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

On 06/11/2019 10:16 AM, Christophe Leroy wrote:

Le 10/06/2019 à 04:39, Anshuman Khandual a écrit :

quoted

On 06/07/2019 09:01 PM, Christophe Leroy wrote:

quoted

Le 07/06/2019 à 12:34, Anshuman Khandual a écrit :

quoted

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@lists.infradead.org
Cc: linux-ia64@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-s390@vger.kernel.org
Cc: linux-sh@vger.kernel.org
Cc: sparclinux@vger.kernel.org
Cc: x86@kernel.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Christophe Leroy <redacted>
Cc: Stephen Rothwell <redacted>
Cc: Andrey Konovalov <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Paul Mackerras <redacted>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <redacted>
Cc: Martin Schwidefsky <redacted>
Cc: Heiko Carstens <redacted>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Gleixner <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>

Signed-off-by: Anshuman Khandual <redacted>
---
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/)

   arch/arm/mm/fault.c      | 24 +-----------------------
   arch/arm64/mm/fault.c    | 24 +-----------------------
   arch/ia64/mm/fault.c     | 24 +-----------------------
   arch/powerpc/mm/fault.c  | 23 ++---------------------
   arch/s390/mm/fault.c     | 16 +---------------
   arch/sh/mm/fault.c       | 18 ++----------------
   arch/sparc/mm/fault_64.c | 16 +---------------
   arch/x86/mm/fault.c      | 21 ++-------------------
   include/linux/kprobes.h  | 16 ++++++++++++++++
   9 files changed, 27 insertions(+), 155 deletions(-)

[...]

quoted

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)

   }
   #endif
   +static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
+                          unsigned int trap)
+{
+    int ret = 0;

ret is pointless.

quoted

+
+    /*
+     * To be potentially processing a kprobe fault and to be allowed
+     * to call kprobe_running(), we have to be non-preemptible.
+     */
+    if (kprobes_built_in() && !preemptible() && !user_mode(regs)) {
+        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 ?

Probably not.

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;

As mentioned before will stick with current x86 implementation.