block: Replace zero-length array with flexible-array member
Subsystems (4)
PATCH 1/1 block: Replace zero-length array with flexible-array member
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]
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")
--- block/partitions/ldm.h | 2 +- drivers/block/drbd/drbd_int.h | 2 +- drivers/block/drbd/drbd_protocol.h | 8 ++++---- drivers/block/rbd_types.h | 2 +- 4 files changed, 7 insertions(+), 7 deletions(-)
block/partitions/ldm.h +1 -1
@@ -93,7 +93,7 @@ struct frag { /* VBLK Fragment handling */u8 num;/* Total number of records */ u8 rec;/* This is record number n */ u8 map;/* Which portions are in use */ -u8 data[0 ]; +u8 data[]; };/* In memory LDM database structures. */
drivers/block/drbd/drbd_int.h +1 -1
@@ -620,7 +620,7 @@ struct fifo_buffer {unsigned int head_index;unsigned int size;int total;/* sum of all values */ -int values[0 ]; +int values[]; };extern struct fifo_buffer *fifo_alloc (unsigned int fifo_size);
drivers/block/drbd/drbd_protocol.h +4 -4
@@ -271,7 +271,7 @@ struct p_rs_param {u32 resync_rate;/* Since protocol version 88 and higher. */ -char verify_alg[0 ]; +char verify_alg[]; } __packed;struct p_rs_param_89 {
@@ -305,7 +305,7 @@ struct p_protocol {u32 two_primaries;/* Since protocol version 87 and higher. */ -char integrity_alg[0 ]; +char integrity_alg[]; } __packed;
@@ -360,7 +360,7 @@ struct p_sizes {u16 dds_flags;/* use enum dds_flags here. */ /* optional queue_limits if (agreed_features & DRBD_FF_WSAME) */ -struct o_qlim qlim[0 ]; +struct o_qlim qlim[]; } __packed;struct p_state {
@@ -409,7 +409,7 @@ struct p_compressed_bm {*/ u8 encoding; -u8 code[0 ]; +u8 code[]; } __packed;struct p_delay_probe93 {
drivers/block/rbd_types.h +1 -1
@@ -93,7 +93,7 @@ struct rbd_image_header_ondisk {__le32 snap_count;__le32 reserved;__le64 snap_names_len; -struct rbd_image_snap_ondisk snaps[0 ]; +struct rbd_image_snap_ondisk snaps[]; } __attribute__((packed));
Signature · 1 line
2.25.0
Quoted 29 lines · Gustavo A. R. Silva
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]
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] commit76497732932f("cxgb3/l2t: Fix undefined behaviour")
Quoted 5 lines
Signed-off-by Gustavo A. R. Silva <[email protected]>--- block/partitions/ldm.h | 2 +- drivers/block/drbd/drbd_int.h | 2 +- drivers/block/drbd/drbd_protocol.h | 8 ++++----
For rbd
Quoted 1 line
drivers/block/rbd_types.h | 2 +-
Thanks,
Ilya