bpf: Replace zero-length array with flexible-array member
Subsystems (4)
PATCH 1/1 bpf: 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")
--- include/linux/bpf-cgroup.h | 2 +- include/linux/bpf.h | 2 +- include/uapi/linux/bpf.h | 2 +- kernel/bpf/bpf_struct_ops.c | 2 +- kernel/bpf/hashtab.c | 2 +- kernel/bpf/lpm_trie.c | 2 +- 6 files changed, 6 insertions(+), 6 deletions(-)
include/linux/bpf-cgroup.h +1 -1
@@ -36,7 +36,7 @@ struct bpf_cgroup_storage_map;struct bpf_storage_buffer {struct rcu_head rcu; -char data[0 ]; +char data[]; };struct bpf_cgroup_storage {
include/linux/bpf.h +1 -1
@@ -859,7 +859,7 @@ struct bpf_prog_array_item {struct bpf_prog_array {struct rcu_head rcu; -struct bpf_prog_array_item items[0 ]; +struct bpf_prog_array_item items[]; };struct bpf_prog_array *bpf_prog_array_alloc (u32 prog_cnt,gfp_t flags);
include/uapi/linux/bpf.h +1 -1
@@ -73,7 +73,7 @@ struct bpf_insn {/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */ struct bpf_lpm_trie_key {__u32 prefixlen;/* up to 32 for AF_INET, 128 for AF_INET6 */ -__u8 data[0 ];/* Arbitrary size */ +__u8 data[];/* Arbitrary size */ };struct bpf_cgroup_storage_key {
kernel/bpf/bpf_struct_ops.c +1 -1
@@ -23,7 +23,7 @@ enum bpf_struct_ops_state {struct bpf_struct_ops_value {BPF_STRUCT_OPS_COMMON_VALUE ; -char data[0 ]____cacheline_aligned_in_smp ; +char data[]____cacheline_aligned_in_smp ; };struct bpf_struct_ops_map {
kernel/bpf/hashtab.c +1 -1
@@ -118,7 +118,7 @@ struct htab_elem {struct bpf_lru_node lru_node; };u32 hash; -char key[0 ]__aligned(8) ; +char key[]__aligned(8) ; };static inline bool htab_is_prealloc (const struct bpf_htab *htab)
kernel/bpf/lpm_trie.c +1 -1
@@ -25,7 +25,7 @@ struct lpm_trie_node {struct lpm_trie_node __rcu *child[2 ];u32 prefixlen;u32 flags; -u8 data[0 ]; +u8 data[]; };struct lpm_trie {
Signature · 1 line
2.25.0
Quoted 23 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]
Quoted 7 lines
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")Signed-off-by Gustavo A. R. Silva <[email protected]>
Quoted 22 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]
Quoted 7 lines
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")Signed-off-by Gustavo A. R. Silva <[email protected]>
Applied, thanks!
Quoted 8 lines · Daniel Borkmann
Quoted 6 lines
[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")Signed-off-by Gustavo A. R. Silva <[email protected]>Applied, thanks!
Thanks, Daniel.
--
Gustavo