| Message ID | 20200213160004.GA4334@embeddedor (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | ceph: cache: Replace zero-length array with flexible-array member | expand |
On Thu, 2020-02-13 at 10:00 -0600, 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://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 <gustavo@embeddedor.com> > --- > fs/ceph/cache.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c > index 270b769607a2..2f5cb6bc78e1 100644 > --- a/fs/ceph/cache.c > +++ b/fs/ceph/cache.c > @@ -32,7 +32,7 @@ struct ceph_fscache_entry { > size_t uniq_len; > /* The following members must be last */ > struct ceph_fsid fsid; > - char uniquifier[0]; > + char uniquifier[]; > }; > > static const struct fscache_cookie_def ceph_fscache_fsid_object_def = { Meh, ok. Merged into the ceph-client/testing branch. Should make v5.7 barring unforseen issues. Thanks,
diff --git a/fs/ceph/cache.c b/fs/ceph/cache.c index 270b769607a2..2f5cb6bc78e1 100644 --- a/fs/ceph/cache.c +++ b/fs/ceph/cache.c @@ -32,7 +32,7 @@ struct ceph_fscache_entry { size_t uniq_len; /* The following members must be last */ struct ceph_fsid fsid; - char uniquifier[0]; + char uniquifier[]; }; static const struct fscache_cookie_def ceph_fscache_fsid_object_def = {
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") Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com> --- fs/ceph/cache.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)