[PATCH][next] nvmem: core: Fix unintentional sign extension issue

[PATCH][next] nvmem: core: Fix unintentional sign extension issue

[Colin King]

From: Colin Ian King <colin.king@canonical.com>

The shifting of the u8 integer buf[3] by 24 bits to the left will
be promoted to a 32 bit signed int and then sign-extended to a
u64. In the event that the top bit of buf[3] is set then all
then all the upper 32 bits of the u64 end up as also being set
because of the sign-extension. Fix this by casting buf[i] to
a u64 before the shift.

Addresses-Coverity: ("Unintended sign extension")
Fixes: 097eb1136ebb ("nvmem: core: Add functions to make number reading easy")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
---
 drivers/nvmem/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 635e3131eb5f..bca671ff4e54 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -1693,7 +1693,7 @@ int nvmem_cell_read_variable_le_u64(struct device *dev, const char *cell_id,
 	/* Copy w/ implicit endian conversion */
 	*val = 0;
 	for (i = 0; i < len; i++)
-		*val |= buf[i] << (8 * i);
+		*val |= (uint64_t)buf[i] << (8 * i);
 
 	kfree(buf);
 
-- 
2.30.2

Re: [PATCH][next] nvmem: core: Fix unintentional sign extension issue

[Doug Anderson]

Hi,

On Thu, Mar 11, 2021 at 1:53 AM Colin King <colin.king@canonical.com> wrote:
>
> From: Colin Ian King <colin.king@canonical.com>
>
> The shifting of the u8 integer buf[3] by 24 bits to the left will
> be promoted to a 32 bit signed int and then sign-extended to a
> u64. In the event that the top bit of buf[3] is set then all
> then all the upper 32 bits of the u64 end up as also being set
> because of the sign-extension. Fix this by casting buf[i] to
> a u64 before the shift.
>
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: 097eb1136ebb ("nvmem: core: Add functions to make number reading easy")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  drivers/nvmem/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Thanks! I had only tested the "u64" version to read smaller data and
store it in a u64. From my understanding of C rules, without your
patch it would have been even worse than just a sign extension though,
right? Shifting "buf[i]" by more than 32 bits would just not have
worked right.

In any case:

Reviewed-by: Douglas Anderson <dianders@chromium.org>

Re: [PATCH][next] nvmem: core: Fix unintentional sign extension issue

[Colin Ian King]

On 11/03/2021 17:12, Doug Anderson wrote:
> Hi,
> 
> On Thu, Mar 11, 2021 at 1:53 AM Colin King <colin.king@canonical.com> wrote:
>>
>> From: Colin Ian King <colin.king@canonical.com>
>>
>> The shifting of the u8 integer buf[3] by 24 bits to the left will
>> be promoted to a 32 bit signed int and then sign-extended to a
>> u64. In the event that the top bit of buf[3] is set then all
>> then all the upper 32 bits of the u64 end up as also being set
>> because of the sign-extension. Fix this by casting buf[i] to
>> a u64 before the shift.
>>
>> Addresses-Coverity: ("Unintended sign extension")
>> Fixes: 097eb1136ebb ("nvmem: core: Add functions to make number reading easy")
>> Signed-off-by: Colin Ian King <colin.king@canonical.com>
>> ---
>>  drivers/nvmem/core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Thanks! I had only tested the "u64" version to read smaller data and
> store it in a u64. From my understanding of C rules, without your
> patch it would have been even worse than just a sign extension though,
> right? Shifting "buf[i]" by more than 32 bits would just not have
> worked right.

yep, that's correct, I forgot to mention that issue too :-/

> 
> In any case:
> 
> Reviewed-by: Douglas Anderson <dianders@chromium.org>
> 

Re: [PATCH][next] nvmem: core: Fix unintentional sign extension issue

[Srinivas Kandagatla]

On 11/03/2021 09:53, Colin King wrote:
> From: Colin Ian King <colin.king@canonical.com>
> 
> The shifting of the u8 integer buf[3] by 24 bits to the left will
> be promoted to a 32 bit signed int and then sign-extended to a
> u64. In the event that the top bit of buf[3] is set then all
> then all the upper 32 bits of the u64 end up as also being set
> because of the sign-extension. Fix this by casting buf[i] to
> a u64 before the shift.
> 
> Addresses-Coverity: ("Unintended sign extension")
> Fixes: 097eb1136ebb ("nvmem: core: Add functions to make number reading easy")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---

Applied thanks,

--srini

>   drivers/nvmem/core.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
> index 635e3131eb5f..bca671ff4e54 100644
> --- a/drivers/nvmem/core.c
> +++ b/drivers/nvmem/core.c
> @@ -1693,7 +1693,7 @@ int nvmem_cell_read_variable_le_u64(struct device *dev, const char *cell_id,
>   	/* Copy w/ implicit endian conversion */
>   	*val = 0;
>   	for (i = 0; i < len; i++)
> -		*val |= buf[i] << (8 * i);
> +		*val |= (uint64_t)buf[i] << (8 * i);
>   
>   	kfree(buf);
>   
>