Gustavo A. R. Silva
2020-Mar-09 20:20 UTC
[Ocfs2-devel] [PATCH][next] ocfs2: dlm: 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://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!OVOYL_CouISa5L1Lw-20EEFQntw6cKMx-j8UdY4z78uYgzKBUFcfpn50GaurvbV5v7YiUA$
[2]
https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!OVOYL_CouISa5L1Lw-20EEFQntw6cKMx-j8UdY4z78uYgzKBUFcfpn50GaurvbXs8Eh8eg$
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo at embeddedor.com>
---
fs/ocfs2/dlm/dlmcommon.h | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/fs/ocfs2/dlm/dlmcommon.h b/fs/ocfs2/dlm/dlmcommon.h
index 0463dce65bb2..c8a444622faa 100644
--- a/fs/ocfs2/dlm/dlmcommon.h
+++ b/fs/ocfs2/dlm/dlmcommon.h
@@ -564,7 +564,7 @@ struct dlm_migratable_lockres
// 48 bytes
u8 lvb[DLM_LVB_LEN];
// 112 bytes
- struct dlm_migratable_lock ml[0]; // 16 bytes each, begins at byte 112
+ struct dlm_migratable_lock ml[]; // 16 bytes each, begins at byte 112
};
#define DLM_MIG_LOCKRES_MAX_LEN \
(sizeof(struct dlm_migratable_lockres) + \
@@ -601,7 +601,7 @@ struct dlm_convert_lock
u8 name[O2NM_MAX_NAME_LEN];
- s8 lvb[0];
+ s8 lvb[];
};
#define DLM_CONVERT_LOCK_MAX_LEN (sizeof(struct dlm_convert_lock)+DLM_LVB_LEN)
@@ -616,7 +616,7 @@ struct dlm_unlock_lock
u8 name[O2NM_MAX_NAME_LEN];
- s8 lvb[0];
+ s8 lvb[];
};
#define DLM_UNLOCK_LOCK_MAX_LEN (sizeof(struct dlm_unlock_lock)+DLM_LVB_LEN)
@@ -632,7 +632,7 @@ struct dlm_proxy_ast
u8 name[O2NM_MAX_NAME_LEN];
- s8 lvb[0];
+ s8 lvb[];
};
#define DLM_PROXY_AST_MAX_LEN (sizeof(struct dlm_proxy_ast)+DLM_LVB_LEN)
--
2.25.0
Joseph Qi
2020-Mar-10 01:07 UTC
[Ocfs2-devel] [PATCH][next] ocfs2: dlm: Replace zero-length array with flexible-array member
On 2020/3/10 04:20, 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] > > This issue was found with the help of Coccinelle. > > [1] https://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!I_pkpK-fMbcZGGP9iPUPG8LvqoSenH6iuL5O5nTqw9zX-e4A8Mjur9Xx4xl8U41yd3A-vA$ > [2] https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!I_pkpK-fMbcZGGP9iPUPG8LvqoSenH6iuL5O5nTqw9zX-e4A8Mjur9Xx4xl8U42h1JbMjw$ > [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour") > > Signed-off-by: Gustavo A. R. Silva <gustavo at embeddedor.com>Acked-by: Joseph Qi <joseph.qi at linux.alibaba.com>> --- > fs/ocfs2/dlm/dlmcommon.h | 8 ++++---- > 1 file changed, 4 insertions(+), 4 deletions(-) > > diff --git a/fs/ocfs2/dlm/dlmcommon.h b/fs/ocfs2/dlm/dlmcommon.h > index 0463dce65bb2..c8a444622faa 100644 > --- a/fs/ocfs2/dlm/dlmcommon.h > +++ b/fs/ocfs2/dlm/dlmcommon.h > @@ -564,7 +564,7 @@ struct dlm_migratable_lockres > // 48 bytes > u8 lvb[DLM_LVB_LEN]; > // 112 bytes > - struct dlm_migratable_lock ml[0]; // 16 bytes each, begins at byte 112 > + struct dlm_migratable_lock ml[]; // 16 bytes each, begins at byte 112 > }; > #define DLM_MIG_LOCKRES_MAX_LEN \ > (sizeof(struct dlm_migratable_lockres) + \ > @@ -601,7 +601,7 @@ struct dlm_convert_lock > > u8 name[O2NM_MAX_NAME_LEN]; > > - s8 lvb[0]; > + s8 lvb[]; > }; > #define DLM_CONVERT_LOCK_MAX_LEN (sizeof(struct dlm_convert_lock)+DLM_LVB_LEN) > > @@ -616,7 +616,7 @@ struct dlm_unlock_lock > > u8 name[O2NM_MAX_NAME_LEN]; > > - s8 lvb[0]; > + s8 lvb[]; > }; > #define DLM_UNLOCK_LOCK_MAX_LEN (sizeof(struct dlm_unlock_lock)+DLM_LVB_LEN) > > @@ -632,7 +632,7 @@ struct dlm_proxy_ast > > u8 name[O2NM_MAX_NAME_LEN]; > > - s8 lvb[0]; > + s8 lvb[]; > }; > #define DLM_PROXY_AST_MAX_LEN (sizeof(struct dlm_proxy_ast)+DLM_LVB_LEN) > >