zram: check that backends array has at least one backend

Make sure that backends array has anything apart from the sentinel NULL
value.

We also select LZO_BACKEND if none backends were selected.

Link: https://lkml.kernel.org/r/20240902105656.1383858-13-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: sergey-senozhatsky

drivers/block/zram/Kconfig

@@ -14,12 +14,6 @@ config ZRAM
 
 	  See Documentation/admin-guide/blockdev/zram.rst for more information.
 
-config ZRAM_BACKEND_LZO
-	bool "lzo and lzo-rle compression support"
-	depends on ZRAM
-	select LZO_COMPRESS
-	select LZO_DECOMPRESS
-
 config ZRAM_BACKEND_LZ4
 	bool "lz4 compression support"
 	depends on ZRAM
@@ -50,6 +44,19 @@ config ZRAM_BACKEND_842
 	select 842_COMPRESS
 	select 842_DECOMPRESS
 
+config ZRAM_BACKEND_FORCE_LZO
+	depends on ZRAM
+	def_bool !ZRAM_BACKEND_LZ4 && !ZRAM_BACKEND_LZ4HC && \
+		!ZRAM_BACKEND_ZSTD && !ZRAM_BACKEND_DEFLATE && \
+		!ZRAM_BACKEND_842
+
+config ZRAM_BACKEND_LZO
+	bool "lzo and lzo-rle compression support" if !ZRAM_BACKEND_FORCE_LZO
+	depends on ZRAM
+	default ZRAM_BACKEND_FORCE_LZO
+	select LZO_COMPRESS
+	select LZO_DECOMPRESS
+
 choice
 	prompt "Default zram compressor"
 	default ZRAM_DEF_COMP_LZORLE

drivers/block/zram/zcomp.c

@@ -192,6 +192,14 @@ struct zcomp *zcomp_create(const char *alg)
 	struct zcomp *comp;
 	int error;
 
+	/*
+	 * The backends array has a sentinel NULL value, so the minimum
+	 * size is 1. In order to be valid the array, apart from the
+	 * sentinel NULL element, should have at least one compression
+	 * backend selected.
+	 */
+	BUILD_BUG_ON(ARRAY_SIZE(backends) <= 1);
+
 	comp = kzalloc(sizeof(struct zcomp), GFP_KERNEL);
 	if (!comp)
 		return ERR_PTR(-ENOMEM);