[PATCH] char: hpet: Use flexible-array member

[PATCH] char: hpet: Use flexible-array member

[Gustavo A. R. Silva]

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, by making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
unadvertenly introduced[3] to the codebase from now on.

[1] https://github.com/KSPP/linux/issues/21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 drivers/char/hpet.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
index 9ac6671bb514..aed2c45f7968 100644
--- a/drivers/char/hpet.c
+++ b/drivers/char/hpet.c
@@ -110,7 +110,7 @@ struct hpets {
 	unsigned long hp_delta;
 	unsigned int hp_ntimer;
 	unsigned int hp_which;
-	struct hpet_dev hp_dev[1];
+	struct hpet_dev hp_dev[];
 };
 
 static struct hpets *hpets;
-- 
2.23.0

Re: [PATCH] char: hpet: Use flexible-array member

[Greg Kroah-Hartman]

On Mon, Jan 20, 2020 at 05:53:26PM -0600, Gustavo A. R. Silva wrote:
> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> presence of a "variable length array":
> 
> struct something {
>     int length;
>     u8 data[1];
> };
> 
> struct something *instance;
> 
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
> 
> There is also 0-byte arrays. Both cases pose confusion for things like
> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> to declare variable-length types such as the one above is a flexible array
> member[2] which need to be the last member of a structure and empty-sized:
> 
> struct something {
>         int stuff;
>         u8 data[];
> };
> 
> Also, by making use of the mechanism above, we will get a compiler warning
> in case the flexible array does not occur last in the structure, which
> will help us prevent some kind of undefined behavior bugs from being
> unadvertenly introduced[3] to the codebase from now on.
> 
> [1] https://github.com/KSPP/linux/issues/21
> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/char/hpet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> index 9ac6671bb514..aed2c45f7968 100644
> --- a/drivers/char/hpet.c
> +++ b/drivers/char/hpet.c
> @@ -110,7 +110,7 @@ struct hpets {
>  	unsigned long hp_delta;
>  	unsigned int hp_ntimer;
>  	unsigned int hp_which;
> -	struct hpet_dev hp_dev[1];
> +	struct hpet_dev hp_dev[];

Are you sure the allocation size is the same again?  Much like the
n_hdlc patch was, I think you need to adjust the variable size here.
Maybe, it's a bit of a pain to figure out at a quick glance, I just want
to make sure you at least do look at that :)

thanks,

greg k-h

Re: [PATCH] char: hpet: Use flexible-array member

[Gustavo A. R. Silva]

On 1/23/20 12:25, Greg Kroah-Hartman wrote:
> On Mon, Jan 20, 2020 at 05:53:26PM -0600, Gustavo A. R. Silva wrote:
>> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
>> presence of a "variable length array":
>>
>> struct something {
>>     int length;
>>     u8 data[1];
>> };
>>
>> struct something *instance;
>>
>> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
>> instance->length = size;
>> memcpy(instance->data, source, size);
>>
>> There is also 0-byte arrays. Both cases pose confusion for things like
>> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
>> to declare variable-length types such as the one above is a flexible array
>> member[2] which need to be the last member of a structure and empty-sized:
>>
>> struct something {
>>         int stuff;
>>         u8 data[];
>> };
>>
>> Also, by making use of the mechanism above, we will get a compiler warning
>> in case the flexible array does not occur last in the structure, which
>> will help us prevent some kind of undefined behavior bugs from being
>> unadvertenly introduced[3] to the codebase from now on.
>>
>> [1] https://github.com/KSPP/linux/issues/21
>> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>>
>> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
>> ---
>>  drivers/char/hpet.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
>> index 9ac6671bb514..aed2c45f7968 100644
>> --- a/drivers/char/hpet.c
>> +++ b/drivers/char/hpet.c
>> @@ -110,7 +110,7 @@ struct hpets {
>>  	unsigned long hp_delta;
>>  	unsigned int hp_ntimer;
>>  	unsigned int hp_which;
>> -	struct hpet_dev hp_dev[1];
>> +	struct hpet_dev hp_dev[];
> 
> Are you sure the allocation size is the same again?  Much like the
> n_hdlc patch was, I think you need to adjust the variable size here.
> Maybe, it's a bit of a pain to figure out at a quick glance, I just want
> to make sure you at least do look at that :)
> 

Yep. The allocation thing was already handled almost a year ago by the
following patch, and it didn't require to increase the size at that time:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=401c9bd10beef4b030eb9e34d16b5341dc6c683b

Thanks
--
Gustavo

Re: [PATCH] char: hpet: Use flexible-array member

[Greg Kroah-Hartman]

On Thu, Jan 23, 2020 at 12:46:42PM -0600, Gustavo A. R. Silva wrote:
> 
> 
> On 1/23/20 12:25, Greg Kroah-Hartman wrote:
> > On Mon, Jan 20, 2020 at 05:53:26PM -0600, Gustavo A. R. Silva wrote:
> >> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> >> presence of a "variable length array":
> >>
> >> struct something {
> >>     int length;
> >>     u8 data[1];
> >> };
> >>
> >> struct something *instance;
> >>
> >> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> >> instance->length = size;
> >> memcpy(instance->data, source, size);
> >>
> >> There is also 0-byte arrays. Both cases pose confusion for things like
> >> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> >> to declare variable-length types such as the one above is a flexible array
> >> member[2] which need to be the last member of a structure and empty-sized:
> >>
> >> struct something {
> >>         int stuff;
> >>         u8 data[];
> >> };
> >>
> >> Also, by making use of the mechanism above, we will get a compiler warning
> >> in case the flexible array does not occur last in the structure, which
> >> will help us prevent some kind of undefined behavior bugs from being
> >> unadvertenly introduced[3] to the codebase from now on.
> >>
> >> [1] https://github.com/KSPP/linux/issues/21
> >> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> >> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> >>
> >> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> >> ---
> >>  drivers/char/hpet.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> >> index 9ac6671bb514..aed2c45f7968 100644
> >> --- a/drivers/char/hpet.c
> >> +++ b/drivers/char/hpet.c
> >> @@ -110,7 +110,7 @@ struct hpets {
> >>  	unsigned long hp_delta;
> >>  	unsigned int hp_ntimer;
> >>  	unsigned int hp_which;
> >> -	struct hpet_dev hp_dev[1];
> >> +	struct hpet_dev hp_dev[];
> > 
> > Are you sure the allocation size is the same again?  Much like the
> > n_hdlc patch was, I think you need to adjust the variable size here.
> > Maybe, it's a bit of a pain to figure out at a quick glance, I just want
> > to make sure you at least do look at that :)
> > 
> 
> Yep. The allocation thing was already handled almost a year ago by the
> following patch, and it didn't require to increase the size at that time:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=401c9bd10beef4b030eb9e34d16b5341dc6c683b

Great, thanks for verifying, I'll go queue this up now.

greg k-h