Subject: Re: Contextual conflict between kspp and rcu trees

From: Kees Cook <[email-masked]>
Date: Wed, 15 Jun 2022 13:25:30 -0700

On Wed, Jun 15, 2022 at 12:55:53PM -0700, Nathan Chancellor wrote:
> Hi Stephen et al.,
> 
> There is a contextual conflict between commit e1d337335207 ("cfi: Fix
> __cfi_slowpath_diag RCU usage with cpuidle") in the kspp tree and commit
> dcc0c11aa87b ("rcu/context-tracking: Remove rcu_irq_enter/exit()") in
> the rcu tree, which is visible when building ARCH=arm64 defconfig +
> CONFIG_LTO_CLANG_THIN=y + CONFIG_CFI_CLANG=y with clang:
> 
>   kernel/cfi.c:298:3: error: call to undeclared function 'rcu_irq_enter'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>                   rcu_irq_enter();
>                   ^
>   kernel/cfi.c:298:3: note: did you mean 'ct_irq_enter'?
>   ./include/linux/context_tracking_irq.h:6:6: note: 'ct_irq_enter' declared here
>   void ct_irq_enter(void);
>        ^
>   kernel/cfi.c:307:3: error: call to undeclared function 'rcu_irq_exit'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
>                   rcu_irq_exit();
>                   ^
>   kernel/cfi.c:307:3: note: did you mean 'ct_irq_exit'?
>   ./include/linux/context_tracking_irq.h:7:6: note: 'ct_irq_exit' declared here
>   void ct_irq_exit(void);
>        ^
>   2 errors generated.
> 
> 
> Per the above RCU commit and commit 6c5218715286 ("context_tracking:
> Take IRQ eqs entrypoints over RCU"), it appears that the following diff
> is the proper fix up. Would you mind applying it to the merge of
> whichever tree comes second if possible? I did build and boot test it
> but it would not be a bad idea for Sami and Frederic to verify that it
> is correct so that Kees/Paul can mention it to Linus :)

Ah! Thanks for tracking this down! Maybe dcc0c11aa87b should be updated
to leave an alias until the next merge window?

-- 
Kees Cook