[patch] ext4: underflow in alignment check

[patch] ext4: underflow in alignment check

[Dan Carpenter]

My static checker complains that this can underflow if arg is negative
which is true.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index b1a3471..a2a17e9 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1576,7 +1576,7 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
 	} else if (token = Opt_min_batch_time) {
 		sbi->s_min_batch_time = arg;
 	} else if (token = Opt_inode_readahead_blks) {
-		if (arg && (arg > (1 << 30) || !is_power_of_2(arg))) {
+		if (arg && (arg > (1U << 30) || !is_power_of_2(arg))) {
 			ext4_msg(sb, KERN_ERR,
 				 "EXT4-fs: inode_readahead_blks must be "
 				 "0 or a power of 2 smaller than 2^31");

Re: [patch] ext4: underflow in alignment check

[Jan Kara]

On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> My static checker complains that this can underflow if arg is negative
> which is true.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>

How come? (1 << 30) fits even into 32-bit signed type. So where's the
problem?

								Honza

> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index b1a3471..a2a17e9 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1576,7 +1576,7 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>  	} else if (token = Opt_min_batch_time) {
>  		sbi->s_min_batch_time = arg;
>  	} else if (token = Opt_inode_readahead_blks) {
> -		if (arg && (arg > (1 << 30) || !is_power_of_2(arg))) {
> +		if (arg && (arg > (1U << 30) || !is_power_of_2(arg))) {
>  			ext4_msg(sb, KERN_ERR,
>  				 "EXT4-fs: inode_readahead_blks must be "
>  				 "0 or a power of 2 smaller than 2^31");
> --
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-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

Re: [patch] ext4: underflow in alignment check

[Dan Carpenter]

On Mon, Jun 20, 2016 at 06:02:04PM +0200, Jan Kara wrote:
> On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> > My static checker complains that this can underflow if arg is negative
> > which is true.
> > 
> > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> 
> How come? (1 << 30) fits even into 32-bit signed type. So where's the
> problem?

Bad changelog...  I was talking about a different issue.  I was casting
it to unsigned to take advantage of type promototion.  Assume we have:

int arg = 1 << 31;

(arg > (1 << 30)) // <-- this is false
(arg > (1U << 30)) // <-- this is true so there is no underflow.

regards,
dan carpenter

Re: [patch] ext4: underflow in alignment check

[Jan Kara]

On Mon 20-06-16 22:53:26, Dan Carpenter wrote:
> On Mon, Jun 20, 2016 at 06:02:04PM +0200, Jan Kara wrote:
> > On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> > > My static checker complains that this can underflow if arg is negative
> > > which is true.
> > > 
> > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > 
> > How come? (1 << 30) fits even into 32-bit signed type. So where's the
> > problem?
> 
> Bad changelog...  I was talking about a different issue.  I was casting
> it to unsigned to take advantage of type promototion.  Assume we have:
> 
> int arg = 1 << 31;
> 
> (arg > (1 << 30)) // <-- this is false
> (arg > (1U << 30)) // <-- this is true so there is no underflow.

I see, but match_int() - or more precisely match_number() returns -ERANGE
when the number is > INT_MAX, subsequently we check whether the number is <
0 (Opt_inode_readahead_blks has flag MOPT_GTE0 set) and bail out if yes. So
at the place you are modifying we are sure the number is in [0, INT_MAX].
So the condition (arg > (1 << 30)) is pointless - just defensive
programming in case we decide e.g. to upgrade the type of 'arg' to long - but
not wrong...

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

Re: [patch] ext4: underflow in alignment check

[Dan Carpenter]

On Tue, Jun 21, 2016 at 09:43:53AM +0200, Jan Kara wrote:
> On Mon 20-06-16 22:53:26, Dan Carpenter wrote:
> > On Mon, Jun 20, 2016 at 06:02:04PM +0200, Jan Kara wrote:
> > > On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> > > > My static checker complains that this can underflow if arg is negative
> > > > which is true.
> > > > 
> > > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > 
> > > How come? (1 << 30) fits even into 32-bit signed type. So where's the
> > > problem?
> > 
> > Bad changelog...  I was talking about a different issue.  I was casting
> > it to unsigned to take advantage of type promototion.  Assume we have:
> > 
> > int arg = 1 << 31;
> > 
> > (arg > (1 << 30)) // <-- this is false
> > (arg > (1U << 30)) // <-- this is true so there is no underflow.
> 
> I see, but match_int() - or more precisely match_number() returns -ERANGE
> when the number is > INT_MAX, subsequently we check whether the number is <
> 0 (Opt_inode_readahead_blks has flag MOPT_GTE0 set) and bail out if yes. So
> at the place you are modifying we are sure the number is in [0, INT_MAX].
> So the condition (arg > (1 << 30)) is pointless - just defensive
> programming in case we decide e.g. to upgrade the type of 'arg' to long - but
> not wrong...

Ah.  Smatch wasn't able to figure out that MOPT_GTE0 was set.

Thanks for reviewing this.

regards,
dan carpenter