Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
On Wed, 24 Jul 2019 at 12:55, Steven Whitehouse wrote: > On 24/07/2019 11:27, Christoph Hellwig wrote: > > On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote: > >> and it would have the same effect, so far as I can tell. I don't mind > >> changing it, if that is perhaps a clearer way to write the same thing, > >> rather than >i_inode; > > The cleanest thing is to not rely on any of that magic and write it > > like all other file systems: > > > > ip = kmem_cache_alloc > > if (!ip) > > retuturn NULL; > > > > ... > > > > return >i_inode; > > > > Absolutely not point in trying to be clever here. > > Yes, that works too, I'll change that ... Andreas
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
Hi, On 24/07/2019 11:27, Christoph Hellwig wrote: On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote: and it would have the same effect, so far as I can tell. I don't mind changing it, if that is perhaps a clearer way to write the same thing, rather than >i_inode; The cleanest thing is to not rely on any of that magic and write it like all other file systems: ip = kmem_cache_alloc if (!ip) retuturn NULL; ... return >i_inode; Absolutely not point in trying to be clever here. Yes, that works too, Steve.
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote: > and it would have the same effect, so far as I can tell. I don't mind > changing it, if that is perhaps a clearer way to write the same thing, > rather than >i_inode; The cleanest thing is to not rely on any of that magic and write it like all other file systems: ip = kmem_cache_alloc if (!ip) retuturn NULL; ... return >i_inode; Absolutely not point in trying to be clever here.
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
Hi, On 24/07/2019 11:02, Christoph Hellwig wrote: On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote: Hi, On 24/07/2019 09:43, Jia-Ju Bai wrote: In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns NULL, ip is assigned to NULL. In this case, "return >i_inode" will cause a null-pointer dereference. To fix this null-pointer dereference, NULL is returned when ip is NULL. This bug is found by a static analysis tool STCheck written by us. The bug is in the tool I'm afraid. Since i_inode is the first element of ip, there is no NULL dereference here. A pointer to ip->i_inode and a pointer to ip are one and the same (bar the differing types) which is the reason that we return >i_inode rather than just ip, But that doesn't help if ip is NULL, as dereferencing a field in in still remains invalid behavior. We are not dereferencing it though really, we are taking the address of the field... we could have written: return (struct inode *)ip; and it would have the same effect, so far as I can tell. I don't mind changing it, if that is perhaps a clearer way to write the same thing, rather than >i_inode; Steve.
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
On 24/07/2019 11:02, Christoph Hellwig wrote: > On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote: >> Hi, >> >> On 24/07/2019 09:43, Jia-Ju Bai wrote: >>> In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns >>> NULL, ip is assigned to NULL. In this case, "return >i_inode" will >>> cause a null-pointer dereference. >>> >>> To fix this null-pointer dereference, NULL is returned when ip is NULL. >>> >>> This bug is found by a static analysis tool STCheck written by us. >> >> The bug is in the tool I'm afraid. Since i_inode is the first element of ip, >> there is no NULL dereference here. A pointer to ip->i_inode and a pointer to >> ip are one and the same (bar the differing types) which is the reason that >> we return >i_inode rather than just ip, > > But that doesn't help if ip is NULL, as dereferencing a field in in > still remains invalid behavior. > According to C99 you may be right that it is undefined behaviour, that just happens to work correctly on current versions of GCC, see [1]. Although as pointed out in [1] this undefined behaviour was relied upon to implement the offsetof macro (nowadays that should be a compiler built-in, see include/linux/stddef.h). I wish that cases like these would be more clearly defined in the C standard, at least as implementation defined behavior such that GCC could then be explicit about what semantics it wants and document it here: https://gcc.gnu.org/onlinedocs/gcc/#toc-C-Implementation-Defined-Behavior It is difficult to write future-proof code if the compiler can keep changing its mind about semantics of code that it has previously accepted. [1] https://software.intel.com/en-us/blogs/2015/04/20/null-pointer-dereferencing-causes-undefined-behavior Best regards, --Edwin
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote: > Hi, > > On 24/07/2019 09:43, Jia-Ju Bai wrote: > > In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns > > NULL, ip is assigned to NULL. In this case, "return >i_inode" will > > cause a null-pointer dereference. > > > > To fix this null-pointer dereference, NULL is returned when ip is NULL. > > > > This bug is found by a static analysis tool STCheck written by us. > > The bug is in the tool I'm afraid. Since i_inode is the first element of ip, > there is no NULL dereference here. A pointer to ip->i_inode and a pointer to > ip are one and the same (bar the differing types) which is the reason that > we return >i_inode rather than just ip, But that doesn't help if ip is NULL, as dereferencing a field in in still remains invalid behavior.
Re: [Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()
Hi, On 24/07/2019 09:43, Jia-Ju Bai wrote: In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns NULL, ip is assigned to NULL. In this case, "return >i_inode" will cause a null-pointer dereference. To fix this null-pointer dereference, NULL is returned when ip is NULL. This bug is found by a static analysis tool STCheck written by us. The bug is in the tool I'm afraid. Since i_inode is the first element of ip, there is no NULL dereference here. A pointer to ip->i_inode and a pointer to ip are one and the same (bar the differing types) which is the reason that we return >i_inode rather than just ip, Steve. Signed-off-by: Jia-Ju Bai --- fs/gfs2/super.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-) diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c index 0acc5834f653..c07c3f4f8451 100644 --- a/fs/gfs2/super.c +++ b/fs/gfs2/super.c @@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb) memset(>i_res, 0, sizeof(ip->i_res)); RB_CLEAR_NODE(>i_res.rs_node); ip->i_rahead = 0; - } - return >i_inode; + return >i_inode; + } else + return NULL; } static void gfs2_free_inode(struct inode *inode)
