[PATCH] ACPI: APEI: Extend BERT driver to provide a character device to access data

[Tony Luck]

The BERT table simply provides the size and address of the error
record in BIOS reserved memory. Currently this driver decodes the
record to the console log. But other users of BERT may want to access
the full binary record.

In an earlier age we might have used /dev/mem to retrieve this error
record, but many systems disable /dev/mem for security reasons.

Extend this driver to provide read-only access to the data via a character
special device "/dev/bert-data".

Cc: Len Brown <lenb@xxxxxxxxxx>
Cc: Boris Petkov <bp@xxxxxxx>
Cc: Tyler Baicar <tbaicar@xxxxxxxxxxxxxx>
Cc: Punit Agrawal <punit.agrawal@xxxxxxx>
Cc: linux-acpi@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>

v2: (suggested by Punit Agrawal) don't make a whole new driver, merge
    this functionality into the existing BERT driver.
---
 drivers/acpi/apei/bert.c | 45 ++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 42 insertions(+), 3 deletions(-)

diff --git a/drivers/acpi/apei/bert.c b/drivers/acpi/apei/bert.c
index 12771fcf0417..9bc39b1bffde 100644
--- a/drivers/acpi/apei/bert.c
+++ b/drivers/acpi/apei/bert.c
@@ -26,12 +26,16 @@
 #include <linux/init.h>
 #include <linux/acpi.h>
 #include <linux/io.h>
+#include <linux/miscdevice.h>
+#include <linux/uaccess.h>
 
 #include "apei-internal.h"
 
 #undef pr_fmt
 #define pr_fmt(fmt) "BERT: " fmt
 
+static __iomem void *bert_data;
+static unsigned int region_len;
 static int bert_disable;
 
 static void __init bert_print_all(struct acpi_bert_region *region,
@@ -95,12 +99,45 @@ static int __init bert_check_table(struct acpi_table_bert *bert_tab)
 	return 0;
 }
 
+static int bert_chrdev_open(struct inode *inode, struct file *file)
+{
+	if (file->f_flags & (O_WRONLY | O_RDWR))
+		return -EPERM;
+	inode->i_size = region_len;
+	return 0;
+}
+
+static ssize_t bert_chrdev_read(struct file *filp, char __user *ubuf,
+				size_t usize, loff_t *off)
+{
+	if (*off > region_len)
+		return -EINVAL;
+	if (*off + usize > region_len)
+		usize = region_len - *off;
+	if (copy_to_user(ubuf, bert_data + *off, usize))
+		return -EFAULT;
+	*off += usize;
+	return usize;
+}
+
+static const struct file_operations bert_chrdev_ops = {
+	.open		= bert_chrdev_open,
+	.read		= bert_chrdev_read,
+	.llseek		= default_llseek,
+};
+
+static struct miscdevice bert_chrdev_device = {
+	.minor		= MISC_DYNAMIC_MINOR,
+	.name		= "bert-data",
+	.fops		= &bert_chrdev_ops,
+	.mode		= 0444,
+};
+
 static int __init bert_init(void)
 {
-	struct apei_resources bert_resources;
 	struct acpi_bert_region *boot_error_region;
+	struct apei_resources bert_resources;
 	struct acpi_table_bert *bert_tab;
-	unsigned int region_len;
 	acpi_status status;
 	int rc = 0;
 
@@ -139,7 +176,9 @@ static int __init bert_init(void)
 	boot_error_region = ioremap_cache(bert_tab->address, region_len);
 	if (boot_error_region) {
 		bert_print_all(boot_error_region, region_len);
-		iounmap(boot_error_region);
+		bert_data = boot_error_region;
+		if (misc_register(&bert_chrdev_device))
+			iounmap(boot_error_region);
 	} else {
 		rc = -ENOMEM;
 	}
-- 
2.11.0