pstore/ram: Refactor ftrace buffer merging

This changes the ftrace record merging code to be agnostic of
pstore/ram, as the first step to making it available as a generic
routine for other backends to use, such as pstore/zone.

Link: https://lore.kernel.org/lkml/20200510202436.63222-6-keescook@chromium.org/
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/ram.c

@@ -168,28 +168,29 @@ static bool prz_ok(struct persistent_ram_zone *prz)
 			   persistent_ram_ecc_string(prz, NULL, 0));
 }
 
-static ssize_t ftrace_log_combine(struct persistent_ram_zone *dest,
-				  struct persistent_ram_zone *src)
+static
+ssize_t ftrace_log_combine(char **dest_log, size_t *dest_log_size,
+			   const char *src_log, size_t src_log_size)
 {
 	size_t dest_size, src_size, total, dest_off, src_off;
 	size_t dest_idx = 0, src_idx = 0, merged_idx = 0;
 	void *merged_buf;
 	struct pstore_ftrace_record *drec, *srec, *mrec;
 	size_t record_size = sizeof(struct pstore_ftrace_record);
 
-	dest_off = dest->old_log_size % record_size;
-	dest_size = dest->old_log_size - dest_off;
+	dest_off = *dest_log_size % record_size;
+	dest_size = *dest_log_size - dest_off;
 
-	src_off = src->old_log_size % record_size;
-	src_size = src->old_log_size - src_off;
+	src_off = src_log_size % record_size;
+	src_size = src_log_size - src_off;
 
 	total = dest_size + src_size;
 	merged_buf = kmalloc(total, GFP_KERNEL);
 	if (!merged_buf)
 		return -ENOMEM;
 
-	drec = (struct pstore_ftrace_record *)(dest->old_log + dest_off);
-	srec = (struct pstore_ftrace_record *)(src->old_log + src_off);
+	drec = (struct pstore_ftrace_record *)(*dest_log + dest_off);
+	srec = (struct pstore_ftrace_record *)(src_log + src_off);
 	mrec = (struct pstore_ftrace_record *)(merged_buf);
 
 	while (dest_size > 0 && src_size > 0) {
@@ -213,9 +214,9 @@ static ssize_t ftrace_log_combine(struct persistent_ram_zone *dest,
 		src_size -= record_size;
 	}
 
-	kfree(dest->old_log);
-	dest->old_log = merged_buf;
-	dest->old_log_size = total;
+	kfree(*dest_log);
+	*dest_log = merged_buf;
+	*dest_log_size = total;
 
 	return 0;
 }
@@ -291,7 +292,11 @@ static ssize_t ramoops_pstore_read(struct pstore_record *record)
 				tmp_prz->corrected_bytes +=
 						prz_next->corrected_bytes;
 				tmp_prz->bad_blocks += prz_next->bad_blocks;
-				size = ftrace_log_combine(tmp_prz, prz_next);
+
+				size = ftrace_log_combine(&tmp_prz->old_log,
+						&tmp_prz->old_log_size,
+						prz_next->old_log,
+						prz_next->old_log_size);
 				if (size)
 					goto out;
 			}