[PATCH bpf] libbpf: clarify batch lookup semantics

[Martin Kelly <martin.kelly@crowdstrike.com>]

The batch lookup APIs copy key memory into out_batch, which is then
supplied in later calls to in_batch. Thus both parameters need to point
to memory large enough to hold a single key (other than an initial NULL
in_batch). For many maps, keys are pointer sized or less, but for larger
maps, it's important to point to a larger block of memory to avoid
memory corruption.

Document these semantics to clarify the API.

Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
---
 include/uapi/linux/bpf.h |  5 ++++-
 tools/lib/bpf/bpf.h      | 15 ++++++++++-----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d96708380e52..dae613b8778a 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -617,7 +617,10 @@ union bpf_iter_link_info {
  *		to NULL to begin the batched operation. After each subsequent
  *		**BPF_MAP_LOOKUP_BATCH**, the caller should pass the resultant
  *		*out_batch* as the *in_batch* for the next operation to
- *		continue iteration from the current point.
+ *		continue iteration from the current point. Both *in_batch* and
+ *		*out_batch* must point to memory large enough to hold a key,
+ *		except for maps of type **BPF_MAP_TYPE_HASH**, for which batch
+ *		parameters must be at least 4 bytes wide regardless of key size.
  *
  *		The *keys* and *values* are output parameters which must point
  *		to memory large enough to hold *count* items based on the key
diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index ab2570d28aec..c7e918ab0a60 100644
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -190,10 +190,13 @@ LIBBPF_API int bpf_map_delete_batch(int fd, const void *keys,
 /**
  * @brief **bpf_map_lookup_batch()** allows for batch lookup of BPF map elements.
  *
- * The parameter *in_batch* is the address of the first element in the batch to read.
- * *out_batch* is an output parameter that should be passed as *in_batch* to subsequent
- * calls to **bpf_map_lookup_batch()**. NULL can be passed for *in_batch* to indicate
- * that the batched lookup starts from the beginning of the map.
+ * The parameter *in_batch* is the address of the first element in the batch to
+ * read. *out_batch* is an output parameter that should be passed as *in_batch*
+ * to subsequent calls to **bpf_map_lookup_batch()**. NULL can be passed for
+ * *in_batch* to indicate that the batched lookup starts from the beginning of
+ * the map. Both *in_batch* and *out_batch* must point to memory large enough to
+ * hold a single key, except for maps of type **BPF_MAP_TYPE_HASH**, for which
+ * the memory pointed to must be at least 4 bytes wide regardless of key size.
  *
  * The *keys* and *values* are output parameters which must point to memory large enough to
  * hold *count* items based on the key and value size of the map *map_fd*. The *keys*
@@ -226,7 +229,9 @@ LIBBPF_API int bpf_map_lookup_batch(int fd, void *in_batch, void *out_batch,
  *
  * @param fd BPF map file descriptor
  * @param in_batch address of the first element in batch to read, can pass NULL to
- * get address of the first element in *out_batch*
+ * get address of the first element in *out_batch*. If not NULL, must be large
+ * enough to hold a key. For **BPF_MAP_TYPE_HASH**, must be large enough to hold
+ * 4 bytes.
  * @param out_batch output parameter that should be passed to next call as *in_batch*
  * @param keys pointer to an array of *count* keys
  * @param values pointer to an array large enough for *count* values
-- 
2.34.1