bpf: Clarify batch lookup/lookup_and_delete semantics

The batch lookup and lookup_and_delete APIs have two parameters,
in_batch and out_batch, to facilitate iterative
lookup/lookup_and_deletion operations for supported maps. Except NULL
for in_batch at the start of these two batch operations, both parameters
need to point to memory equal or larger than the respective map key
size, except for various hashmaps (hash, percpu_hash, lru_hash,
lru_percpu_hash) where the in_batch/out_batch memory size should be
at least 4 bytes.

Document these semantics to clarify the API.

Signed-off-by: Martin Kelly <martin.kelly@crowdstrike.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240221211838.1241578-1-martin.kelly@crowdstrike.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

Author: Martin Kelly

include/uapi/linux/bpf.h

@@ -617,7 +617,11 @@ 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, PERCPU_HASH,
+ *		LRU_HASH, LRU_PERCPU_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

tools/include/uapi/linux/bpf.h

@@ -617,7 +617,11 @@ 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, PERCPU_HASH,
+ *		LRU_HASH, LRU_PERCPU_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

tools/lib/bpf/bpf.h

@@ -190,10 +190,14 @@ 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, PERCPU_HASH,
+ * LRU_HASH, LRU_PERCPU_HASH}**, for which the memory size 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 +230,10 @@ 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, PERCPU_HASH, LRU_HASH,
+ * LRU_PERCPU_HASH}**, the memory size must be at least 4 bytes wide regardless
+ * of key size.
  * @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