the unwinding function polluting function traces.
    
    Another advantage is that this also works with clang.
    
    Reviewed-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
    Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
    Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
    Stable-dep-of: ebd912ff9919 ("s390/stacktrace: Merge perf_callchain_user() and arch_stack_walk_user()")
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/include/asm/ftrace.h b/arch/s390/include/asm/ftrace.h
index 621f23d5ae30a..77e479d44f1e3 100644
--- a/arch/s390/include/asm/ftrace.h
+++ b/arch/s390/include/asm/ftrace.h
@@ -8,12 +8,8 @@
 
 #ifndef __ASSEMBLY__
 
-#ifdef CONFIG_CC_IS_CLANG
-/* https://llvm.org/pr41424 */
-#define ftrace_return_address(n) 0UL
-#else
-#define ftrace_return_address(n) __builtin_return_address(n)
-#endif
+unsigned long return_address(unsigned int n);
+#define ftrace_return_address(n) return_address(n)
 
 void ftrace_caller(void);
 
diff --git a/arch/s390/kernel/Makefile b/arch/s390/kernel/Makefile
index fa029d0dc28ff..db2d9ba5a86d2 100644
--- a/arch/s390/kernel/Makefile
+++ b/arch/s390/kernel/Makefile
@@ -11,6 +11,8 @@ CFLAGS_REMOVE_ftrace.o		= $(CC_FLAGS_FTRACE)
 # Do not trace early setup code
 CFLAGS_REMOVE_early.o		= $(CC_FLAGS_FTRACE)
 CFLAGS_REMOVE_rethook.o		= $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_stacktrace.o	= $(CC_FLAGS_FTRACE)
+CFLAGS_REMOVE_unwind_bc.o	= $(CC_FLAGS_FTRACE)
 
 endif
 
diff --git a/arch/s390/kernel/stacktrace.c b/arch/s390/kernel/stacktrace.c
index 94f440e383031..7c294da45bf52 100644
--- a/arch/s390/kernel/stacktrace.c
+++ b/arch/s390/kernel/stacktrace.c
@@ -101,3 +101,22 @@ void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
 	}
 	pagefault_enable();
 }
+
+unsigned long return_address(unsigned int n)
+{
+	struct unwind_state state;
+	unsigned long addr;
+
+	/* Increment to skip current stack entry */
+	n++;
+
+	unwind_for_each_frame(&state, NULL, NULL, 0) {
+		addr = unwind_get_return_address(&state);
+		if (!addr)
+			break;
+		if (!n--)
+			return addr;
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(return_address);