linux-kernel - Re: [PATCH] reiserfs : fix improper free in reiserfs_get

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Date:   Tue, 14 Jul 2020 14:10:43 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     trix@...hat.com
Cc:     jack@...e.cz, william.kucharski@...cle.com, jeffm@...e.com,
        joseph.qi@...ux.alibaba.com, liao.pingfang@....com.cn,
        reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] reiserfs : fix improper free in reiserfs_get_block

On Tue, Jul 14, 2020 at 06:05:09AM -0700, trix@...hat.com wrote:
> From: Tom Rix <trix@...hat.com>
> 
> clang static analysis flags this error
> 
> inode.c:1083:5: warning: Argument to kfree() is the address of the
>   local variable 'unf_single', which is not memory allocated by
>   malloc() [unix.Malloc]
>                                 kfree(un);
>                                 ^~~~~~~~~
> Assignment of 'un'
> 
> 	/*
> 	 * We use this in case we need to allocate
> 	 * only one block which is a fastpath
> 	 */
> 	unp_t unf_single = 0;
> 
> 	...
> 
> 	if (blocks_needed == 1) {
> 		un = &unf_single;
> 	} else {
> 		un = kcalloc(min(blocks_needed, max_to_insert),
> 			     UNFM_P_SIZE, GFP_NOFS);
> 		if (!un) {
> 			un = &unf_single;
> 			blocks_needed = 1;
> 			max_to_insert = 0;
> 		}
> 	}
> 
> The logic to free 'un'
> 
> 	if (blocks_needed != 1)
> 		kfree(un);
> 
> Because the kcalloc failure falls back to using unf_single,
> the if-check for the free is wrong.

I think you mean "Because clang's static analysis is limited, it
warns incorrectly about this".  There's no path to get to the
kfree with blocks_needed != 1 and un being equal to &unf_single.

> So improve the check.
> 
> Signed-off-by: Tom Rix <trix@...hat.com>
> ---
>  fs/reiserfs/inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/reiserfs/inode.c b/fs/reiserfs/inode.c
> index 1509775da040..4d62148e43e6 100644
> --- a/fs/reiserfs/inode.c
> +++ b/fs/reiserfs/inode.c
> @@ -1079,7 +1079,7 @@ int reiserfs_get_block(struct inode *inode, sector_t block,
>  						     UNFM_P_SIZE *
>  						     blocks_needed);
>  
> -			if (blocks_needed != 1)
> +			if (un != &unf_single)
>  				kfree(un);

I don't actually object to this patch, but your analysis of clang's
analysis is wrong.