[PATCH v10 04/12] scs: disable when function graph tracing is enabled

[Sami Tolvanen]

The graph tracer hooks returns by modifying frame records on the
(regular) stack, but with SCS the return address is taken from the
shadow stack, and the value in the frame record has no effect. As we
don't currently have a mechanism to determine the corresponding slot
on the shadow stack (and to pass this through the ftrace
infrastructure), for now let's disable SCS when the graph tracer is
enabled.

With SCS the return address is taken from the shadow stack and the
value in the frame record has no effect. The mcount based graph tracer
hooks returns by modifying frame records on the (regular) stack, and
thus is not compatible. The patchable-function-entry graph tracer
used for DYNAMIC_FTRACE_WITH_REGS modifies the LR before it is saved
to the shadow stack, and is compatible.

Modifying the mcount based graph tracer to work with SCS would require
a mechanism to determine the corresponding slot on the shadow stack
(and to pass this through the ftrace infrastructure), and we expect
that everyone will eventually move to the patchable-function-entry
based graph tracer anyway, so for now let's disable SCS when the
mcount-based graph tracer is enabled.

SCS and patchable-function-entry are both supported from LLVM 10.x.

Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reviewed-by: Mark Rutland <mark.rutland@xxxxxxx>
---
 arch/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index 691a552c2cc3..c53cb9025ad2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -542,6 +542,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
 
 config SHADOW_CALL_STACK
 	bool "Clang Shadow Call Stack"
+	depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER
 	depends on ARCH_SUPPORTS_SHADOW_CALL_STACK
 	help
 	  This option enables Clang's Shadow Call Stack, which uses a
-- 
2.26.0.292.g33ef6b2f38-goog