isofs compress: Remove VLA usage

As part of the effort to remove VLAs from the kernel[1], this changes
the allocation of the bhs and pages arrays from being on the stack to being
kcalloc()ed. This also allows for the removal of the explicit zeroing
of bhs.

https://lkml.org/lkml/2018/3/7/621

Signed-off-by: Kyle Spiers <ksspiers@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/isofs/compress.c

@@ -20,6 +20,7 @@
 #include <linux/init.h>
 #include <linux/bio.h>
 
+#include <linux/slab.h>
 #include <linux/vmalloc.h>
 #include <linux/zlib.h>
 
@@ -59,7 +60,7 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 				>> bufshift;
 	int haveblocks;
 	blkcnt_t blocknum;
-	struct buffer_head *bhs[needblocks + 1];
+	struct buffer_head **bhs;
 	int curbh, curpage;
 
 	if (block_size > deflateBound(1UL << zisofs_block_shift)) {
@@ -80,7 +81,11 @@ static loff_t zisofs_uncompress_block(struct inode *inode, loff_t block_start,
 
 	/* Because zlib is not thread-safe, do all the I/O at the top. */
 	blocknum = block_start >> bufshift;
-	memset(bhs, 0, (needblocks + 1) * sizeof(struct buffer_head *));
+	bhs = kcalloc(needblocks + 1, sizeof(*bhs), GFP_KERNEL);
+	if (!bhs) {
+		*errp = -ENOMEM;
+		return 0;
+	}
 	haveblocks = isofs_get_blocks(inode, blocknum, bhs, needblocks);
 	ll_rw_block(REQ_OP_READ, 0, haveblocks, bhs);
 
@@ -190,6 +195,7 @@ z_eio:
 b_eio:
 	for (i = 0; i < haveblocks; i++)
 		brelse(bhs[i]);
+	kfree(bhs);
 	return stream.total_out;
 }
 
@@ -305,7 +311,7 @@ static int zisofs_readpage(struct file *file, struct page *page)
 	unsigned int zisofs_pages_per_cblock =
 		PAGE_SHIFT <= zisofs_block_shift ?
 		(1 << (zisofs_block_shift - PAGE_SHIFT)) : 0;
-	struct page *pages[max_t(unsigned, zisofs_pages_per_cblock, 1)];
+	struct page **pages;
 	pgoff_t index = page->index, end_index;
 
 	end_index = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
@@ -330,6 +336,12 @@ static int zisofs_readpage(struct file *file, struct page *page)
 		full_page = 0;
 		pcount = 1;
 	}
+	pages = kcalloc(max_t(unsigned int, zisofs_pages_per_cblock, 1),
+					sizeof(*pages), GFP_KERNEL);
+	if (!pages) {
+		unlock_page(page);
+		return -ENOMEM;
+	}
 	pages[full_page] = page;
 
 	for (i = 0; i < pcount; i++, index++) {
@@ -357,6 +369,7 @@ static int zisofs_readpage(struct file *file, struct page *page)
 	}			
 
 	/* At this point, err contains 0 or -EIO depending on the "critical" page */
+	kfree(pages);
 	return err;
 }