Re: [RFC PATCH v1 24/28] riscv: select config for shadow stack and landing pad instr support

[Conor Dooley]

On Thu, Jan 25, 2024 at 10:12:16AM -0800, Deepak Gupta wrote:
> On Thu, Jan 25, 2024 at 06:04:26PM +0000, Conor Dooley wrote:
> > On Wed, Jan 24, 2024 at 10:21:49PM -0800, debug@xxxxxxxxxxxx wrote:
> > > From: Deepak Gupta <debug@xxxxxxxxxxxx>
> > > 
> > > This patch selects config shadow stack support and landing pad instr
> > > support. Shadow stack support and landing instr support is hidden behind
> > > `CONFIG_RISCV_USER_CFI`. Selecting `CONFIG_RISCV_USER_CFI` wires up path
> > > to enumerate CPU support and if cpu support exists, kernel will support
> > > cpu assisted user mode cfi.
> > > 
> > > Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> > > ---
> > >  arch/riscv/Kconfig | 15 +++++++++++++++
> > >  1 file changed, 15 insertions(+)
> > > 
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > index 9d386e9edc45..437b2f9abf3e 100644
> > > --- a/arch/riscv/Kconfig
> > > +++ b/arch/riscv/Kconfig
> > > @@ -163,6 +163,7 @@ config RISCV
> > >  	select SYSCTL_EXCEPTION_TRACE
> > >  	select THREAD_INFO_IN_TASK
> > >  	select TRACE_IRQFLAGS_SUPPORT
> > > +	select RISCV_USER_CFI
> > 
> > This select makes no sense to me, it will unconditionally enable
> > RISCV_USER_CFI. I don't think that that is your intent, since you have a
> > detailed option below that allows the user to turn it on or off.
> > 
> > If you remove it, the commit message will need to change too FYI.
> > 
> 
> Selecting this config puts support in Kernel so that it can run tasks who wants
> to enable hardware assisted control flow integrity for themselves. But apps still
> always need to optin using `prctls`. Those prctls are stubs and return EINVAL when
> this config is not selected. Not selecting this config means, kernel will not support
> enabling this feature for user mode.

I don't think you understand me. "select RISCV_USER_CFI" will
unconditionally build it into the kernel, making stubs etc useless.
You're talking like (and the rest of your commit implements it!) that
this feature can be enabled in menuconfig etc. Having this select
will always enable the config option, rendering the choice below
redundant. Try turning it off in menuconfig.

Oh and if it were valid to have here, you put it in out of order. That's
an alphanumerically sorted list :)

Cheers,
Conor.

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