[PATCH 2/2] s390: add pt_regs register and stack access API

[PATCH 2/2] s390: add pt_regs register and stack access API

[Heiko Carstens]

From: Heiko Carstens <heiko.carstens@de.ibm.com>

This API is needed for the kprobe-based event tracer.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
 arch/s390/Kconfig              |    1 
 arch/s390/include/asm/ptrace.h |   14 +++++++
 arch/s390/kernel/ptrace.c      |   76 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 90 insertions(+), 1 deletion(-)

--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -492,13 +492,25 @@ struct user_regs_struct
 struct task_struct;
 extern void user_enable_single_step(struct task_struct *);
 extern void user_disable_single_step(struct task_struct *);
+extern void show_regs(struct pt_regs * regs);
 
 #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
 #define instruction_pointer(regs) ((regs)->psw.addr & PSW_ADDR_INSN)
 #define user_stack_pointer(regs)((regs)->gprs[15])
 #define regs_return_value(regs)((regs)->gprs[2])
 #define profile_pc(regs) instruction_pointer(regs)
-extern void show_regs(struct pt_regs * regs);
+
+int regs_query_register_offset(const char *name);
+const char *regs_query_register_name(unsigned int offset);
+unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned int n);
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset);
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n);
+
+static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
+{
+	return regs->gprs[15] & PSW_ADDR_INSN;
+}
+
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -87,6 +87,7 @@ config S390
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_DEFAULT_NO_SPIN_MUTEXES
 	select HAVE_OPROFILE
 	select HAVE_KPROBES
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -992,3 +992,79 @@ const struct user_regset_view *task_user
 #endif
 	return &user_s390_view;
 }
+
+static const char *gpr_names[NUM_GPRS] = {
+	"r0", "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
+	"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",
+};
+
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return 0;
+	return regs->gprs[offset];
+}
+
+int regs_query_register_offset(const char *name)
+{
+	unsigned long offset;
+
+	if (!name || *name != 'r')
+		return -EINVAL;
+	if (strict_strtoul(name + 1, 10, &offset))
+		return -EINVAL;
+	if (offset >= NUM_GPRS)
+		return -EINVAL;
+	return offset;
+}
+
+const char *regs_query_register_name(unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return NULL;
+	return gpr_names[offset];
+}
+
+static int regs_within_kernel_stack(struct pt_regs *regs, unsigned long addr)
+{
+	unsigned long ksp = kernel_stack_pointer(regs);
+
+	return (addr & ~(THREAD_SIZE - 1)) == (ksp & ~(THREAD_SIZE - 1));
+}
+
+/**
+ * regs_get_kernel_stack_nth() - get Nth entry of the stack
+ * @regs:pt_regs which contains kernel stack pointer.
+ * @n:stack entry number.
+ *
+ * regs_get_kernel_stack_nth() returns @n th entry of the kernel stack which
+ * is specifined by @regs. If the @n th entry is NOT in the kernel stack,
+ * this returns 0.
+ */
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n)
+{
+	unsigned long addr;
+
+	addr = kernel_stack_pointer(regs) + n * sizeof(long);
+	if (!regs_within_kernel_stack(regs, addr))
+		return 0;
+	return *(unsigned long *)addr;
+}
+
+/**
+ * regs_get_argument_nth() - get Nth argument at function call
+ * @regs:pt_regs which contains registers at function entry.
+ * @n:argument number.
+ *
+ * regs_get_argument_nth() returns @n th argument of a function call.
+ * Since usually the kernel stack will be changed right after function entry,
+ * you must use this at function entry. If the @n th entry is NOT in the
+ * kernel stack or pt_regs, this returns 0.
+ */
+unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned int n)
+{
+	if (n < 5)
+		return regs->gprs[2 + n];
+	n += STACK_FRAME_OVERHEAD / sizeof(long);
+	return regs_get_kernel_stack_nth(regs, n - 5);
+}

Re: [PATCH 2/2] s390: add pt_regs register and stack access API

[Masami Hiramatsu]

Heiko Carstens wrote:
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
> 
> This API is needed for the kprobe-based event tracer.
> 
> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
>  arch/s390/Kconfig              |    1 
>  arch/s390/include/asm/ptrace.h |   14 +++++++
>  arch/s390/kernel/ptrace.c      |   76 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 90 insertions(+), 1 deletion(-)
> 
> --- a/arch/s390/include/asm/ptrace.h
> +++ b/arch/s390/include/asm/ptrace.h
> @@ -492,13 +492,25 @@ struct user_regs_struct
>  struct task_struct;
>  extern void user_enable_single_step(struct task_struct *);
>  extern void user_disable_single_step(struct task_struct *);
> +extern void show_regs(struct pt_regs * regs);
>  
>  #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
>  #define instruction_pointer(regs) ((regs)->psw.addr & PSW_ADDR_INSN)
>  #define user_stack_pointer(regs)((regs)->gprs[15])
>  #define regs_return_value(regs)((regs)->gprs[2])
>  #define profile_pc(regs) instruction_pointer(regs)
> -extern void show_regs(struct pt_regs * regs);
> +
> +int regs_query_register_offset(const char *name);
> +const char *regs_query_register_name(unsigned int offset);
> +unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned int n);

Hmm, the latest kprobe-tracer on -tip/master doesn't support
regs_get_argument_nth() anymore, because the function ABI
strongly depends on each function interface, compile option
etc. So I removed it (14640106f243a3b29944d7198569090fa6546f2d
and aa5add93e92019018e905146f8c3d3f8e3c08300).

Yeah, those patches are still on -tip tree. Since I'm working on
-tip/master for kprobe-tracer etc., please work on -tip tree.

Thank you!

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com

Re: [PATCH 2/2] s390: add pt_regs register and stack access API

[Heiko Carstens]

On Wed, Feb 10, 2010 at 09:19:06PM -0500, Masami Hiramatsu wrote:
> > +int regs_query_register_offset(const char *name);
> > +const char *regs_query_register_name(unsigned int offset);
> > +unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned int n);
> 
> Hmm, the latest kprobe-tracer on -tip/master doesn't support
> regs_get_argument_nth() anymore, because the function ABI
> strongly depends on each function interface, compile option
> etc. So I removed it (14640106f243a3b29944d7198569090fa6546f2d
> and aa5add93e92019018e905146f8c3d3f8e3c08300).
> 
> Yeah, those patches are still on -tip tree. Since I'm working on
> -tip/master for kprobe-tracer etc., please work on -tip tree.

Ok, here is an updated patch without regs_get_argument_nth:

Subject: [PATCH] s390: add pt_regs register and stack access API

From: Heiko Carstens <heiko.carstens@de.ibm.com>

This API is needed for the kprobe-based event tracer.

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
---
 arch/s390/Kconfig              |    1 
 arch/s390/include/asm/ptrace.h |   13 ++++++++-
 arch/s390/kernel/ptrace.c      |   58 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+), 1 deletion(-)

--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -492,13 +492,24 @@ struct user_regs_struct
 struct task_struct;
 extern void user_enable_single_step(struct task_struct *);
 extern void user_disable_single_step(struct task_struct *);
+extern void show_regs(struct pt_regs * regs);
 
 #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
 #define instruction_pointer(regs) ((regs)->psw.addr & PSW_ADDR_INSN)
 #define user_stack_pointer(regs)((regs)->gprs[15])
 #define regs_return_value(regs)((regs)->gprs[2])
 #define profile_pc(regs) instruction_pointer(regs)
-extern void show_regs(struct pt_regs * regs);
+
+int regs_query_register_offset(const char *name);
+const char *regs_query_register_name(unsigned int offset);
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset);
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n);
+
+static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
+{
+	return regs->gprs[15] & PSW_ADDR_INSN;
+}
+
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -90,6 +90,7 @@ config S390
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_DEFAULT_NO_SPIN_MUTEXES
 	select HAVE_OPROFILE
 	select HAVE_KPROBES
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -992,3 +992,61 @@ const struct user_regset_view *task_user
 #endif
 	return &user_s390_view;
 }
+
+static const char *gpr_names[NUM_GPRS] = {
+	"r0", "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
+	"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",
+};
+
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return 0;
+	return regs->gprs[offset];
+}
+
+int regs_query_register_offset(const char *name)
+{
+	unsigned long offset;
+
+	if (!name || *name != 'r')
+		return -EINVAL;
+	if (strict_strtoul(name + 1, 10, &offset))
+		return -EINVAL;
+	if (offset >= NUM_GPRS)
+		return -EINVAL;
+	return offset;
+}
+
+const char *regs_query_register_name(unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return NULL;
+	return gpr_names[offset];
+}
+
+static int regs_within_kernel_stack(struct pt_regs *regs, unsigned long addr)
+{
+	unsigned long ksp = kernel_stack_pointer(regs);
+
+	return (addr & ~(THREAD_SIZE - 1)) == (ksp & ~(THREAD_SIZE - 1));
+}
+
+/**
+ * regs_get_kernel_stack_nth() - get Nth entry of the stack
+ * @regs:pt_regs which contains kernel stack pointer.
+ * @n:stack entry number.
+ *
+ * regs_get_kernel_stack_nth() returns @n th entry of the kernel stack which
+ * is specifined by @regs. If the @n th entry is NOT in the kernel stack,
+ * this returns 0.
+ */
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n)
+{
+	unsigned long addr;
+
+	addr = kernel_stack_pointer(regs) + n * sizeof(long);
+	if (!regs_within_kernel_stack(regs, addr))
+		return 0;
+	return *(unsigned long *)addr;
+}

Re: [PATCH 2/2] s390: add pt_regs register and stack access API

[Masami Hiramatsu]

Heiko Carstens wrote:
> On Wed, Feb 10, 2010 at 09:19:06PM -0500, Masami Hiramatsu wrote:
>>> +int regs_query_register_offset(const char *name);
>>> +const char *regs_query_register_name(unsigned int offset);
>>> +unsigned long regs_get_argument_nth(struct pt_regs *regs, unsigned int n);
>>
>> Hmm, the latest kprobe-tracer on -tip/master doesn't support
>> regs_get_argument_nth() anymore, because the function ABI
>> strongly depends on each function interface, compile option
>> etc. So I removed it (14640106f243a3b29944d7198569090fa6546f2d
>> and aa5add93e92019018e905146f8c3d3f8e3c08300).
>>
>> Yeah, those patches are still on -tip tree. Since I'm working on
>> -tip/master for kprobe-tracer etc., please work on -tip tree.
> 
> Ok, here is an updated patch without regs_get_argument_nth:
> 
> Subject: [PATCH] s390: add pt_regs register and stack access API
> 
> From: Heiko Carstens <heiko.carstens@de.ibm.com>
> 
> This API is needed for the kprobe-based event tracer.

Thank you,
At least interfaces looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@redhat.com>

> 
> Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> ---
>  arch/s390/Kconfig              |    1 
>  arch/s390/include/asm/ptrace.h |   13 ++++++++-
>  arch/s390/kernel/ptrace.c      |   58 +++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 71 insertions(+), 1 deletion(-)
> 
> --- a/arch/s390/include/asm/ptrace.h
> +++ b/arch/s390/include/asm/ptrace.h
> @@ -492,13 +492,24 @@ struct user_regs_struct
>  struct task_struct;
>  extern void user_enable_single_step(struct task_struct *);
>  extern void user_disable_single_step(struct task_struct *);
> +extern void show_regs(struct pt_regs * regs);
>  
>  #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
>  #define instruction_pointer(regs) ((regs)->psw.addr & PSW_ADDR_INSN)
>  #define user_stack_pointer(regs)((regs)->gprs[15])
>  #define regs_return_value(regs)((regs)->gprs[2])
>  #define profile_pc(regs) instruction_pointer(regs)
> -extern void show_regs(struct pt_regs * regs);
> +
> +int regs_query_register_offset(const char *name);
> +const char *regs_query_register_name(unsigned int offset);
> +unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset);
> +unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n);
> +
> +static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
> +{
> +	return regs->gprs[15] & PSW_ADDR_INSN;
> +}
> +
>  #endif /* __KERNEL__ */
>  #endif /* __ASSEMBLY__ */
>  
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -90,6 +90,7 @@ config S390
>  	select HAVE_SYSCALL_TRACEPOINTS
>  	select HAVE_DYNAMIC_FTRACE
>  	select HAVE_FUNCTION_GRAPH_TRACER
> +	select HAVE_REGS_AND_STACK_ACCESS_API
>  	select HAVE_DEFAULT_NO_SPIN_MUTEXES
>  	select HAVE_OPROFILE
>  	select HAVE_KPROBES
> --- a/arch/s390/kernel/ptrace.c
> +++ b/arch/s390/kernel/ptrace.c
> @@ -992,3 +992,61 @@ const struct user_regset_view *task_user
>  #endif
>  	return &user_s390_view;
>  }
> +
> +static const char *gpr_names[NUM_GPRS] = {
> +	"r0", "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
> +	"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",
> +};
> +
> +unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset)
> +{
> +	if (offset >= NUM_GPRS)
> +		return 0;
> +	return regs->gprs[offset];
> +}
> +
> +int regs_query_register_offset(const char *name)
> +{
> +	unsigned long offset;
> +
> +	if (!name || *name != 'r')
> +		return -EINVAL;
> +	if (strict_strtoul(name + 1, 10, &offset))
> +		return -EINVAL;
> +	if (offset >= NUM_GPRS)
> +		return -EINVAL;
> +	return offset;
> +}
> +
> +const char *regs_query_register_name(unsigned int offset)
> +{
> +	if (offset >= NUM_GPRS)
> +		return NULL;
> +	return gpr_names[offset];
> +}
> +
> +static int regs_within_kernel_stack(struct pt_regs *regs, unsigned long addr)
> +{
> +	unsigned long ksp = kernel_stack_pointer(regs);
> +
> +	return (addr & ~(THREAD_SIZE - 1)) == (ksp & ~(THREAD_SIZE - 1));
> +}
> +
> +/**
> + * regs_get_kernel_stack_nth() - get Nth entry of the stack
> + * @regs:pt_regs which contains kernel stack pointer.
> + * @n:stack entry number.
> + *
> + * regs_get_kernel_stack_nth() returns @n th entry of the kernel stack which
> + * is specifined by @regs. If the @n th entry is NOT in the kernel stack,
> + * this returns 0.
> + */
> +unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n)
> +{
> +	unsigned long addr;
> +
> +	addr = kernel_stack_pointer(regs) + n * sizeof(long);
> +	if (!regs_within_kernel_stack(regs, addr))
> +		return 0;
> +	return *(unsigned long *)addr;
> +}
> --
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-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America), Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com

[tip:tracing/core] s390: Add pt_regs register and stack access API

[tip-bot for Heiko Carstens]

Commit-ID:  952974ac61f686896bd4134dae106a886a5589f1
Gitweb:     http://git.kernel.org/tip/952974ac61f686896bd4134dae106a886a5589f1
Author:     Heiko Carstens <heiko.carstens@de.ibm.com>
AuthorDate: Fri, 12 Feb 2010 13:38:40 +0100
Committer:  Frederic Weisbecker <fweisbec@gmail.com>
CommitDate: Wed, 17 Feb 2010 13:19:26 +0100

s390: Add pt_regs register and stack access API

This API is needed for the kprobe-based event tracer.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Reviewed-by: Masami Hiramatsu <mhiramat@redhat.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
LKML-Reference: <20100212123840.GB27548@osiris.boeblingen.de.ibm.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 arch/s390/Kconfig              |    1 +
 arch/s390/include/asm/ptrace.h |   13 ++++++++-
 arch/s390/kernel/ptrace.c      |   58 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 71 insertions(+), 1 deletions(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index c802352..2590ce2 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -87,6 +87,7 @@ config S390
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_FUNCTION_GRAPH_TRACER
+	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_DEFAULT_NO_SPIN_MUTEXES
 	select HAVE_OPROFILE
 	select HAVE_KPROBES
diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
index 95dcf18..dd2d913 100644
--- a/arch/s390/include/asm/ptrace.h
+++ b/arch/s390/include/asm/ptrace.h
@@ -492,13 +492,24 @@ struct user_regs_struct
 struct task_struct;
 extern void user_enable_single_step(struct task_struct *);
 extern void user_disable_single_step(struct task_struct *);
+extern void show_regs(struct pt_regs * regs);
 
 #define user_mode(regs) (((regs)->psw.mask & PSW_MASK_PSTATE) != 0)
 #define instruction_pointer(regs) ((regs)->psw.addr & PSW_ADDR_INSN)
 #define user_stack_pointer(regs)((regs)->gprs[15])
 #define regs_return_value(regs)((regs)->gprs[2])
 #define profile_pc(regs) instruction_pointer(regs)
-extern void show_regs(struct pt_regs * regs);
+
+int regs_query_register_offset(const char *name);
+const char *regs_query_register_name(unsigned int offset);
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset);
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n);
+
+static inline unsigned long kernel_stack_pointer(struct pt_regs *regs)
+{
+	return regs->gprs[15] & PSW_ADDR_INSN;
+}
+
 #endif /* __KERNEL__ */
 #endif /* __ASSEMBLY__ */
 
diff --git a/arch/s390/kernel/ptrace.c b/arch/s390/kernel/ptrace.c
index 13815d3..1720f38 100644
--- a/arch/s390/kernel/ptrace.c
+++ b/arch/s390/kernel/ptrace.c
@@ -984,3 +984,61 @@ const struct user_regset_view *task_user_regset_view(struct task_struct *task)
 #endif
 	return &user_s390_view;
 }
+
+static const char *gpr_names[NUM_GPRS] = {
+	"r0", "r1",  "r2",  "r3",  "r4",  "r5",  "r6",  "r7",
+	"r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15",
+};
+
+unsigned long regs_get_register(struct pt_regs *regs, unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return 0;
+	return regs->gprs[offset];
+}
+
+int regs_query_register_offset(const char *name)
+{
+	unsigned long offset;
+
+	if (!name || *name != 'r')
+		return -EINVAL;
+	if (strict_strtoul(name + 1, 10, &offset))
+		return -EINVAL;
+	if (offset >= NUM_GPRS)
+		return -EINVAL;
+	return offset;
+}
+
+const char *regs_query_register_name(unsigned int offset)
+{
+	if (offset >= NUM_GPRS)
+		return NULL;
+	return gpr_names[offset];
+}
+
+static int regs_within_kernel_stack(struct pt_regs *regs, unsigned long addr)
+{
+	unsigned long ksp = kernel_stack_pointer(regs);
+
+	return (addr & ~(THREAD_SIZE - 1)) == (ksp & ~(THREAD_SIZE - 1));
+}
+
+/**
+ * regs_get_kernel_stack_nth() - get Nth entry of the stack
+ * @regs:pt_regs which contains kernel stack pointer.
+ * @n:stack entry number.
+ *
+ * regs_get_kernel_stack_nth() returns @n th entry of the kernel stack which
+ * is specifined by @regs. If the @n th entry is NOT in the kernel stack,
+ * this returns 0.
+ */
+unsigned long regs_get_kernel_stack_nth(struct pt_regs *regs, unsigned int n)
+{
+	unsigned long addr;
+
+	addr = kernel_stack_pointer(regs) + n * sizeof(long);
+	if (!regs_within_kernel_stack(regs, addr))
+		return 0;
+	return *(unsigned long *)addr;
+}