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

[Daniel Xu]

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?

Thanks,
Daniel