linux-kernel - Re: [PATCH v2] isofs compress: Remove VLA usage

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Date:   Tue, 10 Apr 2018 12:57:44 +0200
From:   Jan Kara <jack@...e.cz>
To:     Kyle Spiers <ksspiers@...gle.com>
Cc:     jack@...e.cz, arnd@...db.de, dhowells@...hat.com,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        keescook@...omium.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] isofs compress: Remove VLA usage

On Thu 05-04-18 11:17:20, Kyle Spiers wrote:
> 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@...gle.com>

This is a good cleanup but the error recovery is hosed. See below...

> ---
>  fs/isofs/compress.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/isofs/compress.c b/fs/isofs/compress.c
> index 9bb2fe35799d..4eba16bf173c 100644
> --- a/fs/isofs/compress.c
> +++ b/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 +81,9 @@ 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)
> +		return -ENOMEM;

As Joe pointed out this needs to be:

	if (!bhs) {
		*errp = -ENOMEM;
		return 0;
	}

> @@ -330,6 +334,10 @@ 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)
> +		return -ENOMEM;

And this is wrong as well. You need to do:

	if (!pages) {
		unlock_page(page);
		return -ENOMEM;
	}


								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR