Messages in this thread Patch in this message | | | Date | Mon, 12 Dec 2022 10:46:12 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH] ftrace: Allow WITH_ARGS flavour of graph tracer with shadow call stack |
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On Sun, Dec 11, 2022 at 12:27:31PM +0900, Masami Hiramatsu wrote: > On Fri, 9 Dec 2022 15:34:02 +0100 > Ard Biesheuvel <ardb@kernel.org> wrote: > > > The recent switch on arm64 from DYNAMIC_FTRACE_WITH_REGS to > > DYNAMIC_FTRACE_WITH_ARGS failed to take into account that we currently > > require the former in order to allow the function graph tracer to be > > enabled in combination with shadow call stacks. This means that this is > > no longer permitted at all, in spite of the fact that either flavour of > > ftrace works perfectly fine in this combination. > > > > Given that arm64 is the only arch that implements shadow call stacks in > > the first place, let's update the condition to just reflect the arm64 > > change. When other architectures adopt shadow call stack support, this > > can be revisited if needed. > > This brings a question. Is the SCS safe if kretprobe(rethook) is enabled? > it also changes the stack entry after a calling function.
That should be safe.
The SCS push is just an instruction somewhere in the function, and since kretprobe instruments the first instruction of a function, that intrumentation will run *before* the SCS push occurs, and so it can safely override the return address.
The difficulty with ftrace is that the old mcount implementation calls into ftrace *after* the function prologue, after saving some GPRs to the stack, signing the return address with pointer authentication, and/or pushing the return address to the SCS.
The DYNAMIC_FTRACE_WITH_{ARGS,REGS} forms use patchable-function-entry to hook functions *before* any of that happens, and are safe for the same reason as kretprobes.
Thanks, Mark.
> > Thank you, > > > > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> > > --- > > arch/Kconfig | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/Kconfig b/arch/Kconfig > > index 072a1b39e3afd0d1..683f365b5e31c856 100644 > > --- a/arch/Kconfig > > +++ b/arch/Kconfig > > @@ -635,7 +635,7 @@ config ARCH_SUPPORTS_SHADOW_CALL_STACK > > config SHADOW_CALL_STACK > > bool "Shadow Call Stack" > > depends on ARCH_SUPPORTS_SHADOW_CALL_STACK > > - depends on DYNAMIC_FTRACE_WITH_REGS || !FUNCTION_GRAPH_TRACER > > + depends on DYNAMIC_FTRACE_WITH_ARGS || !FUNCTION_GRAPH_TRACER > > help > > This option enables the compiler's Shadow Call Stack, which > > uses a shadow stack to protect function return addresses from > > -- > > 2.35.1 > > > > > -- > Masami Hiramatsu (Google) <mhiramat@kernel.org>
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