pstore: Refactor compression initialization

This refactors compression initialization slightly to better handle
getting potentially called twice (via early pstore_register() calls
and later pstore_init()) and improves the comments and reporting to be
more verbose.
Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>

fs/pstore/platform.c

@@ -274,36 +274,56 @@ static int pstore_decompress(void *in, void *out,
 
 static void allocate_buf_for_compression(void)
 {
+	struct crypto_comp *ctx;
+	int size;
+	char *buf;
+
+	/* Skip if not built-in or compression backend not selected yet. */
 	if (!IS_ENABLED(CONFIG_PSTORE_COMPRESS) || !zbackend)
 		return;
 
+	/* Skip if no pstore backend yet or compression init already done. */
+	if (!psinfo || tfm)
+		return;
+
 	if (!crypto_has_comp(zbackend->name, 0, 0)) {
-		pr_err("No %s compression\n", zbackend->name);
+		pr_err("Unknown compression: %s\n", zbackend->name);
 		return;
 	}
 
-	big_oops_buf_sz = zbackend->zbufsize(psinfo->bufsize);
-	if (big_oops_buf_sz <= 0)
+	size = zbackend->zbufsize(psinfo->bufsize);
+	if (size <= 0) {
+		pr_err("Invalid compression size for %s: %d\n",
+		       zbackend->name, size);
 		return;
+	}
 
-	big_oops_buf = kmalloc(big_oops_buf_sz, GFP_KERNEL);
-	if (!big_oops_buf) {
-		pr_err("allocate compression buffer error!\n");
+	buf = kmalloc(size, GFP_KERNEL);
+	if (!buf) {
+		pr_err("Failed %d byte compression buffer allocation for: %s\n",
+		       size, zbackend->name);
 		return;
 	}
 
-	tfm = crypto_alloc_comp(zbackend->name, 0, 0);
-	if (IS_ERR_OR_NULL(tfm)) {
-		kfree(big_oops_buf);
-		big_oops_buf = NULL;
-		pr_err("crypto_alloc_comp() failed!\n");
+	ctx = crypto_alloc_comp(zbackend->name, 0, 0);
+	if (IS_ERR_OR_NULL(ctx)) {
+		kfree(buf);
+		pr_err("crypto_alloc_comp('%s') failed: %ld\n", zbackend->name,
+		       PTR_ERR(ctx));
 		return;
 	}
+
+	/* A non-NULL big_oops_buf indicates compression is available. */
+	tfm = ctx;
+	big_oops_buf_sz = size;
+	big_oops_buf = buf;
+
+	pr_info("Using compression: %s\n", zbackend->name);
 }
 
 static void free_buf_for_compression(void)
 {
-	if (IS_ENABLED(CONFIG_PSTORE_COMPRESS) && !IS_ERR_OR_NULL(tfm))
+	if (IS_ENABLED(CONFIG_PSTORE_COMPRESS) && tfm)
 		crypto_free_comp(tfm);
 	kfree(big_oops_buf);
 	big_oops_buf = NULL;
@@ -774,7 +794,6 @@ void __init pstore_choose_compression(void)
 	for (step = zbackends; step->name; step++) {
 		if (!strcmp(compress, step->name)) {
 			zbackend = step;
-			pr_info("using %s compression\n", zbackend->name);
 			return;
 		}
 	}
@@ -791,7 +810,6 @@ static int __init pstore_init(void)
 	 * initialize compression because crypto was not ready. If so,
 	 * initialize compression now.
 	 */
-	if (psinfo && !tfm)
 	allocate_buf_for_compression();
 
 	ret = pstore_init_fs();