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

[Christophe LEROY]

Le 03/10/2018 Ã 07:52, Nicholas Piggin a ÃcritÂ:
> On Wed, 3 Oct 2018 07:47:05 +0200
> Christophe LEROY <christophe.leroy@xxxxxx> wrote:
>
> > Le 03/10/2018 Ã 07:30, Nicholas Piggin a ÃcritÂ:
> > > On Mon,  1 Oct 2018 12:30:23 +0000 (UTC)
> > > Christophe Leroy <christophe.leroy@xxxxxx> 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. And I didn't want my serie to imply big-bangs in 
unrelated or not directly related topics.

Christophe

> Thanks,
> Nick