Re: [RFC PATCH v3 3/7] powerpc: Activate CONFIG_THREAD_INFO_IN_TASK

[Nicholas Piggin <npiggin@gmail.com>]

On Wed, 3 Oct 2018 08:04:49 +0200
Christophe LEROY <christophe.leroy@c-s.fr> wrote:

> Le 03/10/2018 à 07:52, Nicholas Piggin a écrit :
> > On Wed, 3 Oct 2018 07:47:05 +0200
> > Christophe LEROY <christophe.leroy@c-s.fr> wrote:
> >   
> >> Le 03/10/2018 à 07:30, Nicholas Piggin a écrit :  
> >>> On Mon,  1 Oct 2018 12:30:23 +0000 (UTC)
> >>> Christophe Leroy <christophe.leroy@c-s.fr> wrote:
> >>>      
> >>>> This patch activates CONFIG_THREAD_INFO_IN_TASK which
> >>>> moves the thread_info into task_struct.
> >>>>
> >>>> Moving thread_info into task_struct has the following advantages:
> >>>> - It protects thread_info from corruption in the case of stack
> >>>> overflows.
> >>>> - Its address is harder to determine if stack addresses are
> >>>> leaked, making a number of attacks more difficult.
> >>>>
> >>>> This has the following consequences:
> >>>> - thread_info is now located at the top of task_struct.  
> >>>
> >>> "top"... I got confused for a minute thinking high address and
> >>> wondering how you can change CURRENT_THREAD_INFO just to point
> >>> to current :)  
> >>
> >> Would 'beginning' be less confusing ?  
> > 
> > Yes, good idea.
> >   
> >>>> @@ -83,7 +83,13 @@ int is_cpu_dead(unsigned int cpu);
> >>>>    /* 32-bit */
> >>>>    extern int smp_hw_index[];
> >>>>    
> >>>> -#define raw_smp_processor_id()	(current_thread_info()->cpu)
> >>>> +/*
> >>>> + * This is particularly ugly: it appears we can't actually get the definition
> >>>> + * of task_struct here, but we need access to the CPU this task is running on.
> >>>> + * Instead of using task_struct we're using _TASK_CPU which is extracted from
> >>>> + * asm-offsets.h by kbuild to get the current processor ID.
> >>>> + */
> >>>> +#define raw_smp_processor_id()		(*(unsigned int*)((void*)current + _TASK_CPU))  
> >>>
> >>> This is clever but yes ugly. Can't you include asm-offsets.h? riscv
> >>> seems to.  
> >>
> >> riscv has a clean asm-offsets.h . Our's defines constant with the same
> >> name as those defined in other headers which are included in C files. So
> >> including asm-offsets in C files does create conflicts like:
> >>
> >> ./include/generated/asm-offsets.h:71:0: warning: "TASK_SIZE" redefined
> >>    #define TASK_SIZE -2147483648 /* TASK_SIZE */
> >> ./arch/powerpc/include/asm/processor.h:95:0: note: this is the location
> >> of the previous definition
> >>    #define TASK_SIZE (CONFIG_TASK_SIZE)
> >>
> >> ./include/generated/asm-offsets.h:98:0: warning: "NSEC_PER_SEC" redefined
> >>    #define NSEC_PER_SEC 1000000000 /* NSEC_PER_SEC */
> >> ./include/linux/time64.h:36:0: note: this is the location of the
> >> previous definition
> >>    #define NSEC_PER_SEC 1000000000L
> >>
> >> ./arch/powerpc/include/asm/nohash/32/pgtable.h:34:0: warning:
> >> "PGD_TABLE_SIZE" redefined
> >>    #define PGD_TABLE_SIZE (sizeof(pgd_t) << PGD_INDEX_SIZE)
> >> ./include/generated/asm-offsets.h:101:0: note: this is the location of
> >> the previous definition
> >>    #define PGD_TABLE_SIZE 256 /* PGD_TABLE_SIZE */
> >>
> >> ...  
> > 
> > Okay.
> >   
> >>
> >> In v2, I had a patch to fix those redundancies
> >> (https://patchwork.ozlabs.org/patch/974363/) but I found it unconvenient.  
> > 
> > Because of merge conflicts, or you did not like the new names?  
> 
> Both, because of the amount of changes it implies, and also because of 
> the new names. I find it quite convenient to be able to use same names 
> both in C and ASM.

Yeah that's true. I guess this is okay for a one-off hack.

Thanks,
Nick