Re: [PATCH 2/2] misc: Replace zero-length arrays with flexible array member (manual)

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 3/4/20 1:58 AM, Philippe Mathieu-Daudé wrote:
> Description copied from Linux kernel commit from Gustavo A. R. Silva
> (see [3]):
> 
> --v-- description start --v--
> 
>    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], 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 unadvertenly introduced [2] to the
>    Linux codebase from now on.
> 
> --^-- description end --^--
> 
> Do the similar housekeeping in the QEMU codebase (which uses
> C99 since commit 7be41675f7cb).
> 
> All these instances of code were found with the help of the
> following command (then manual analysis):
> 
>    git grep -F '[0];'
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=76497732932f
> [3] https://git.kernel.org/pub/scm/linux/kernel/git/gustavoars/linux.git/commit/?id=17642a2fbd2c1
> 
> Inspired-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>   docs/interop/vhost-user.rst       | 4 ++--
>   block/qed.h                       | 2 +-
>   include/hw/acpi/acpi-defs.h       | 4 ++--
>   include/hw/boards.h               | 2 +-
>   include/hw/s390x/event-facility.h | 2 +-
>   include/hw/s390x/sclp.h           | 8 ++++----
>   block/vmdk.c                      | 2 +-
>   hw/char/sclpconsole-lm.c          | 2 +-
>   hw/char/sclpconsole.c             | 2 +-
>   hw/s390x/virtio-ccw.c             | 2 +-
>   target/s390x/ioinst.c             | 2 +-
>   11 files changed, 16 insertions(+), 16 deletions(-)
> 
> diff --git a/docs/interop/vhost-user.rst b/docs/interop/vhost-user.rst
> index 401652397c..3b1b6602c7 100644
> --- a/docs/interop/vhost-user.rst
> +++ b/docs/interop/vhost-user.rst
> @@ -568,7 +568,7 @@ For split virtqueue, queue region can be implemented as:
>         uint16_t used_idx;
>   
>         /* Used to track the state of each descriptor in descriptor table */
> -      DescStateSplit desc[0];
> +      DescStateSplit desc[];
>     } QueueRegionSplit;
>   
>   To track inflight I/O, the queue region should be processed as follows:
> @@ -690,7 +690,7 @@ For packed virtqueue, queue region can be implemented as:
>         uint8_t padding[7];
>   
>         /* Used to track the state of each descriptor fetched from descriptor ring */
> -      DescStatePacked desc[0];
> +      DescStatePacked desc[];
>     } QueueRegionPacked;
>   
>   To track inflight I/O, the queue region should be processed as follows:
> diff --git a/block/qed.h b/block/qed.h
> index 42c115d822..87428ba00e 100644
> --- a/block/qed.h
> +++ b/block/qed.h
> @@ -103,7 +103,7 @@ typedef struct {
>   } QEMU_PACKED QEDHeader;
>   
>   typedef struct {
> -    uint64_t offsets[0];            /* in bytes */
> +    uint64_t offsets[];             /* in bytes */

Apparently this one is incorrect, it triggers:

GCC:
block/qed.h:106:14: error: flexible array member in otherwise empty struct

Clang:
block/qed.h:106:14: error: flexible array member 'offsets' not allowed 
in otherwise empty struct

>   } QEDTable;
>   
>   /* The L2 cache is a simple write-through cache for L2 structures */
> diff --git a/include/hw/acpi/acpi-defs.h b/include/hw/acpi/acpi-defs.h
> index 19f7ba7b70..c13327fa78 100644
> --- a/include/hw/acpi/acpi-defs.h
> +++ b/include/hw/acpi/acpi-defs.h
> @@ -152,7 +152,7 @@ typedef struct AcpiSerialPortConsoleRedirection
>    */
>   struct AcpiRsdtDescriptorRev1 {
>       ACPI_TABLE_HEADER_DEF       /* ACPI common table header */
> -    uint32_t table_offset_entry[0];  /* Array of pointers to other */
> +    uint32_t table_offset_entry[];  /* Array of pointers to other */
>       /* ACPI tables */
>   } QEMU_PACKED;
>   typedef struct AcpiRsdtDescriptorRev1 AcpiRsdtDescriptorRev1;
> @@ -162,7 +162,7 @@ typedef struct AcpiRsdtDescriptorRev1 AcpiRsdtDescriptorRev1;
>    */
>   struct AcpiXsdtDescriptorRev2 {
>       ACPI_TABLE_HEADER_DEF       /* ACPI common table header */
> -    uint64_t table_offset_entry[0];  /* Array of pointers to other */
> +    uint64_t table_offset_entry[];  /* Array of pointers to other */
>       /* ACPI tables */
>   } QEMU_PACKED;
>   typedef struct AcpiXsdtDescriptorRev2 AcpiXsdtDescriptorRev2;
> diff --git a/include/hw/boards.h b/include/hw/boards.h
> index 9bc42dfb22..c96120d15f 100644
> --- a/include/hw/boards.h
> +++ b/include/hw/boards.h
> @@ -71,7 +71,7 @@ typedef struct CPUArchId {
>    */
>   typedef struct {
>       int len;
> -    CPUArchId cpus[0];
> +    CPUArchId cpus[];
>   } CPUArchIdList;
>   
>   /**
> diff --git a/include/hw/s390x/event-facility.h b/include/hw/s390x/event-facility.h
> index bdc32a3c09..700a610f33 100644
> --- a/include/hw/s390x/event-facility.h
> +++ b/include/hw/s390x/event-facility.h
> @@ -122,7 +122,7 @@ typedef struct MDBO {
>   
>   typedef struct MDB {
>       MdbHeader header;
> -    MDBO mdbo[0];
> +    MDBO mdbo[];
>   } QEMU_PACKED MDB;
>   
>   typedef struct SclpMsg {
> diff --git a/include/hw/s390x/sclp.h b/include/hw/s390x/sclp.h
> index c54413b78c..cd7b24359f 100644
> --- a/include/hw/s390x/sclp.h
> +++ b/include/hw/s390x/sclp.h
> @@ -132,7 +132,7 @@ typedef struct ReadInfo {
>       uint16_t highest_cpu;
>       uint8_t  _reserved5[124 - 122];     /* 122-123 */
>       uint32_t hmfai;
> -    struct CPUEntry entries[0];
> +    struct CPUEntry entries[];
>   } QEMU_PACKED ReadInfo;
>   
>   typedef struct ReadCpuInfo {
> @@ -142,7 +142,7 @@ typedef struct ReadCpuInfo {
>       uint16_t nr_standby;            /* 12-13 */
>       uint16_t offset_standby;        /* 14-15 */
>       uint8_t reserved0[24-16];       /* 16-23 */
> -    struct CPUEntry entries[0];
> +    struct CPUEntry entries[];
>   } QEMU_PACKED ReadCpuInfo;
>   
>   typedef struct ReadStorageElementInfo {
> @@ -151,7 +151,7 @@ typedef struct ReadStorageElementInfo {
>       uint16_t assigned;
>       uint16_t standby;
>       uint8_t _reserved0[16 - 14]; /* 14-15 */
> -    uint32_t entries[0];
> +    uint32_t entries[];
>   } QEMU_PACKED ReadStorageElementInfo;
>   
>   typedef struct AttachStorageElement {
> @@ -159,7 +159,7 @@ typedef struct AttachStorageElement {
>       uint8_t _reserved0[10 - 8];  /* 8-9 */
>       uint16_t assigned;
>       uint8_t _reserved1[16 - 12]; /* 12-15 */
> -    uint32_t entries[0];
> +    uint32_t entries[];
>   } QEMU_PACKED AttachStorageElement;
>   
>   typedef struct AssignStorage {
> diff --git a/block/vmdk.c b/block/vmdk.c
> index 20e909d997..8466051bc9 100644
> --- a/block/vmdk.c
> +++ b/block/vmdk.c
> @@ -187,7 +187,7 @@ typedef struct VmdkMetaData {
>   typedef struct VmdkGrainMarker {
>       uint64_t lba;
>       uint32_t size;
> -    uint8_t  data[0];
> +    uint8_t  data[];
>   } QEMU_PACKED VmdkGrainMarker;
>   
>   enum {
> diff --git a/hw/char/sclpconsole-lm.c b/hw/char/sclpconsole-lm.c
> index c420dc066e..2b5f37b6a2 100644
> --- a/hw/char/sclpconsole-lm.c
> +++ b/hw/char/sclpconsole-lm.c
> @@ -31,7 +31,7 @@
>   typedef struct OprtnsCommand {
>       EventBufferHeader header;
>       MDMSU message_unit;
> -    char data[0];
> +    char data[];
>   } QEMU_PACKED OprtnsCommand;
>   
>   /* max size for line-mode data in 4K SCCB page */
> diff --git a/hw/char/sclpconsole.c b/hw/char/sclpconsole.c
> index 1fa124dab9..5c7664905e 100644
> --- a/hw/char/sclpconsole.c
> +++ b/hw/char/sclpconsole.c
> @@ -25,7 +25,7 @@
>   
>   typedef struct ASCIIConsoleData {
>       EventBufferHeader ebh;
> -    char data[0];
> +    char data[];
>   } QEMU_PACKED ASCIIConsoleData;
>   
>   /* max size for ASCII data in 4K SCCB page */
> diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> index 50cf95b781..64f928fc7d 100644
> --- a/hw/s390x/virtio-ccw.c
> +++ b/hw/s390x/virtio-ccw.c
> @@ -193,7 +193,7 @@ typedef struct VirtioThinintInfo {
>   typedef struct VirtioRevInfo {
>       uint16_t revision;
>       uint16_t length;
> -    uint8_t data[0];
> +    uint8_t data[];
>   } QEMU_PACKED VirtioRevInfo;
>   
>   /* Specify where the virtqueues for the subchannel are in guest memory. */
> diff --git a/target/s390x/ioinst.c b/target/s390x/ioinst.c
> index c437a1d8c6..0e840cc579 100644
> --- a/target/s390x/ioinst.c
> +++ b/target/s390x/ioinst.c
> @@ -347,7 +347,7 @@ typedef struct ChscResp {
>       uint16_t len;
>       uint16_t code;
>       uint32_t param;
> -    char data[0];
> +    char data[];
>   } QEMU_PACKED ChscResp;
>   
>   #define CHSC_MIN_RESP_LEN 0x0008
>