| Message ID | 20200213152416.GA1873@embeddedor |
|---|---|
| State | Changes Requested |
| Delegated to: | BPF Maintainers |
| Headers | show |
| Series | bpf: queue_stack_maps: Replace zero-length array with flexible-array member | expand |
On Thu, Feb 13, 2020 at 7:22 AM Gustavo A. R. Silva <gustavo@embeddedor.com> 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> > --- Sure, why not, though I don't think that's the only one (e.g., bpf_storage_buffer's data is zero-length as well). Acked-by: Andrii Nakryiko <andriin@fb.com> > kernel/bpf/queue_stack_maps.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c > index f697647ceb54..30e1373fd437 100644 > --- a/kernel/bpf/queue_stack_maps.c > +++ b/kernel/bpf/queue_stack_maps.c > @@ -19,7 +19,7 @@ struct bpf_queue_stack { > u32 head, tail; > u32 size; /* max_entries + 1 */ > > - char elements[0] __aligned(8); > + char elements[] __aligned(8); > }; > > static struct bpf_queue_stack *bpf_queue_stack(struct bpf_map *map) > -- > 2.25.0 >
On 2/13/20 6:47 PM, Andrii Nakryiko wrote: > On Thu, Feb 13, 2020 at 7:22 AM Gustavo A. R. Silva > <gustavo@embeddedor.com> 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> > > Sure, why not, though I don't think that's the only one (e.g., > bpf_storage_buffer's data is zero-length as well). > > Acked-by: Andrii Nakryiko <andriin@fb.com> +1, Gustavo, there are several such instances in the whole BPF subsystem. Please combine them all into a single patch, including https://patchwork.ozlabs.org/patch/1239563/, and resubmit. Thanks, Daniel
diff --git a/kernel/bpf/queue_stack_maps.c b/kernel/bpf/queue_stack_maps.c index f697647ceb54..30e1373fd437 100644 --- a/kernel/bpf/queue_stack_maps.c +++ b/kernel/bpf/queue_stack_maps.c @@ -19,7 +19,7 @@ struct bpf_queue_stack { u32 head, tail; u32 size; /* max_entries + 1 */ - char elements[0] __aligned(8); + char elements[] __aligned(8); }; static struct bpf_queue_stack *bpf_queue_stack(struct bpf_map *map)
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> --- kernel/bpf/queue_stack_maps.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)