Re: [RFC V3] mm: Generalize and rename notify_page_fault() as kprobe_page_fault()

[Christophe Leroy]

Le 10/06/2019 Ã 04:39, Anshuman Khandual a ÃcritÂ:
> On 06/07/2019 09:01 PM, Christophe Leroy wrote:
> > Le 07/06/2019 Ã 12:34, Anshuman Khandual a ÃcritÂ:
> > > Very similar definitions for notify_page_fault() are being used by multiple
> > > architectures duplicating much of the same code. This attempts to unify all
> > > of them into a generic implementation, rename it as kprobe_page_fault() and
> > > then move it to a common header.
> > >
> > > kprobes_built_in() can detect CONFIG_KPROBES, hence new kprobe_page_fault()
> > > need not be wrapped again within CONFIG_KPROBES. Trap number argument can
> > > now contain upto an 'unsigned int' accommodating all possible platforms.
> > >
> > > kprobe_page_fault() goes the x86 way while dealing with preemption context.
> > > As explained in these following commits the invoking context in itself must
> > > be non-preemptible for kprobes processing context irrespective of whether
> > > kprobe_running() or perhaps smp_processor_id() is safe or not. It does not
> > > make much sense to continue when original context is preemptible. Instead
> > > just bail out earlier.
> > >
> > > commit a980c0ef9f6d
> > > ("x86/kprobes: Refactor kprobes_fault() like kprobe_exceptions_notify()")
> > >
> > > commit b506a9d08bae ("x86: code clarification patch to Kprobes arch code")
> > >
> > > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> > > Cc: linux-ia64@xxxxxxxxxxxxxxx
> > > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> > > Cc: linux-s390@xxxxxxxxxxxxxxx
> > > Cc: linux-sh@xxxxxxxxxxxxxxx
> > > Cc: sparclinux@xxxxxxxxxxxxxxx
> > > Cc: x86@xxxxxxxxxx
> > > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > > Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > > Cc: Mark Rutland <mark.rutland@xxxxxxx>
> > > Cc: Christophe Leroy <christophe.leroy@xxxxxx>
> > > Cc: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
> > > Cc: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> > > Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > > Cc: Paul Mackerras <paulus@xxxxxxxxx>
> > > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Cc: Tony Luck <tony.luck@xxxxxxxxx>
> > > Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> > > Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> > > Cc: Heiko Carstens <heiko.carstens@xxxxxxxxxx>
> > > Cc: Yoshinori Sato <ysato@xxxxxxxxxxxxxxxxxxxx>
> > > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > >
> > > Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> > > ---
> > > Testing:
> > >
> > > - Build and boot tested on arm64 and x86
> > > - Build tested on some other archs (arm, sparc64, alpha, powerpc etc)
> > >
> > > Changes in RFC V3:
> > >
> > > - Updated the commit message with an explaination for new preemption behaviour
> > > - Moved notify_page_fault() to kprobes.h with 'static nokprobe_inline' per Matthew
> > > - Changed notify_page_fault() return type from int to bool per Michael Ellerman
> > > - Renamed notify_page_fault() as kprobe_page_fault() per Peterz
> > >
> > > Changes in RFC V2: (https://patchwork.kernel.org/patch/10974221/)
> > >
> > > - Changed generic notify_page_fault() per Mathew Wilcox
> > > - Changed x86 to use new generic notify_page_fault()
> > > - s/must not/need not/ in commit message per Matthew Wilcox
> > >
> > > Changes in RFC V1: (https://patchwork.kernel.org/patch/10968273/)
> > >
> > >  Â arch/arm/mm/fault.cÂÂÂÂÂ | 24 +-----------------------
> > >  Â arch/arm64/mm/fault.cÂÂÂ | 24 +-----------------------
> > >  Â arch/ia64/mm/fault.cÂÂÂÂ | 24 +-----------------------
> > >   arch/powerpc/mm/fault.c | 23 ++---------------------
> > >  Â arch/s390/mm/fault.cÂÂÂÂ | 16 +---------------
> > >  Â arch/sh/mm/fault.cÂÂÂÂÂÂ | 18 ++----------------
> > >  Â arch/sparc/mm/fault_64.c | 16 +---------------
> > >  Â arch/x86/mm/fault.cÂÂÂÂÂ | 21 ++-------------------
> > >   include/linux/kprobes.h | 16 ++++++++++++++++
> > >  Â 9 files changed, 27 insertions(+), 155 deletions(-)
> >
> > [...]
> >
> > > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > > index 443d980..064dd15 100644
> > > --- a/include/linux/kprobes.h
> > > +++ b/include/linux/kprobes.h
> > > @@ -458,4 +458,20 @@ static inline bool is_kprobe_optinsn_slot(unsigned long addr)
> > >  Â }
> > >  Â #endif
> > >  Â +static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
> > > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ unsigned int trap)
> > > +{
> > > +ÂÂÂ int ret = 0;
> >
> > ret is pointless.
> >
> > > +
> > > +ÂÂÂ /*
> > > +ÂÂÂÂ * To be potentially processing a kprobe fault and to be allowed
> > > +ÂÂÂÂ * to call kprobe_running(), we have to be non-preemptible.
> > > +ÂÂÂÂ */
> > > +ÂÂÂ if (kprobes_built_in() && !preemptible() && !user_mode(regs)) {
> > > +ÂÂÂÂÂÂÂ if (kprobe_running() && kprobe_fault_handler(regs, trap))
> >
> > don't need an 'if A if B', can do 'if A && B'
>
> Which will make it a very lengthy condition check.

Yes. But is that a problem at all ?

For me the following would be easier to read.

if (kprobes_built_in() && !preemptible() && !user_mode(regs) &&
    kprobe_running() && kprobe_fault_handler(regs, trap))
	ret = 1;

Christophe

> > > +ÂÂÂÂÂÂÂÂÂÂÂ ret = 1;
> >
> > can do 'return true;' directly here
> >
> > > +ÂÂÂ }
> > > +ÂÂÂ return ret;
> >
> > And 'return false' here.
>
> Makes sense, will drop ret.