Re: [RFC PATCH v1 00/17] ban the use of _PAGE_XXX flags outside platform specific code

[Christophe Leroy]

On 09/05/2018 02:03 PM, Aneesh Kumar K.V wrote:
> On 09/05/2018 06:06 PM, Christophe Leroy wrote:
> > Today flags like for instance _PAGE_RW or _PAGE_USER are used through
> > common parts of code.
> > Using those directly in common parts of code have proven to lead to
> > mistakes or misbehaviour, because their use is not always as trivial
> > as one could think.
> >
> > For instance, (flags & _PAGE_USER) == 0 isn't enough to tell
> > that a page is a kernel page, because some targets are using
> > _PAGE_PRIVILEDGED and not _PAGE_USER, so the test has to be
> > (flags & (_PAGE_USER | _PAGE_PRIVILEDGED)) == _PAGE_PRIVILEDGED
> > This has to (bad) consequences:
> >
> > Â - All targets must define every bit, even the unsupported ones,
> > ÂÂÂ leading to a lot of useless #define _PAGE_XXX 0
> > Â - If someone forgets to take into account all possible _PAGE_XXX bits
> > ÂÂÂ for the case, we can get unexpected behaviour on some targets.
> >
> > This becomes even more complex when we come to using _PAGE_RW.
> > Testing (flags & _PAGE_RW) is not enough to test whether a page
> > if writable or not, because:
> >
> > Â - Some targets have _PAGE_RO instead, which has to be unset to tell
> > ÂÂÂ a page is writable
> > Â - Some targets have _PAGE_R and _PAGE_W, in which case
> > ÂÂÂ _PAGE_RW = _PAGE_R | _PAGE_W
> > Â - Even knowing whether a page is readable is not always trivial 
> > because:
> > ÂÂÂ - Some targets requires to check that _PAGE_R is set to ensure page
> > ÂÂÂ is readable
> > ÂÂÂ - Some targets requires to check that _PAGE_NA is not set
> > ÂÂÂ - Some targets requires to check that _PAGE_RO or _PAGE_RW is set
> >
> > Etc ....
> >
> > In order to work around all those issues and minimise the risks of 
> > errors,
> > this serie aims at removing all use of _PAGE_XXX flags from powerpc code
> > and always use pte_xxx() and pte_mkxxx() accessors instead. Those 
> > accessors
> > are then defined in target specific parts of the kernel code.
>
> We recently did on book3s 64.
>
> static inline int pte_present(pte_t pte)
> {
>  ÂÂÂÂ/*
>  ÂÂÂÂ * A pte is considerent present if _PAGE_PRESENT is set.
>  ÂÂÂÂ * We also need to consider the pte present which is marked
>  ÂÂÂÂ * invalid during ptep_set_access_flags. Hence we look for 
> _PAGE_INVALID
>  ÂÂÂÂ * if we find _PAGE_PRESENT cleared.
>  ÂÂÂÂ */
>  ÂÂÂÂreturn !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID));
> }
>
> So I guess with that pte_present conversion we need to be careful.
>
> Do you have a git tree which I can use to double check?

I pushed on branch 'helpers' on https://github.com/chleroy/linux.git

Christophe