diff --git a/xen/arch/x86/cpu/mcheck/mce-apei.c b/xen/arch/x86/cpu/mcheck/mce-apei.c
index 31f2302ed5..08f6401055 100644
--- a/xen/arch/x86/cpu/mcheck/mce-apei.c
+++ b/xen/arch/x86/cpu/mcheck/mce-apei.c
@@ -89,10 +89,12 @@ int apei_write_mce(struct mce *m)
 	return erst_write(&rcd.hdr);
 }
 
-size_t apei_read_mce(struct mce *m, u64 *record_id)
+#ifndef NDEBUG /* currently dead code */
+
+ssize_t apei_read_mce(struct mce *m, u64 *record_id)
 {
 	struct cper_mce_record rcd;
-	size_t len;
+	ssize_t len;
 
 	if (!m || !record_id)
 		return -EINVAL;
@@ -115,12 +117,14 @@ size_t apei_read_mce(struct mce *m, u64 *record_id)
 }
 
 /* Check whether there is record in ERST */
-int apei_check_mce(void)
+bool_t apei_check_mce(void)
 {
-	return erst_get_record_count();
+	return erst_get_record_count() > 0;
 }
 
 int apei_clear_mce(u64 record_id)
 {
 	return erst_clear(record_id);
 }
+
+#endif /* currently dead code */
diff --git a/xen/common/lib.c b/xen/common/lib.c
index e3cb3f4730..a758025964 100644
--- a/xen/common/lib.c
+++ b/xen/common/lib.c
@@ -487,6 +487,9 @@ void __init init_constructors(void)
     const ctor_func_t *f;
     for ( f = __ctors_start; f < __ctors_end; ++f )
         (*f)();
+
+    /* Putting this here seems as good (or bad) as any other place. */
+    BUILD_BUG_ON(sizeof(size_t) != sizeof(ssize_t));
 }
 
 /*
diff --git a/xen/drivers/acpi/apei/erst.c b/xen/drivers/acpi/apei/erst.c
index 98f73d9061..2fafe7fcf7 100644
--- a/xen/drivers/acpi/apei/erst.c
+++ b/xen/drivers/acpi/apei/erst.c
@@ -383,21 +383,24 @@ static int erst_get_erange(struct erst_erange *range)
 	return 0;
 }
 
-static size_t __erst_get_record_count(void)
+static ssize_t __erst_get_record_count(void)
 {
 	struct apei_exec_context ctx;
 	int rc;
+	u64 output;
+	ssize_t count;
 
 	erst_exec_ctx_init(&ctx);
 	rc = apei_exec_run(&ctx, ACPI_ERST_GET_RECORD_COUNT);
 	if (rc)
 		return rc;
-	return apei_exec_ctx_get_output(&ctx);
+	count = output = apei_exec_ctx_get_output(&ctx);
+	return count >= 0 && count == output ? count : -ERANGE;
 }
 
-size_t erst_get_record_count(void)
+ssize_t erst_get_record_count(void)
 {
-	size_t count;
+	ssize_t count;
 	unsigned long flags;
 
 	if (!erst_enabled)
@@ -483,6 +486,8 @@ static int __erst_write_to_storage(u64 offset)
 	return erst_errno(val);
 }
 
+#ifndef NDEBUG /* currently dead code */
+
 static int __erst_read_from_storage(u64 record_id, u64 offset)
 {
 	struct apei_exec_context ctx;
@@ -565,6 +570,8 @@ static int __erst_clear_from_storage(u64 record_id)
 	return erst_errno(val);
 }
 
+#endif /* currently dead code */
+
 /* NVRAM ERST Error Log Address Range is not supported yet */
 static int __erst_write_to_nvram(const struct cper_record_header *record)
 {
@@ -572,6 +579,8 @@ static int __erst_write_to_nvram(const struct cper_record_header *record)
 	return -ENOSYS;
 }
 
+#ifndef NDEBUG /* currently dead code */
+
 static int __erst_read_to_erange_from_nvram(u64 record_id, u64 *offset)
 {
 	printk(KERN_WARNING
@@ -586,6 +595,8 @@ static int __erst_clear_from_nvram(u64 record_id)
 	return -ENOSYS;
 }
 
+#endif /* currently dead code */
+
 int erst_write(const struct cper_record_header *record)
 {
 	int rc;
@@ -625,6 +636,8 @@ int erst_write(const struct cper_record_header *record)
 	return rc;
 }
 
+#ifndef NDEBUG /* currently dead code */
+
 static int __erst_read_to_erange(u64 record_id, u64 *offset)
 {
 	int rc;
@@ -641,22 +654,24 @@ static int __erst_read_to_erange(u64 record_id, u64 *offset)
 	return 0;
 }
 
-static size_t __erst_read(u64 record_id, struct cper_record_header *record,
+static ssize_t __erst_read(u64 record_id, struct cper_record_header *record,
 			   size_t buflen)
 {
 	int rc;
-	u64 offset, len = 0;
+	u64 offset;
+	ssize_t len;
 	struct cper_record_header *rcd_tmp;
 
 	rc = __erst_read_to_erange(record_id, &offset);
 	if (rc)
 		return rc;
 	rcd_tmp = erst_erange.vaddr + offset;
+	if (rcd_tmp->record_length > buflen)
+		return -ENOBUFS;
 	len = rcd_tmp->record_length;
-	if (len <= buflen)
-		memcpy(record, rcd_tmp, len);
+	memcpy(record, rcd_tmp, len);
 
-	return len;
+	return len >= 0 ? len : -ERANGE;
 }
 
 /*
@@ -664,10 +679,10 @@ static size_t __erst_read(u64 record_id, struct cper_record_header *record,
  * else if return value < 0, something goes wrong,
  * else everything is OK, and return value is record length
  */
-size_t erst_read(u64 record_id, struct cper_record_header *record,
+ssize_t erst_read(u64 record_id, struct cper_record_header *record,
 		  size_t buflen)
 {
-	size_t len;
+	ssize_t len;
 	unsigned long flags;
 
 	if (!erst_enabled)
@@ -685,10 +700,10 @@ size_t erst_read(u64 record_id, struct cper_record_header *record,
  * else if return value < 0, something goes wrong,
  * else everything is OK, and return value is record length
  */
-size_t erst_read_next(struct cper_record_header *record, size_t buflen)
+ssize_t erst_read_next(struct cper_record_header *record, size_t buflen)
 {
 	int rc;
-	size_t len;
+	ssize_t len;
 	unsigned long flags;
 	u64 record_id;
 
@@ -731,6 +746,8 @@ int erst_clear(u64 record_id)
 	return rc;
 }
 
+#endif /* currently dead code */
+
 static int __init erst_check_table(struct acpi_table_erst *erst_tab)
 {
 	if (erst_tab->header.length < sizeof(*erst_tab))
diff --git a/xen/include/acpi/apei.h b/xen/include/acpi/apei.h
index 162f616ef0..d228595708 100644
--- a/xen/include/acpi/apei.h
+++ b/xen/include/acpi/apei.h
@@ -13,11 +13,11 @@
 #define FIX_APEI_RANGE_MAX 64
 
 int erst_write(const struct cper_record_header *record);
-size_t erst_get_record_count(void);
+ssize_t erst_get_record_count(void);
 int erst_get_next_record_id(u64 *record_id);
-size_t erst_read(u64 record_id, struct cper_record_header *record,
+ssize_t erst_read(u64 record_id, struct cper_record_header *record,
 		  size_t buflen);
-size_t erst_read_next(struct cper_record_header *record, size_t buflen);
+ssize_t erst_read_next(struct cper_record_header *record, size_t buflen);
 int erst_clear(u64 record_id);
 
 void __iomem *apei_pre_map(paddr_t paddr, unsigned long size);
diff --git a/xen/include/asm-arm/types.h b/xen/include/asm-arm/types.h
index 183f3e0733..09e5455364 100644
--- a/xen/include/asm-arm/types.h
+++ b/xen/include/asm-arm/types.h
@@ -60,6 +60,7 @@ typedef __SIZE_TYPE__ size_t;
 #else
 typedef unsigned long size_t;
 #endif
+typedef signed long ssize_t;
 
 typedef char bool_t;
 #define test_and_set_bool(b)   xchg(&(b), 1)
diff --git a/xen/include/asm-x86/types.h b/xen/include/asm-x86/types.h
index 9da305346b..b82fa58aa0 100644
--- a/xen/include/asm-x86/types.h
+++ b/xen/include/asm-x86/types.h
@@ -39,6 +39,7 @@ typedef __SIZE_TYPE__ size_t;
 #else
 typedef unsigned long size_t;
 #endif
+typedef signed long ssize_t;
 
 typedef char bool_t;
 #define test_and_set_bool(b)   xchg(&(b), 1)