Re: [PATCH v8 04/12] scs: disable when function graph tracing is enabled

Mark Rutland
2020-02-19T11:34:24Z

On Tue, Feb 18, 2020 at 04:08:09PM -0800, Sami Tolvanen wrote:
> 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.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> arch/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 66b34fd0df54..4102b8e0eea9 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -535,6 +535,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK
>
> config SHADOW_CALL_STACK
> bool "Clang Shadow Call Stack"
> + depends on !FUNCTION_GRAPH_TRACER

Fangrui Song has implemented `-fpatchable-function-entry` in LLVM (for
10.x onwards), so we can support this when DYNAMIC_FTRACE_WITH_REGS is
selected.

This can be:

depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER

... and we can update the commit message to something like:

| 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.

Assuming you're happy with that:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.