introduction of regset ->get() wrappers, switching ELF coredumps to those

Two new helpers: given a process and regset, dump into a buffer.
regset_get() takes a buffer and size, regset_get_alloc() takes size
and allocates a buffer.

Return value in both cases is the amount of data actually dumped in
case of success or -E...  on error.

In both cases the size is capped by regset->n * regset->size, so
->get() is called with offset 0 and size no more than what regset
expects.

binfmt_elf.c callers of ->get() are switched to using those; the other
caller (copy_regset_to_user()) will need some preparations to switch.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/binfmt_elf.c

@@ -1821,7 +1821,7 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 				 long signr, size_t *total)
 {
 	unsigned int i;
-	unsigned int regset0_size = regset_size(t->task, &view->regsets[0]);
+	int regset0_size;
 
 	/*
 	 * NT_PRSTATUS is the one special case, because the regset data
@@ -1830,8 +1830,10 @@ 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, regset0_size,
-				    &t->prstatus.pr_reg, NULL);
+	regset0_size = regset_get(t->task, &view->regsets[0],
+		   sizeof(t->prstatus.pr_reg), &t->prstatus.pr_reg);
+	if (regset0_size < 0)
+		return 0;
 
 	fill_note(&t->notes[0], "CORE", NT_PRSTATUS,
 		  PRSTATUS_SIZE(t->prstatus, regset0_size), &t->prstatus);
@@ -1846,33 +1848,29 @@ static int fill_thread_core_info(struct elf_thread_core_info *t,
 	 */
 	for (i = 1; i < view->n; ++i) {
 		const struct user_regset *regset = &view->regsets[i];
-		do_thread_regset_writeback(t->task, regset);
-		if (regset->core_note_type && regset->get &&
-		    (!regset->active || regset->active(t->task, regset) > 0)) {
+		int note_type = regset->core_note_type;
+		bool is_fpreg = note_type == NT_PRFPREG;
+		void *data;
 		int ret;
-			size_t size = regset_size(t->task, regset);
-			void *data = kzalloc(size, GFP_KERNEL);
-			if (unlikely(!data))
-				return 0;
-			ret = regset->get(t->task, regset,
-					  0, size, data, NULL);
-			if (unlikely(ret))
-				kfree(data);
-			else {
-				if (regset->core_note_type != NT_PRFPREG)
-					fill_note(&t->notes[i], "LINUX",
-						  regset->core_note_type,
-						  size, data);
-				else {
-					SET_PR_FPVALID(&t->prstatus,
-							1, regset0_size);
-					fill_note(&t->notes[i], "CORE",
-						  NT_PRFPREG, size, data);
-				}
+
+		do_thread_regset_writeback(t->task, regset);
+		if (!note_type) // not for coredumps
+			continue;
+		if (regset->active && regset->active(t->task, regset) <= 0)
+			continue;
+
+		ret = regset_get_alloc(t->task, regset, ~0U, &data);
+		if (ret < 0)
+			continue;
+
+		if (is_fpreg)
+			SET_PR_FPVALID(&t->prstatus, 1, regset0_size);
+
+		fill_note(&t->notes[i], is_fpreg ? "CORE" : "LINUX",
+			  note_type, ret, data);
+
 		*total += notesize(&t->notes[i]);
 	}
-		}
-	}
 
 	return 1;
 }

include/linux/regset.h

@@ -353,6 +353,15 @@ static inline int user_regset_copyin_ignore(unsigned int *pos,
 	return 0;
 }
 
+extern int regset_get(struct task_struct *target,
+		      const struct user_regset *regset,
+		      unsigned int size, void *data);
+
+extern int regset_get_alloc(struct task_struct *target,
+			    const struct user_regset *regset,
+			    unsigned int size,
+			    void **data);
+
 /**
  * copy_regset_to_user - fetch a thread's user_regset data into user memory
  * @target:	thread to be examined

kernel/Makefile

@@ -10,7 +10,7 @@ obj-y     = fork.o exec_domain.o panic.o \
 	    extable.o params.o \
 	    kthread.o sys_ni.o nsproxy.o \
 	    notifier.o ksysfs.o cred.o reboot.o \
-	    async.o range.o smpboot.o ucount.o
+	    async.o range.o smpboot.o ucount.o regset.o
 
 obj-$(CONFIG_MODULES) += kmod.o
 obj-$(CONFIG_MULTIUSER) += groups.o

kernel/regset.c

@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#include <linux/export.h>
+#include <linux/slab.h>
+#include <linux/regset.h>
+
+static int __regset_get(struct task_struct *target,
+			const struct user_regset *regset,
+			unsigned int size,
+			void **data)
+{
+	void *p = *data, *to_free = NULL;
+	int res;
+
+	if (!regset->get)
+		return -EOPNOTSUPP;
+	if (size > regset->n * regset->size)
+		size = regset->n * regset->size;
+	if (!p) {
+		to_free = p = kzalloc(size, GFP_KERNEL);
+		if (!p)
+			return -ENOMEM;
+	}
+	res = regset->get(target, regset, 0, size, p, NULL);
+	if (unlikely(res < 0)) {
+		kfree(to_free);
+		return res;
+	}
+	*data = p;
+	if (regset->get_size) { // arm64-only kludge, will go away
+		unsigned max_size = regset->get_size(target, regset);
+		if (size > max_size)
+			size = max_size;
+	}
+	return size;
+}
+
+int regset_get(struct task_struct *target,
+	       const struct user_regset *regset,
+	       unsigned int size,
+	       void *data)
+{
+	return __regset_get(target, regset, size, &data);
+}
+EXPORT_SYMBOL(regset_get);
+
+int regset_get_alloc(struct task_struct *target,
+		     const struct user_regset *regset,
+		     unsigned int size,
+		     void **data)
+{
+	*data = NULL;
+	return __regset_get(target, regset, size, data);
+}
+EXPORT_SYMBOL(regset_get_alloc);