Re: [RFCv1 1/4] arm64: Use static key for tracing PID in CONTEXTIDR

[Kees Cook <keescook@chromium.org>]

On Thu, Oct 21, 2021 at 03:33:01PM +0100, James Clark wrote:
> 
> 
> On 21/10/2021 14:45, Leo Yan wrote:
> > The kernel provides CONFIG_PID_IN_CONTEXTIDR for tracing PID into system
> > register CONTEXTIDR; we need to statically enable this configuration
> > when build kernel image to use this feature.
> > 
> > On the other hand, hardware tracing modules (e.g. Arm CoreSight, SPE,
> > etc) rely on this feature to provide context info in their tracing data.
> > If kernel has not enabled configuration CONFIG_PID_IN_CONTEXTIDR, then
> > tracing modules have no chance to capture PID related info.
> > 
> > This patch introduces static key for tracing PID in CONTEXTIDR, it
> > provides a possibility for device driver to dynamically enable and
> > disable tracing PID into CONTEXTIDR as needed.
> > 
> > As the first step, the kernel increases the static key if
> > CONFIG_PID_IN_CONTEXTIDR is enabled when booting kernel, in this case
> > kernel will always trace PID into CONTEXTIDR at the runtime.  This means
> > before and after applying this patch, the semantics for
> > CONFIG_PID_IN_CONTEXTIDR are consistent.
> > 
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> > ---
> >  arch/arm64/include/asm/mmu_context.h |  4 +++-
> >  arch/arm64/kernel/process.c          | 11 +++++++++++
> >  2 files changed, 14 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> > index f4ba93d4ffeb..e1f33616f83a 100644
> > --- a/arch/arm64/include/asm/mmu_context.h
> > +++ b/arch/arm64/include/asm/mmu_context.h
> > @@ -26,9 +26,11 @@
> >  
> >  extern bool rodata_full;
> >  
> > +DECLARE_STATIC_KEY_FALSE(contextidr_in_use);
> > +
> >  static inline void contextidr_thread_switch(struct task_struct *next)
> >  {
> > -	if (!IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR))
> > +	if (!static_branch_unlikely(&contextidr_in_use))
> >  		return;
> >  
> >  	write_sysreg(task_pid_nr(next), contextidr_el1);
> > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > index 40adb8cdbf5a..d744c0c7e4c4 100644
> > --- a/arch/arm64/kernel/process.c
> > +++ b/arch/arm64/kernel/process.c
> > @@ -61,6 +61,9 @@ unsigned long __stack_chk_guard __ro_after_init;
> >  EXPORT_SYMBOL(__stack_chk_guard);
> >  #endif
> >  
> > +DEFINE_STATIC_KEY_FALSE(contextidr_in_use);
> > +EXPORT_SYMBOL_GPL(contextidr_in_use);
> > +
> >  /*
> >   * Function pointers to optional machine specific functions
> >   */
> > @@ -721,3 +724,11 @@ int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> >  	return prot;
> >  }
> >  #endif
> > +
> > +static int __init contextidr_init(void)
> > +{
> > +	if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR))
> > +		static_branch_inc(&contextidr_in_use);
> > +	return 0;
> > +}
> > +early_initcall(contextidr_init);
> 
> Hi Leo,
> 
> Can you skip this early_initcall() part if you do something like this:
> 
>     DECLARE_STATIC_KEY_MAYBE(CONFIG_PID_IN_CONTEXTIDR, contextidr_in_use)
> 
> It seems like there is a way to conditionally initialise it to true.

I was going to suggest the same thing. :) With this change, it looks
good:

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook