powerpc/ptrace: Simplify gpr_get()/tm_cgpr_get()

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>
[mpe: Fixup some minor whitespace issues noticed by Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201119160221.GA5188@redhat.com

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;
 
@@ -96,16 +98,10 @@ int tm_cgpr_get(struct task_struct *target, const struct user_regset *regset,
 	flush_fp_to_thread(target);
 	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));
+	membuf_write(&to, &target->thread.ckpt_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_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));
 }

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)
@@ -228,15 +229,10 @@ 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));
 }

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)				\
 ({							\