Re: [PATCH bpf-next v4 3/4] bpftool: btf: Support dumping a specific types from file

[Daniel Xu]

On Fri, Dec 13, 2024 at 04:55:34PM GMT, Quentin Monnet wrote:
> 2024-12-13 09:45 UTC-0700 ~ Daniel Xu <dxu@xxxxxxxxx>
> > Hi Quentin,
> > 
> > On Fri, Dec 13, 2024 at 03:17:36PM GMT, Quentin Monnet wrote:
> >> 2024-12-12 18:24 UTC-0700 ~ Daniel Xu <dxu@xxxxxxxxx>
> >>> Some projects, for example xdp-tools [0], prefer to check in a minimized
> >>> vmlinux.h rather than the complete file which can get rather large.
> >>>
> >>> However, when you try to add a minimized version of a complex struct (eg
> >>> struct xfrm_state), things can get quite complex if you're trying to
> >>> manually untangle and deduplicate the dependencies.
> >>>
> >>> This commit teaches bpftool to do a minimized dump of a specific types by
> >>> providing a optional root_id argument(s).
> >>>
> >>> Example usage:
> >>>
> >>>     $ ./bpftool btf dump file ~/dev/linux/vmlinux | rg "STRUCT 'xfrm_state'"
> >>>     [12643] STRUCT 'xfrm_state' size=912 vlen=58
> >>>
> >>>     $ ./bpftool btf dump file ~/dev/linux/vmlinux root_id 12643 format c
> >>>     #ifndef __VMLINUX_H__
> >>>     #define __VMLINUX_H__
> >>>
> >>>     [..]
> >>>
> >>>     struct xfrm_type_offload;
> >>>
> >>>     struct xfrm_sec_ctx;
> >>>
> >>>     struct xfrm_state {
> >>>             possible_net_t xs_net;
> >>>             union {
> >>>                     struct hlist_node gclist;
> >>>                     struct hlist_node bydst;
> >>>             };
> >>>             union {
> >>>                     struct hlist_node dev_gclist;
> >>>                     struct hlist_node bysrc;
> >>>             };
> >>>             struct hlist_node byspi;
> >>>     [..]
> >>>
> >>> [0]: https://github.com/xdp-project/xdp-tools/blob/master/headers/bpf/vmlinux.h
> >>>
> >>> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> >>> ---
> >>>  .../bpf/bpftool/Documentation/bpftool-btf.rst |  8 +++-
> >>>  tools/bpf/bpftool/btf.c                       | 39 ++++++++++++++++++-
> >>>  2 files changed, 43 insertions(+), 4 deletions(-)
> >>>
> >>> diff --git a/tools/bpf/bpftool/Documentation/bpftool-btf.rst b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> >>> index 245569f43035..dbe6d6d94e4c 100644
> >>> --- a/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> >>> +++ b/tools/bpf/bpftool/Documentation/bpftool-btf.rst
> >>> @@ -24,7 +24,7 @@ BTF COMMANDS
> >>>  =============
> >>>  
> >>>  | **bpftool** **btf** { **show** | **list** } [**id** *BTF_ID*]
> >>> -| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*]
> >>> +| **bpftool** **btf dump** *BTF_SRC* [**format** *FORMAT*] [**root_id** *ROOT_ID*]
> >>>  | **bpftool** **btf help**
> >>>  |
> >>>  | *BTF_SRC* := { **id** *BTF_ID* | **prog** *PROG* | **map** *MAP* [{**key** | **value** | **kv** | **all**}] | **file** *FILE* }
> >>> @@ -43,7 +43,7 @@ bpftool btf { show | list } [id *BTF_ID*]
> >>>      that hold open file descriptors (FDs) against BTF objects. On such kernels
> >>>      bpftool will automatically emit this information as well.
> >>>  
> >>> -bpftool btf dump *BTF_SRC* [format *FORMAT*]
> >>> +bpftool btf dump *BTF_SRC* [format *FORMAT*] [root_id *ROOT_ID*]
> >>>      Dump BTF entries from a given *BTF_SRC*.
> >>>  
> >>>      When **id** is specified, BTF object with that ID will be loaded and all
> >>> @@ -67,6 +67,10 @@ bpftool btf dump *BTF_SRC* [format *FORMAT*]
> >>>      formatting, the output is sorted by default. Use the **unsorted** option
> >>>      to avoid sorting the output.
> >>>  
> >>> +    **root_id** option can be used to filter a dump to a single type and all
> >>> +    its dependent types. It cannot be used with any other types of filtering.
> >>> +    It can be passed multiple times to dump multiple types.
> >>> +
> >>>  bpftool btf help
> >>>      Print short help message.
> >>>  
> >>> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> >>> index 3e995faf9efa..2636655ac180 100644
> >>> --- a/tools/bpf/bpftool/btf.c
> >>> +++ b/tools/bpf/bpftool/btf.c
> >>> @@ -27,6 +27,8 @@
> >>>  #define KFUNC_DECL_TAG		"bpf_kfunc"
> >>>  #define FASTCALL_DECL_TAG	"bpf_fastcall"
> >>>  
> >>> +#define MAX_ROOT_IDS		16
> >>> +
> >>>  static const char * const btf_kind_str[NR_BTF_KINDS] = {
> >>>  	[BTF_KIND_UNKN]		= "UNKNOWN",
> >>>  	[BTF_KIND_INT]		= "INT",
> >>> @@ -880,7 +882,8 @@ static int do_dump(int argc, char **argv)
> >>>  {
> >>>  	bool dump_c = false, sort_dump_c = true;
> >>>  	struct btf *btf = NULL, *base = NULL;
> >>> -	__u32 root_type_ids[2];
> >>> +	__u32 root_type_ids[MAX_ROOT_IDS];
> >>> +	bool have_id_filtering;
> >>>  	int root_type_cnt = 0;
> >>>  	__u32 btf_id = -1;
> >>>  	const char *src;
> >>> @@ -974,6 +977,8 @@ static int do_dump(int argc, char **argv)
> >>>  		goto done;
> >>>  	}
> >>>  
> >>> +	have_id_filtering = !!root_type_cnt;
> >>> +
> >>>  	while (argc) {
> >>>  		if (is_prefix(*argv, "format")) {
> >>>  			NEXT_ARG();
> >>> @@ -993,6 +998,36 @@ static int do_dump(int argc, char **argv)
> >>>  				goto done;
> >>>  			}
> >>>  			NEXT_ARG();
> >>> +		} else if (is_prefix(*argv, "root_id")) {
> >>> +			__u32 root_id;
> >>> +			char *end;
> >>> +
> >>> +			if (have_id_filtering) {
> >>> +				p_err("cannot use root_id with other type filtering");
> >>> +				err = -EINVAL;
> >>> +				goto done;
> >>> +			} else if (root_type_cnt == MAX_ROOT_IDS) {
> >>> +				p_err("only %d root_id are supported", MAX_ROOT_IDS);
> >>
> >>
> >> I doubt users will often reach this limit, but if they do, the message
> >> can be confusing, because MAX_ROOT_IDS also accounts for root_type_ids[]
> >> cells used when we pass map arguments ("key" or "value" or "kv"), so you
> >> could pass 15 "root_id" on the command line and get a message telling
> >> only 16 are supported.
> >>
> >> Maybe add a counter to tell how many were defined from the rest of the
> >> command line, and adjust the value in the error message?
> > 
> > The above `if (have_id_filtering)` check prevents mixing key/value/kv
> > map args with root_id. That ought to prevent overcounting, right?
> 
> Ah, you're right, you even mentioned it in the docs, sorry. All good on
> that side, then.
> 
> Regarding the restriction, would you mind making the mention from the
> man page a bit more explicit, please? Something along:
> 
>     [...] It cannot be used with any other types of filtering, such as
>     the "key", "value", or "kv" arguments when dumping BTF for a map.
>     **root_id** can be passed multiple times...

Ack, will do.