[RFC][PATCH 2/2] write callback: Check if existing entry is erasable

[Seiji Aguchi <seiji.aguchi@xxxxxxx>]

This patch introduces a following rule checking if an existing entry in NVRAM are erasable to avoid missing 
a critical message ,such as panic, before an user check it.

    - In case where an existing entry is panic or emergency
      -  It is not erasable because if panic/emergency event is lost, we have no way to detect 
         the root cause. We shouldn't overwrite them for any reason.

    - In case where an existing entry is oops/shutdown/halt/poweroff
      -  It is erasable if an error ,panic, emergency or oops, happen in new event.
           - Oops is erasable because system may still be running and its message may be saved 
             into /var/log/messages.
           - Also, normal event ,shutdown/halt/poweroff, is erasable because error message is 
             more important than normal message.

    - In case where an existing entry is unknown event
      -  It is erasable because any event supported by pstore outweighs unsupported events.

Signed-off-by: Seiji Aguchi <seiji.aguchi@xxxxxxx>
---
 drivers/firmware/efivars.c |   48 +++++++++++++++++++++++++++++++++++++++++++-
 fs/pstore/platform.c       |    4 +-
 include/linux/pstore.h     |    5 ++++
 3 files changed, 54 insertions(+), 3 deletions(-)

diff --git a/drivers/firmware/efivars.c b/drivers/firmware/efivars.c index 4929254..54d9bc6 100644
--- a/drivers/firmware/efivars.c
+++ b/drivers/firmware/efivars.c
@@ -685,6 +685,45 @@ static ssize_t efi_pstore_read(u64 *id, enum pstore_type_id *type,
 	return 0;
 }
 
+
+static bool can_erase_entry(struct efivar_entry *entry, enum kmsg_dump_reason
+			   new_reason)
+{
+	int prev_reason = 0;
+	const char *prev_why;
+	bool is_erasable = 0;
+
+	/* Get a reason of previous message */
+	while (1) {
+		prev_why =  pstore_get_reason_str(prev_reason);
+		if (!strncmp(entry->var.Data, prev_why, strlen(prev_why)))
+			break;
+		prev_reason++;
+	}
+
+	/* check if exisiting message is erasable */
+
+	switch (prev_reason) {
+	case KMSG_DUMP_PANIC:
+	case KMSG_DUMP_EMERG:
+		/* Never erase panic or emergency message */
+		break;
+	case KMSG_DUMP_OOPS:
+	case KMSG_DUMP_RESTART:
+	case KMSG_DUMP_HALT:
+	case KMSG_DUMP_POWEROFF:
+		/* Can erase if new one is error message */
+		if (new_reason <= KMSG_DUMP_EMERG)
+			is_erasable = 1;
+		break;
+	default:
+		/* Can erase unknown message */
+		is_erasable = 1;
+	}
+
+	return is_erasable;
+}
+
 static int efi_pstore_write(enum pstore_type_id type,
 		enum kmsg_dump_reason reason, u64 *id,
 		unsigned int part, size_t size, struct pstore_info *psi) @@ -706,7 +745,7 @@ static int efi_pstore_write(enum pstore_type_id type,
 		efi_name[i] = stub_name[i];
 
 	/*
-	 * Clean up any entries with the same name
+	 * Search for erasable entry
 	 */
 
 	list_for_each_entry(entry, &efivars->list, list) { @@ -721,6 +760,13 @@ static int efi_pstore_write(enum pstore_type_id type,
 		if (entry->var.VariableName[utf16_strlen(efi_name)] == 0)
 			continue;
 
+		if (!can_erase_entry(entry, reason)) {
+			/* return without writing new entry */
+			spin_unlock(&efivars->lock);
+			*id = part;
+			return ret;
+		}
+
 		/* found */
 		found = entry;
 		efivars->ops->set_variable(entry->var.VariableName,
diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c index 03ce7a9..32715eb 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -68,7 +68,7 @@ void pstore_set_kmsg_bytes(int bytes)
 /* Tag each group of saved records with a sequence number */
 static int	oopscount;
 
-static const char *get_reason_str(enum kmsg_dump_reason reason)
+const char *pstore_get_reason_str(enum kmsg_dump_reason reason)
 {
 	switch (reason) {
 	case KMSG_DUMP_PANIC:
@@ -104,7 +104,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	int		is_locked = 0;
 	int		ret;
 
-	why = get_reason_str(reason);
+	why = pstore_get_reason_str(reason);
 
 	if (in_nmi()) {
 		is_locked = spin_trylock(&psinfo->buf_lock); diff --git a/include/linux/pstore.h b/include/linux/pstore.h index e1461e1..e9347e9 100644
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -54,12 +54,17 @@ struct pstore_info {
 
 #ifdef CONFIG_PSTORE
 extern int pstore_register(struct pstore_info *);
+extern const char *pstore_get_reason_str(enum kmsg_dump_reason reason);
 #else
 static inline int
 pstore_register(struct pstore_info *psi)  {
 	return -ENODEV;
 }
+static const char *pstore_get_reason_str(enum kmsg_dump_reason reason) 
+{
+	return NULL;
+}
 #endif
 
 #endif /*_LINUX_PSTORE_H*/
--
1.7.1


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