[PATCH] mm/kprobes: Add generic kprobe_fault_handler() fallback definition

Architectures like parisc enable CONFIG_KROBES without having a definition
for kprobe_fault_handler() which results in a build failure. Arch needs to
provide kprobe_fault_handler() as it is platform specific and cannot have
a generic working alternative. But in the event when platform lacks such a
definition there needs to be a fallback.

This adds a stub kprobe_fault_handler() definition which not only prevents
a build failure but also makes sure that kprobe_page_fault() if called will
always return negative in absence of a sane platform specific alternative.

While here wrap kprobe_page_fault() in CONFIG_KPROBES. This enables stud
definitions for generic kporbe_fault_handler() and kprobes_built_in() can
just be dropped. Only on x86 it needs to be added back locally as it gets
used in a !CONFIG_KPROBES function do_general_protection().

Cc: Vineet Gupta <redacted>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <redacted>
Cc: Ralf Baechle <redacted>
Cc: Paul Burton <redacted>
Cc: James Hogan <jhogan@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Heiko Carstens <redacted>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <redacted>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Naveen N. Rao" <redacted>
Cc: Anil S Keshavamurthy <redacted>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Allison Randal <redacted>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Enrico Weigelt <info@metux.net>
Cc: Richard Fontana <redacted>
Cc: Kate Stewart <redacted>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: x86@kernel.org
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-ia64@vger.kernel.org
Cc: linux-mips@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

Signed-off-by: Anshuman Khandual <redacted>
---
 arch/arc/include/asm/kprobes.h     |  1 +
 arch/arm/include/asm/kprobes.h     |  1 +
 arch/arm64/include/asm/kprobes.h   |  1 +
 arch/ia64/include/asm/kprobes.h    |  1 +
 arch/mips/include/asm/kprobes.h    |  1 +
 arch/powerpc/include/asm/kprobes.h |  1 +
 arch/s390/include/asm/kprobes.h    |  1 +
 arch/sh/include/asm/kprobes.h      |  1 +
 arch/sparc/include/asm/kprobes.h   |  1 +
 arch/x86/include/asm/kprobes.h     |  6 ++++++
 include/linux/kprobes.h            | 32 ++++++++++++++++++------------
 11 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/arch/arc/include/asm/kprobes.h b/arch/arc/include/asm/kprobes.h
index 2134721dce44..ee8efe256675 100644
--- a/arch/arc/include/asm/kprobes.h
+++ b/arch/arc/include/asm/kprobes.h

@@ -45,6 +45,7 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 int kprobe_fault_handler(struct pt_regs *regs, unsigned long cause);
 void kretprobe_trampoline(void);
 void trap_is_kprobe(unsigned long address, struct pt_regs *regs);

diff --git a/arch/arm/include/asm/kprobes.h b/arch/arm/include/asm/kprobes.h
index 213607a1f45c..660f877b989f 100644
--- a/arch/arm/include/asm/kprobes.h
+++ b/arch/arm/include/asm/kprobes.h

@@ -38,6 +38,7 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 void arch_remove_kprobe(struct kprobe *);
 int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
 int kprobe_exceptions_notify(struct notifier_block *self,

diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
index 97e511d645a2..667773f75616 100644
--- a/arch/arm64/include/asm/kprobes.h
+++ b/arch/arm64/include/asm/kprobes.h

@@ -42,6 +42,7 @@ struct kprobe_ctlblk {
 	struct kprobe_step_ctx ss_ctx;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 void arch_remove_kprobe(struct kprobe *);
 int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
 int kprobe_exceptions_notify(struct notifier_block *self,

diff --git a/arch/ia64/include/asm/kprobes.h b/arch/ia64/include/asm/kprobes.h
index c5cf5e4fb338..c321e8585089 100644
--- a/arch/ia64/include/asm/kprobes.h
+++ b/arch/ia64/include/asm/kprobes.h

@@ -106,6 +106,7 @@ struct arch_specific_insn {
 	unsigned short slot;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 				    unsigned long val, void *data);

diff --git a/arch/mips/include/asm/kprobes.h b/arch/mips/include/asm/kprobes.h
index 68b1e5d458cf..d1efe991ea22 100644
--- a/arch/mips/include/asm/kprobes.h
+++ b/arch/mips/include/asm/kprobes.h

@@ -40,6 +40,7 @@ do {									\
 
 #define kretprobe_blacklist_size 0
 
+#define kprobe_fault_handler kprobe_fault_handler
 void arch_remove_kprobe(struct kprobe *p);
 int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 

diff --git a/arch/powerpc/include/asm/kprobes.h b/arch/powerpc/include/asm/kprobes.h
index 66b3f2983b22..c94f375ec957 100644
--- a/arch/powerpc/include/asm/kprobes.h
+++ b/arch/powerpc/include/asm/kprobes.h

@@ -84,6 +84,7 @@ struct arch_optimized_insn {
 	kprobe_opcode_t *insn;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 					unsigned long val, void *data);
 extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);

diff --git a/arch/s390/include/asm/kprobes.h b/arch/s390/include/asm/kprobes.h
index b106aa29bf55..0ecaebb78092 100644
--- a/arch/s390/include/asm/kprobes.h
+++ b/arch/s390/include/asm/kprobes.h

@@ -73,6 +73,7 @@ struct kprobe_ctlblk {
 void arch_remove_kprobe(struct kprobe *p);
 void kretprobe_trampoline(void);
 
+#define kprobe_fault_handler kprobe_fault_handler
 int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 int kprobe_exceptions_notify(struct notifier_block *self,
 	unsigned long val, void *data);

diff --git a/arch/sh/include/asm/kprobes.h b/arch/sh/include/asm/kprobes.h
index 6171682f7798..637a698393c0 100644
--- a/arch/sh/include/asm/kprobes.h
+++ b/arch/sh/include/asm/kprobes.h

@@ -45,6 +45,7 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 				    unsigned long val, void *data);

diff --git a/arch/sparc/include/asm/kprobes.h b/arch/sparc/include/asm/kprobes.h
index bfcaa6326c20..9aa4d25a45a8 100644
--- a/arch/sparc/include/asm/kprobes.h
+++ b/arch/sparc/include/asm/kprobes.h

@@ -47,6 +47,7 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 int kprobe_exceptions_notify(struct notifier_block *self,
 			     unsigned long val, void *data);
 int kprobe_fault_handler(struct pt_regs *regs, int trapnr);

diff --git a/arch/x86/include/asm/kprobes.h b/arch/x86/include/asm/kprobes.h
index 5dc909d9ad81..1af2b6db13bd 100644
--- a/arch/x86/include/asm/kprobes.h
+++ b/arch/x86/include/asm/kprobes.h

@@ -101,11 +101,17 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 };
 
+#define kprobe_fault_handler kprobe_fault_handler
 extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
 extern int kprobe_exceptions_notify(struct notifier_block *self,
 				    unsigned long val, void *data);
 extern int kprobe_int3_handler(struct pt_regs *regs);
 extern int kprobe_debug_handler(struct pt_regs *regs);
+#else
+static inline int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
+{
+	return 0;
+}
 
 #endif /* CONFIG_KPROBES */
 #endif /* _ASM_X86_KPROBES_H */

diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 04bdaf01112c..e106f3018804 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h

@@ -182,11 +182,19 @@ DECLARE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
 /*
  * For #ifdef avoidance:
  */
-static inline int kprobes_built_in(void)
+
+/*
+ * Architectures need to override this with their own implementation
+ * if they care to call kprobe_page_fault(). This will just ensure
+ * that kprobe_page_fault() returns false when called without having
+ * a proper platform specific definition for kprobe_fault_handler().
+ */
+#ifndef kprobe_fault_handler
+static inline int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 {
-	return 1;
+	return 0;
 }
-
+#endif
 #ifdef CONFIG_KRETPROBES
 extern void arch_prepare_kretprobe(struct kretprobe_instance *ri,
 				   struct pt_regs *regs);

@@ -375,14 +383,6 @@ void free_insn_page(void *page);
 
 #else /* !CONFIG_KPROBES: */
 
-static inline int kprobes_built_in(void)
-{
-	return 0;
-}
-static inline int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
-{
-	return 0;
-}
 static inline struct kprobe *get_kprobe(void *addr)
 {
 	return NULL;

@@ -458,12 +458,11 @@ static inline bool is_kprobe_optinsn_slot(unsigned long addr)
 }
 #endif
 
+#ifdef CONFIG_KPROBES
 /* Returns true if kprobes handled the fault */
 static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
 					      unsigned int trap)
 {
-	if (!kprobes_built_in())
-		return false;
 	if (user_mode(regs))
 		return false;
 	/*

@@ -476,5 +475,12 @@ static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
 		return false;
 	return kprobe_fault_handler(regs, trap);
 }
+#else
+static nokprobe_inline bool kprobe_page_fault(struct pt_regs *regs,
+					      unsigned int trap)
+{
+	return false;
+}
+#endif
 
 #endif /* _LINUX_KPROBES_H */

-- 
2.20.1

Hi Anshuman,

On Fri,  5 Jul 2019 11:00:29 +0530
Anshuman Khandual [off-list ref] wrote:

Architectures like parisc enable CONFIG_KROBES without having a definition
for kprobe_fault_handler() which results in a build failure.

Hmm, as far as I can see, kprobe_fault_handler() is closed inside each arch
specific code. The reason why include/linux/kprobes.h defines
dummy inline function is only for !CONFIG_KPROBES case.

Arch needs to
provide kprobe_fault_handler() as it is platform specific and cannot have
a generic working alternative. But in the event when platform lacks such a
definition there needs to be a fallback.

Wait, indeed that each arch need to implement it, but that is for calling
kprobe->fault_handler() as user expected.
Hmm, why not fixing those architecture implementations?

This adds a stub kprobe_fault_handler() definition which not only prevents
a build failure but also makes sure that kprobe_page_fault() if called will
always return negative in absence of a sane platform specific alternative.

I don't like introducing this complicated macro only for avoiding (not fixing)
build error. To fix that, kprobes on parisc should implement kprobe_fault_handler
correctly (and call kprobe->fault_handler).

BTW, even if you need such generic stub, please use a weak function instead
of macros for every arch headers.

While here wrap kprobe_page_fault() in CONFIG_KPROBES. This enables stud
definitions for generic kporbe_fault_handler() and kprobes_built_in() can
just be dropped. Only on x86 it needs to be added back locally as it gets
used in a !CONFIG_KPROBES function do_general_protection().

If you want to remove kprobes_built_in(), you should replace it with
IS_ENABLED(CONFIG_KPROBES), instead of this...

Thank you,

-- 
Masami Hiramatsu [off-list ref]

On 07/05/2019 04:00 PM, Masami Hiramatsu wrote:

Hi Anshuman,

Hello Masami,

On Fri,  5 Jul 2019 11:00:29 +0530
Anshuman Khandual [off-list ref] wrote:

quoted

Architectures like parisc enable CONFIG_KROBES without having a definition
for kprobe_fault_handler() which results in a build failure.

Hmm, as far as I can see, kprobe_fault_handler() is closed inside each arch
specific code. The reason why include/linux/kprobes.h defines
dummy inline function is only for !CONFIG_KPROBES case.

IIRC Andrew mentioned [1] that we should remove this stub from the generic kprobes
header because this is very much architecture specific. As we see in this proposed
patch, except x86 there is no other current user which actually calls this from
some where when CONFIG_KPROBES is not enabled.

[1] https://www.spinics.net/lists/linux-mm/msg182649.html

quoted

Arch needs to
provide kprobe_fault_handler() as it is platform specific and cannot have
a generic working alternative. But in the event when platform lacks such a
definition there needs to be a fallback.

Wait, indeed that each arch need to implement it, but that is for calling
kprobe->fault_handler() as user expected.
Hmm, why not fixing those architecture implementations?

After the recent change which introduced a generic kprobe_page_fault() every
architecture enabling CONFIG_KPROBES must have a kprobe_fault_handler() which
was not the case earlier. Architectures like parisc which does enable KPROBES but
never used (kprobe_page_fault or kprobe->fault_handler) kprobe_fault_handler() now
needs one as well. I am not sure and will probably require inputs from arch parsic
folks whether it at all needs one. We dont have a stub or fallback definition for
kprobe_fault_handler() when CONFIG_KPROBES is enabled just to prevent a build
failure in such cases.

In such a situation it might be better defining a stub symbol fallback than to try
to implement one definition which the architecture previously never needed or used.
AFAICS there is no generic MM callers for kprobe_fault_handler() as well which would
have made it mandatory for parisc to define a real one.

quoted

This adds a stub kprobe_fault_handler() definition which not only prevents
a build failure but also makes sure that kprobe_page_fault() if called will
always return negative in absence of a sane platform specific alternative.

I don't like introducing this complicated macro only for avoiding (not fixing)
build error. To fix that, kprobes on parisc should implement kprobe_fault_handler
correctly (and call kprobe->fault_handler).

As I mentioned before parsic might not need a real one. But you are right this
complicated (if perceived as such) change can be just avoided at least for the
build failure problem by just defining a stub definition kprobe_fault_handler()
for arch parsic when CONFIG_KPROBES is enabled. But this patch does some more
and solves the kprobe_fault_handler() symbol dependency in a more generic way and
forces kprobe_page_fault() to fail in absence a real arch kprobe_fault_handler().
Is not it worth solving in this way ?

BTW, even if you need such generic stub, please use a weak function instead
of macros for every arch headers.

There is a bit problem with that. The existing definitions are with different
signatures and an weak function will need them to be exact same for override
requiring more code changes. Hence choose to go with a macro in each header.

arch/arc/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned long cause);
arch/arm/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
arch/arm64/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
arch/ia64/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/powerpc/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/s390/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/sh/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/sparc/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/x86/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);

quoted

While here wrap kprobe_page_fault() in CONFIG_KPROBES. This enables stud
definitions for generic kporbe_fault_handler() and kprobes_built_in() can
just be dropped. Only on x86 it needs to be added back locally as it gets
used in a !CONFIG_KPROBES function do_general_protection().

If you want to remove kprobes_built_in(), you should replace it with
IS_ENABLED(CONFIG_KPROBES), instead of this...

Apart from kprobes_built_in() the intent was to remove !CONFIG_KPROBES
stub for kprobe_fault_handler() as well which required making generic
kprobe_page_fault() to be empty in such case.

On 07/05/2019 11:00 AM, Anshuman Khandual wrote:

Architectures like parisc enable CONFIG_KROBES without having a definition
for kprobe_fault_handler() which results in a build failure. Arch needs to
provide kprobe_fault_handler() as it is platform specific and cannot have
a generic working alternative. But in the event when platform lacks such a
definition there needs to be a fallback.

This adds a stub kprobe_fault_handler() definition which not only prevents
a build failure but also makes sure that kprobe_page_fault() if called will
always return negative in absence of a sane platform specific alternative.

While here wrap kprobe_page_fault() in CONFIG_KPROBES. This enables stud
definitions for generic kporbe_fault_handler() and kprobes_built_in() can
just be dropped. Only on x86 it needs to be added back locally as it gets
used in a !CONFIG_KPROBES function do_general_protection().

Cc: Vineet Gupta <redacted>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Fenghua Yu <redacted>
Cc: Ralf Baechle <redacted>
Cc: Paul Burton <redacted>
Cc: James Hogan <jhogan@kernel.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Heiko Carstens <redacted>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <redacted>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Naveen N. Rao" <redacted>
Cc: Anil S Keshavamurthy <redacted>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Allison Randal <redacted>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Enrico Weigelt <info@metux.net>
Cc: Richard Fontana <redacted>
Cc: Kate Stewart <redacted>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: x86@kernel.org
Cc: linux-snps-arc@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-ia64@vger.kernel.org
Cc: linux-mips@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

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

Any updates or suggestions on this patch ? Currently there is a build failure on
parisc architecture due to the lack of a kprobe_fault_handler() definition when
CONFIG_KPROBES is enabled and this build failure needs to be fixed.

This patch solves the build problem. But otherwise I am also happy to just define
a stub definition for kprobe_fault_handler() on parisc arch when CONFIG_KPROBES
is enabled, which will avoid the build failure. Please suggest.

Hi Anshuman,

On Mon, 8 Jul 2019 09:03:13 +0530
Anshuman Khandual [off-list ref] wrote:

quoted

quoted

Architectures like parisc enable CONFIG_KROBES without having a definition
for kprobe_fault_handler() which results in a build failure.

Hmm, as far as I can see, kprobe_fault_handler() is closed inside each arch
specific code. The reason why include/linux/kprobes.h defines
dummy inline function is only for !CONFIG_KPROBES case.

IIRC Andrew mentioned [1] that we should remove this stub from the generic kprobes
header because this is very much architecture specific. As we see in this proposed
patch, except x86 there is no other current user which actually calls this from
some where when CONFIG_KPROBES is not enabled.

[1] https://www.spinics.net/lists/linux-mm/msg182649.html

Ah, OK. I saw another branch. Also, this is a bugfix patch against
commit 4dd635bce90e ("mm, kprobes: generalize and rename notify_page_fault() as
 kprobe_page_fault()"), please add Fixes: tag on it.

In this case, we should just add a prototype of kprobe_fault_handler() in
include/linux/kprobes.h, and maybe add a stub of kprobe_fault_handler()
as a weak function, something like below.

int __weak kprobe_fault_handler(struct pt_regs *regs, int trapnr)
{
	/*
	 * Each architecture which uses kprobe_page_fault() must define
	 * a fault handler to handle page fault in kprobe correctly.
	 */
	WARN_ON_ONCE(1);
	return 0;
}

quoted

quoted

Arch needs to
provide kprobe_fault_handler() as it is platform specific and cannot have
a generic working alternative. But in the event when platform lacks such a
definition there needs to be a fallback.

Wait, indeed that each arch need to implement it, but that is for calling
kprobe->fault_handler() as user expected.
Hmm, why not fixing those architecture implementations?

After the recent change which introduced a generic kprobe_page_fault() every
architecture enabling CONFIG_KPROBES must have a kprobe_fault_handler() which
was not the case earlier.

As far as I can see, gcc complains it because there is no prototype of
kprobe_fault_handler(). Actually no need to define empty kprobe_fault_handler()
on each arch. If we have a prototype, but no actual function, gcc stops the
error unless the arch depending code uses it. So actually, we don't need above
__weak function.

Architectures like parisc which does enable KPROBES but
never used (kprobe_page_fault or kprobe->fault_handler) kprobe_fault_handler() now
needs one as well.

(Hmm, it sounds like the kprobes porting is incomplete on parisc...)

I am not sure and will probably require inputs from arch parsic
folks whether it at all needs one. We dont have a stub or fallback definition for
kprobe_fault_handler() when CONFIG_KPROBES is enabled just to prevent a build
failure in such cases.

Yeah, that is a bug, and fixed by adding a prototype, not introducing new macro.

In such a situation it might be better defining a stub symbol fallback than to try
to implement one definition which the architecture previously never needed or used.
AFAICS there is no generic MM callers for kprobe_fault_handler() as well which would
have made it mandatory for parisc to define a real one.

quoted

quoted

This adds a stub kprobe_fault_handler() definition which not only prevents
a build failure but also makes sure that kprobe_page_fault() if called will
always return negative in absence of a sane platform specific alternative.

I don't like introducing this complicated macro only for avoiding (not fixing)
build error. To fix that, kprobes on parisc should implement kprobe_fault_handler
correctly (and call kprobe->fault_handler).

As I mentioned before parsic might not need a real one. But you are right this
complicated (if perceived as such) change can be just avoided at least for the
build failure problem by just defining a stub definition kprobe_fault_handler()
for arch parsic when CONFIG_KPROBES is enabled. But this patch does some more
and solves the kprobe_fault_handler() symbol dependency in a more generic way and
forces kprobe_page_fault() to fail in absence a real arch kprobe_fault_handler().
Is not it worth solving in this way ?

quoted

BTW, even if you need such generic stub, please use a weak function instead
of macros for every arch headers.

There is a bit problem with that. The existing definitions are with different
signatures and an weak function will need them to be exact same for override
requiring more code changes. Hence choose to go with a macro in each header.

arch/arc/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned long cause);
arch/arm/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
arch/arm64/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr);
arch/ia64/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/powerpc/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/s390/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/sh/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/sparc/include/asm/kprobes.h:int kprobe_fault_handler(struct pt_regs *regs, int trapnr);
arch/x86/include/asm/kprobes.h:extern int kprobe_fault_handler(struct pt_regs *regs, int trapnr);

OK, in that case, original commit is wrong way. it should be reverted and
should introduce something like below


/* Returns true if arch should call kprobes_fault_handler() */
static nokprobe_inline bool is_kprobe_page_fault(struct pt_regs *regs)
{
	if (!kprobes_built_in())
		return false;
	if (user_mode(regs))
		return false;
	/*
	 * To be potentially processing a kprobe fault and to be allowed
	 * to call kprobe_running(), we have to be non-preemptible.
	 */
	if (preemptible())
		return false;
	if (!kprobe_running())
		return false;
	return true;
}

Since it silently casts the type of trapnr, which is strongly depends
on architecture.

quoted

quoted

While here wrap kprobe_page_fault() in CONFIG_KPROBES. This enables stud
definitions for generic kporbe_fault_handler() and kprobes_built_in() can
just be dropped. Only on x86 it needs to be added back locally as it gets
used in a !CONFIG_KPROBES function do_general_protection().

If you want to remove kprobes_built_in(), you should replace it with
IS_ENABLED(CONFIG_KPROBES), instead of this...

Apart from kprobes_built_in() the intent was to remove !CONFIG_KPROBES
stub for kprobe_fault_handler() as well which required making generic
kprobe_page_fault() to be empty in such case.

No, I meant that "IS_ENABLED(CONFIG_KPROBES)" is generic and is equal to
what kprobes_built_in() does.

Thank you,

-- 
Masami Hiramatsu [off-list ref]