linux-kernel - [RFC] Coverity 1128444 - Dereference after null check (FORWARD

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Message-ID: <CAGG-pUSSBi7+AmN09hVLeD0Qu-uA7iRPepMtr7X154yfbK4C6A@mail.gmail.com>
Date:	Tue, 12 Nov 2013 12:53:09 -0200
From:	Geyslan Gregório Bem <geyslan@...il.com>
To:	swhiteho@...hat.com
Cc:	cluster-devel@...hat.com, LKML <linux-kernel@...r.kernel.org>
Subject: [RFC] Coverity 1128444 - Dereference after null check (FORWARD_NULL)
 in glock.c

Hello,

Coverity detected in 'fs/gfs2/glock.c' a possible dereference after
null check. Maybe a positive one.

There is a initial check for possible 'gl' NULL. After that 'gl' is
dereferenced in the looping check by __lockref_is_dead().

What do you think?


1875static int gfs2_glock_iter_next(struct gfs2_glock_iter *gi)
1876{
1877        struct gfs2_glock *gl;
1878
1879        do {
1880                gl = gi->gl;

1. Condition "gl", taking false branch

2. var_compare_op: Comparing "gl" to null implies that "gl" might be null.
1881                if (gl) {
1882                        gi->gl = glock_hash_next(gl);
1883                        gi->nhash++;
1884                } else {

3. Condition "gi->hash >= (32768 /* 1 << 15 */)", taking false branch
1885                        if (gi->hash >= GFS2_GL_HASH_SIZE) {
1886                                rcu_read_unlock();
1887                                return 1;
1888                        }
1889                        gi->gl = glock_hash_chain(gi->hash);
1890                        gi->nhash = 0;
1891                }

4. Condition "gi->gl == NULL", taking true branch

7. Condition "gi->gl == NULL", taking false branch
1892                while (gi->gl == NULL) {
1893                        gi->hash++;

5. Condition "gi->hash >= (32768 /* 1 << 15 */)", taking false branch
1894                        if (gi->hash >= GFS2_GL_HASH_SIZE) {
1895                                rcu_read_unlock();
1896                                return 1;
1897                        }
1898                        gi->gl = glock_hash_chain(gi->hash);
1899                        gi->nhash = 0;

6. Jumping back to the beginning of the loop
1900                }
1901        /* Skip entries for other sb and dead entries */

8. Condition "gi->sdp != gi->gl->gl_sbd", taking false branch

CID 1128444 (#1 of 1): Dereference after null check (FORWARD_NULL)9.
var_deref_model: Passing null pointer "&gl->gl_lockref" to function
"__lockref_is_dead(struct lockref const *)", which dereferences
it.[show details]
1902        } while (gi->sdp != gi->gl->gl_sbd ||
__lockref_is_dead(&gl->gl_lockref));
1903
1904        return 0;
1905}

-- 
Regards,

Geyslan G. Bem
hackingbits.com
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