Merge branch 'Use __GFP_ZERO in bpf memory allocator'

Hou Tao says:

====================

From: Hou Tao <houtao1@huawei.com>

Hi,

The patchset tries to fix the hard-up problem found when checking how htab
handles element reuse in bpf memory allocator. The immediate reuse of
freed elements will reinitialize special fields (e.g., bpf_spin_lock) in
htab map value and it may corrupt lookup procedure with BFP_F_LOCK flag
which acquires bpf-spin-lock during value copying, and lead to hard-lock
as shown in patch #2. Patch #1 fixes it by using __GFP_ZERO when allocating
the object from slab and the behavior is similar with the preallocated
hash-table case. Please see individual patches for more details. And comments
are always welcome.

Regards,

Change Log:
v1:
  * Use __GFP_ZERO instead of ctor to avoid retpoline overhead (from Alexei)
  * Add comments for check_and_init_map_value() (from Alexei)
  * split __GFP_ZERO patches out of the original patchset to unblock
    the development work of others.

RFC: https://lore.kernel.org/bpf/20221230041151.1231169-1-houtao@huaweicloud.com
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index be34f7deb6c3..520b238abd5a 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -363,6 +363,13 @@ static inline void bpf_obj_init(const struct btf_field_offs *foffs, void *obj)
 		memset(obj + foffs->field_off[i], 0, foffs->field_sz[i]);
 }
 
+/* 'dst' must be a temporary buffer and should not point to memory that is being
+ * used in parallel by a bpf program or bpf syscall, otherwise the access from
+ * the bpf program or bpf syscall may be corrupted by the reinitialization,
+ * leading to weird problems. Even 'dst' is newly-allocated from bpf memory
+ * allocator, it is still possible for 'dst' to be used in parallel by a bpf
+ * program or bpf syscall.
+ */
 static inline void check_and_init_map_value(struct bpf_map *map, void *dst)
 {
 	bpf_obj_init(map->field_offs, dst);