regset: Add support for dynamically sized regsets

Currently the regset API doesn't allow for the possibility that
regsets (or at least, the amount of meaningful data in a regset)
may change in size.

In particular, this results in useless padding being added to
coredumps if a regset's current size is smaller than its
theoretical maximum size.

This patch adds a get_size() function to struct user_regset.
Individual regset implementations can implement this function to
return the current size of the regset data.  A regset_size()
function is added to provide callers with an abstract interface for
determining the size of a regset without needing to know whether
the regset is dynamically sized or not.

The only affected user of this interface is the ELF coredump code:
This patch ports ELF coredump to dump regsets with their actual
size in the coredump.  This has no effect except for new regsets
that are dynamically sized and provide a get_size() implementation.
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Dmitry Safonov <dsafonov@virtuozzo.com>
Cc: H. J. Lu <hjl.tools@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

fs/binfmt_elf.c

@@ -1699,7 +1699,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 				 long signr, size_t *total)
 {
 	unsigned int i;
-	unsigned int regset_size = view->regsets[0].n * view->regsets[0].size;
+	unsigned int regset0_size = regset_size(t->task, &view->regsets[0]);
 
 	/*
 	 * NT_PRSTATUS is the one special case, because the regset data
@@ -1708,11 +1708,11 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 	 * We assume that regset 0 is NT_PRSTATUS.
 	 */
 	fill_prstatus(&t->prstatus, t->task, signr);
-	(void) view->regsets[0].get(t->task, &view->regsets[0], 0, regset_size,
+	(void) view->regsets[0].get(t->task, &view->regsets[0], 0, regset0_size,
 				    &t->prstatus.pr_reg, NULL);
 
 	fill_note(&t->notes[0], "CORE", NT_PRSTATUS,
-		  PRSTATUS_SIZE(t->prstatus, regset_size), &t->prstatus);
+		  PRSTATUS_SIZE(t->prstatus, regset0_size), &t->prstatus);
 	*total += notesize(&t->notes[0]);
 
 	do_thread_regset_writeback(t->task, &view->regsets[0]);
@@ -1728,7 +1728,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 		if (regset->core_note_type && regset->get &&
 		    (!regset->active || regset->active(t->task, regset))) {
 			int ret;
-			size_t size = regset->n * regset->size;
+			size_t size = regset_size(t->task, regset);
 			void *data = kmalloc(size, GFP_KERNEL);
 			if (unlikely(!data))
 				return 0;
@@ -1743,7 +1743,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 						  size, data);
 				else {
 					SET_PR_FPVALID(&t->prstatus,
-							1, regset_size);
+							1, regset0_size);
 					fill_note(&t->notes[i], "CORE",
 						  NT_PRFPREG, size, data);
 				}

include/linux/regset.h

@@ -106,6 +106,28 @@ typedef int user_regset_writeback_fn(struct task_struct *target,
 				     const struct user_regset *regset,
 				     int immediate);
 
+/**
+ * user_regset_get_size_fn - type of @get_size function in &struct user_regset
+ * @target:	thread being examined
+ * @regset:	regset being examined
+ *
+ * This call is optional; usually the pointer is %NULL.
+ *
+ * When provided, this function must return the current size of regset
+ * data, as observed by the @get function in &struct user_regset.  The
+ * value returned must be a multiple of @size.  The returned size is
+ * required to be valid only until the next time (if any) @regset is
+ * modified for @target.
+ *
+ * This function is intended for dynamically sized regsets.  A regset
+ * that is statically sized does not need to implement it.
+ *
+ * This function should not be called directly: instead, callers should
+ * call regset_size() to determine the current size of a regset.
+ */
+typedef unsigned int user_regset_get_size_fn(struct task_struct *target,
+					     const struct user_regset *regset);
+
 /**
  * struct user_regset - accessible thread CPU state
  * @n:			Number of slots (registers).
@@ -117,19 +139,33 @@ typedef int user_regset_writeback_fn(struct task_struct *target,
  * @set:		Function to store values.
  * @active:		Function to report if regset is active, or %NULL.
  * @writeback:		Function to write data back to user memory, or %NULL.
+ * @get_size:		Function to return the regset's size, or %NULL.
  *
  * This data structure describes a machine resource we call a register set.
  * This is part of the state of an individual thread, not necessarily
  * actual CPU registers per se.  A register set consists of a number of
  * similar slots, given by @n.  Each slot is @size bytes, and aligned to
- * @align bytes (which is at least @size).
+ * @align bytes (which is at least @size).  For dynamically-sized
+ * regsets, @n must contain the maximum possible number of slots for the
+ * regset, and @get_size must point to a function that returns the
+ * current regset size.
  *
- * These functions must be called only on the current thread or on a
- * thread that is in %TASK_STOPPED or %TASK_TRACED state, that we are
- * guaranteed will not be woken up and return to user mode, and that we
- * have called wait_task_inactive() on.  (The target thread always might
- * wake up for SIGKILL while these functions are working, in which case
- * that thread's user_regset state might be scrambled.)
+ * Callers that need to know only the current size of the regset and do
+ * not care about its internal structure should call regset_size()
+ * instead of inspecting @n or calling @get_size.
+ *
+ * For backward compatibility, the @get and @set methods must pad to, or
+ * accept, @n * @size bytes, even if the current regset size is smaller.
+ * The precise semantics of these operations depend on the regset being
+ * accessed.
+ *
+ * The functions to which &struct user_regset members point must be
+ * called only on the current thread or on a thread that is in
+ * %TASK_STOPPED or %TASK_TRACED state, that we are guaranteed will not
+ * be woken up and return to user mode, and that we have called
+ * wait_task_inactive() on.  (The target thread always might wake up for
+ * SIGKILL while these functions are working, in which case that
+ * thread's user_regset state might be scrambled.)
  *
  * The @pos argument must be aligned according to @align; the @count
  * argument must be a multiple of @size.  These functions are not
@@ -156,6 +192,7 @@ struct user_regset {
 	user_regset_set_fn		*set;
 	user_regset_active_fn		*active;
 	user_regset_writeback_fn	*writeback;
+	user_regset_get_size_fn		*get_size;
 	unsigned int			n;
 	unsigned int 			size;
 	unsigned int 			align;
@@ -371,5 +408,21 @@ static inline int copy_regset_from_user(struct task_struct *target,
 	return regset->set(target, regset, offset, size, NULL, data);
 }
 
+/**
+ * regset_size - determine the current size of a regset
+ * @target:	thread to be examined
+ * @regset:	regset to be examined
+ *
+ * Note that the returned size is valid only until the next time
+ * (if any) @regset is modified for @target.
+ */
+static inline unsigned int regset_size(struct task_struct *target,
+				       const struct user_regset *regset)
+{
+	if (!regset->get_size)
+		return regset->n * regset->size;
+	else
+		return regset->get_size(target, regset);
+}
 
 #endif	/* <linux/regset.h> */