[PATCH v3 1/2] powerpc/ptrace: simplify gpr_get/tm_cgpr_get

[Oleg Nesterov <oleg@redhat.com>]

gpr_get() does membuf_write() twice to override pt_regs->msr in between.
We can call membuf_write() once and change ->msr in the kernel buffer,
this simplifies the code and the next fix.

The patch adds a new simple helper, membuf_at(offs), it returns the new
membuf which can be safely used after membuf_write().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/powerpc/kernel/ptrace/ptrace-tm.c   | 13 +++++--------
 arch/powerpc/kernel/ptrace/ptrace-view.c | 13 +++++--------
 include/linux/regset.h                   | 12 ++++++++++++
 3 files changed, 22 insertions(+), 16 deletions(-)

diff --git a/arch/powerpc/kernel/ptrace/ptrace-tm.c b/arch/powerpc/kernel/ptrace/ptrace-tm.c
index 54f2d076206f..f8fcbd85d4cb 100644
--- a/arch/powerpc/kernel/ptrace/ptrace-tm.c
+++ b/arch/powerpc/kernel/ptrace/ptrace-tm.c
@@ -86,6 +86,8 @@ int tm_cgpr_active(struct task_struct *target, const struct user_regset *regset)
 int tm_cgpr_get(struct task_struct *target, const struct user_regset *regset,
 		struct membuf to)
 {
+	struct membuf to_msr = membuf_at(&to, offsetof(struct pt_regs, msr));
+
 	if (!cpu_has_feature(CPU_FTR_TM))
 		return -ENODEV;
 
@@ -97,17 +99,12 @@ int tm_cgpr_get(struct task_struct *target, const struct user_regset *regset,
 	flush_altivec_to_thread(target);
 
 	membuf_write(&to, &target->thread.ckpt_regs,
-			offsetof(struct pt_regs, msr));
-	membuf_store(&to, get_user_ckpt_msr(target));
+				sizeof(struct user_pt_regs));
 
-	BUILD_BUG_ON(offsetof(struct pt_regs, orig_gpr3) !=
-		     offsetof(struct pt_regs, msr) + sizeof(long));
+	membuf_store(&to_msr, get_user_ckpt_msr(target));
 
-	membuf_write(&to, &target->thread.ckpt_regs.orig_gpr3,
-			sizeof(struct user_pt_regs) -
-			offsetof(struct pt_regs, orig_gpr3));
 	return membuf_zero(&to, ELF_NGREG * sizeof(unsigned long) -
-			sizeof(struct user_pt_regs));
+				sizeof(struct user_pt_regs));
 }
 
 /*
diff --git a/arch/powerpc/kernel/ptrace/ptrace-view.c b/arch/powerpc/kernel/ptrace/ptrace-view.c
index 7e6478e7ed07..39686ede40b3 100644
--- a/arch/powerpc/kernel/ptrace/ptrace-view.c
+++ b/arch/powerpc/kernel/ptrace/ptrace-view.c
@@ -217,6 +217,7 @@ int ptrace_put_reg(struct task_struct *task, int regno, unsigned long data)
 static int gpr_get(struct task_struct *target, const struct user_regset *regset,
 		   struct membuf to)
 {
+	struct membuf to_msr = membuf_at(&to, offsetof(struct pt_regs, msr));
 	int i;
 
 	if (target->thread.regs == NULL)
@@ -228,17 +229,13 @@ static int gpr_get(struct task_struct *target, const struct user_regset *regset,
 			target->thread.regs->gpr[i] = NV_REG_POISON;
 	}
 
-	membuf_write(&to, target->thread.regs, offsetof(struct pt_regs, msr));
-	membuf_store(&to, get_user_msr(target));
+	membuf_write(&to, target->thread.regs,
+				sizeof(struct user_pt_regs));
 
-	BUILD_BUG_ON(offsetof(struct pt_regs, orig_gpr3) !=
-		     offsetof(struct pt_regs, msr) + sizeof(long));
+	membuf_store(&to_msr, get_user_msr(target));
 
-	membuf_write(&to, &target->thread.regs->orig_gpr3,
-			sizeof(struct user_pt_regs) -
-			offsetof(struct pt_regs, orig_gpr3));
 	return membuf_zero(&to, ELF_NGREG * sizeof(unsigned long) -
-				 sizeof(struct user_pt_regs));
+				sizeof(struct user_pt_regs));
 }
 
 static int gpr_set(struct task_struct *target, const struct user_regset *regset,
diff --git a/include/linux/regset.h b/include/linux/regset.h
index c3403f328257..a00765f0e8cf 100644
--- a/include/linux/regset.h
+++ b/include/linux/regset.h
@@ -46,6 +46,18 @@ static inline int membuf_write(struct membuf *s, const void *v, size_t size)
 	return s->left;
 }
 
+static inline struct membuf membuf_at(const struct membuf *s, size_t offs)
+{
+	struct membuf n = *s;
+
+	if (offs > n.left)
+		offs = n.left;
+	n.p += offs;
+	n.left -= offs;
+
+	return n;
+}
+
 /* current s->p must be aligned for v; v must be a scalar */
 #define membuf_store(s, v)				\
 ({							\
-- 
2.25.1.362.g51ebf55