Re: [PATCH] treewide: Replace zero-length array with flexible-array
On Thu, May 7, 2020 at 1:49 PM Gustavo A. R. Silva
wrote:
>
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> 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
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva
> ---
> include/linux/fsl/bestcomm/bestcomm.h |2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
Applied for next. Thanks.
Regards,
Leo
>
> diff --git a/include/linux/fsl/bestcomm/bestcomm.h
> b/include/linux/fsl/bestcomm/bestcomm.h
> index a0e2e6b19b57..154e541ce57e 100644
> --- a/include/linux/fsl/bestcomm/bestcomm.h
> +++ b/include/linux/fsl/bestcomm/bestcomm.h
> @@ -27,7 +27,7 @@
> */
> struct bcom_bd {
> u32 status;
> - u32 data[0];/* variable payload size */
> + u32 data[]; /* variable payload size */
> };
>
> /*
> */
>
Re: [PATCH] treewide: Replace zero-length array with flexible-array
On Thu, May 07, 2020 at 01:53:13PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> 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
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva
Applied for -next, thanks.
Johan
RE: [PATCH] treewide: Replace zero-length array with flexible-array
> -Original Message-
> From: Gustavo A. R. Silva
> Sent: Thursday, May 07, 2020 21:54
> To: Winkler, Tomas
> Cc: linux-kernel@vger.kernel.org
> Subject: [PATCH] treewide: Replace zero-length array with flexible-array
Ack.
>
> The current codebase makes use of the zero-length array language extension to
> the C90 standard, but the preferred mechanism to declare variable-length types
> such as these ones is a flexible array member[1][2], introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> 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 inadvertently introduced[3]
> to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by this
> change:
>
> "Flexible array members have incomplete type, and so the sizeof operator may
> not be applied. As a quirk of the original implementation of zero-length
> arrays,
> sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in which
> the sizeof operator is being incorrectly/erroneously applied to zero-length
> arrays and the result is zero. Such instances may be hiding some bugs. So,
> this
> work (flexible-array member conversions) will also help to get completely rid
> of
> those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva
> ---
> samples/mei/mei-amt-version.c |2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/samples/mei/mei-amt-version.c b/samples/mei/mei-amt-version.c
> index 32234481ad7d..ad3e56042f96 100644
> --- a/samples/mei/mei-amt-version.c
> +++ b/samples/mei/mei-amt-version.c
> @@ -267,7 +267,7 @@ struct amt_host_if_msg_header { struct
> amt_host_if_resp_header {
> struct amt_host_if_msg_header header;
> uint32_t status;
> - unsigned char data[0];
> + unsigned char data[];
> } __attribute__((packed));
>
> const uuid_le MEI_IAMTHIF = UUID_LE(0x12f80028, 0xb4b7, 0x4b2d, \
Re: [PATCH] treewide: Replace zero-length array with flexible-array
On Thu, May 07, 2020 at 01:53:34PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of the zero-length array language
> extension to the C90 standard, but the preferred mechanism to declare
> variable-length types such as these ones is a flexible array member[1][2],
> introduced in C99:
>
> struct foo {
> int stuff;
> struct boo array[];
> };
>
> 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
> inadvertently introduced[3] to the codebase from now on.
>
> Also, notice that, dynamic memory allocations won't be affected by
> this change:
>
> "Flexible array members have incomplete type, and so the sizeof operator
> may not be applied. As a quirk of the original implementation of
> zero-length arrays, sizeof evaluates to zero."[1]
>
> sizeof(flexible-array-member) triggers a warning because flexible array
> members have incomplete type[1]. There are some instances of code in
> which the sizeof operator is being incorrectly/erroneously applied to
> zero-length arrays and the result is zero. Such instances may be hiding
> some bugs. So, this work (flexible-array member conversions) will also
> help to get completely rid of those sorts of issues.
>
> This issue was found with the help of Coccinelle.
>
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://github.com/KSPP/linux/issues/21
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
>
> Signed-off-by: Gustavo A. R. Silva
I'm fine with this specific change ... so
Acked-by: Tony Luck
for anyone picking up all of these treewide: patches
(if they don't, then I can apply this to the ia64 tree)
But a question ... is sizeof still ok on a structure that
contains a flexible-array-member?
E.g. I'd expect:
struct foo {
int stuff;
struct boo array[];
};
printk("size of foo = %d\n", sizeof(struct foo));
To not give any complilation warnings and to tell me that
the size of the structure is 4 bytes.
Is that still true and OK?
-Tony
