LinuxLists.cc - [PATCH] ext4: fix undefined bit shift result in ext4_fill_flex

2012-10-03 07:11:55

Subject: [PATCH] ext4: fix undefined bit shift result in ext4_fill_flex_info

The result of the bit shift expression in
'1 << sbi->s_log_groups_per_flex' can be undefined in the case that
s_log_groups_per_flex is 31 because the result of the shift is bigger
than INT_MAX. In reality this probably should not cause much problems
since we'll end up with INT_MIN which will then be converted into
'unsigned int' type, but nevertheless according to the ISO C99 the
result is actually undefined.

Fix this by changing the left operand to 'unsigned int' type.

Note that the commit d50f2ab6f050311dbf7b8f5501b25f0bf64a439b already
tried to fix the undefined behaviour, but this was missed.

Thanks to Laszlo Ersek for pointing this out and suggesting the fix.

Signed-off-by: Lukas Czerner <[email protected]>
Reported-by: Laszlo Ersek <[email protected]>
---
fs/ext4/super.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 69c55d4..95b9c8e 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1929,7 +1929,7 @@ static int ext4_fill_flex_info(struct super_block *sb)
sbi->s_log_groups_per_flex = 0;
return 1;
}
- groups_per_flex = 1 << sbi->s_log_groups_per_flex;
+ groups_per_flex = 1U << sbi->s_log_groups_per_flex;

/* We allocate both existing and potentially added groups */
flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +
--
1.7.7.6

2012-10-03 14:18:58

by Carlos Maiolino

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Subject: Re: [PATCH] ext4: fix undefined bit shift result in ext4_fill_flex_info

On Wed, Oct 03, 2012 at 09:11:47AM +0200, Lukas Czerner wrote:
> The result of the bit shift expression in
> '1 << sbi->s_log_groups_per_flex' can be undefined in the case that
> s_log_groups_per_flex is 31 because the result of the shift is bigger
> than INT_MAX. In reality this probably should not cause much problems
> since we'll end up with INT_MIN which will then be converted into
> 'unsigned int' type, but nevertheless according to the ISO C99 the
> result is actually undefined.
>
> Fix this by changing the left operand to 'unsigned int' type.
>
> Note that the commit d50f2ab6f050311dbf7b8f5501b25f0bf64a439b already
> tried to fix the undefined behaviour, but this was missed.
>
> Thanks to Laszlo Ersek for pointing this out and suggesting the fix.
>
> Signed-off-by: Lukas Czerner <[email protected]>
> Reported-by: Laszlo Ersek <[email protected]>
> ---
> fs/ext4/super.c | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 69c55d4..95b9c8e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1929,7 +1929,7 @@ static int ext4_fill_flex_info(struct super_block *sb)
> sbi->s_log_groups_per_flex = 0;
> return 1;
> }
> - groups_per_flex = 1 << sbi->s_log_groups_per_flex;
> + groups_per_flex = 1U << sbi->s_log_groups_per_flex;
>
> /* We allocate both existing and potentially added groups */
> flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +

Looks good,

Reviewed-by: Carlos Maiolino <[email protected]>
--
--Carlos

2012-10-15 17:00:13

by Theodore Ts'o

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