Re: [PATCH bpf-next v2] bpf: clarify batch lookup semantics

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

On 2/21/24 13:18, Martin Kelly wrote:
> 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>
> ---
>   include/uapi/linux/bpf.h       |  6 +++++-
>   tools/include/uapi/linux/bpf.h |  6 +++++-
>   tools/lib/bpf/bpf.h            | 17 ++++++++++++-----
>   3 files changed, 22 insertions(+), 7 deletions(-)

Yonghong, looks like I missed your comment to change from "clarify batch 
lookup semantics" to "Clarify batch lookup/lookup_and_delete semantics"; 
sorry about that. Feel free to change it if you merge this, or I can 
include it in a v3 if needed.

>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index d96708380e52..d2e6c5fcec01 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/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
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index d96708380e52..d2e6c5fcec01 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/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
> diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
> index ab2570d28aec..df0db2f0cdb7 100644
> --- a/tools/lib/bpf/bpf.h
> +++ b/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