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

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Message-ID: <20200715080451.GK23073@quack2.suse.cz>
Date:   Wed, 15 Jul 2020 10:04:51 +0200
From:   Jan Kara <jack@...e.cz>
To:     Tom Rix <trix@...hat.com>
Cc:     Matthew Wilcox <willy@...radead.org>, 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 14-07-20 06:12:47, Tom Rix wrote:
> 
> On 7/14/20 6:10 AM, Matthew Wilcox wrote:
> > 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.
> 
> Ok.

I agree with Matthew the patch will make the code more obviously correct so
it's a sensible cleanup. But the changelog needs to redone to reflect this
is just a cleanup before the patch can be merged.

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