diff mbox series

USB: serial: garmin_gps: Use flexible-array member

Message ID 20200116220327.GA12537@embeddedor.com (mailing list archive)
State Mainlined
Commit 50c3c5e1c1b000d6a321ffdc0003bc6b7ac0b0e5
Headers show
Series USB: serial: garmin_gps: Use flexible-array member | expand

Commit Message

Gustavo A. R. Silva Jan. 16, 2020, 10:03 p.m. UTC
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/usb/serial/garmin_gps.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Johan Hovold Jan. 17, 2020, 10:46 a.m. UTC | #1
On Thu, Jan 16, 2020 at 04:03:27PM -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>

Applied, thanks.

Johan
diff mbox series

Patch

diff --git a/drivers/usb/serial/garmin_gps.c b/drivers/usb/serial/garmin_gps.c
index 633550ec3025..ffd984142171 100644
--- a/drivers/usb/serial/garmin_gps.c
+++ b/drivers/usb/serial/garmin_gps.c
@@ -104,7 +104,7 @@  struct garmin_packet {
 	int               seq;
 	/* the real size of the data array, always > 0 */
 	int               size;
-	__u8              data[1];
+	__u8              data[];
 };
 
 /* structure used to keep the current state of the driver */