Re: [RFC bpf-next 8/9] libbpf: support llvm-generated indirect jumps

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2025-06-15 at 08:59 +0000, Anton Protopopov wrote:

[...]

> @@ -698,6 +712,14 @@ struct bpf_object {
>  	bool has_subcalls;
>  	bool has_rodata;
>  
> +	const void *rodata;
> +	size_t rodata_size;
> +	int rodata_map_fd;

This is sort-of strange, that jump table metadata resides in one
section, while jump section itself is in .rodata. Wouldn't it be
simpler make LLVM emit all jump tables info in one section?
Also note that Elf_Sym has name, section index, value and size,
hence symbols defined for jump table section can encode jump tables.
E.g. the following implementation seems more intuitive:

  .jumptables
    <subprog-rel-off-0>
    <subprog-rel-off-1> | <--- jump table #1 symbol:
    <subprog-rel-off-2> |        .size = 2   // number of entries in the jump table
    ...                          .value = 1  // offset within .jumptables
    <subprog-rel-off-N>                          ^
                                                 |
  .text                                          |
    ...                                          |
    <insn-N>     <------ relocation referencing -'
    ...                  jump table #1 symbol

> +
> +	/* Jump Tables */
> +	struct jt **jt;
> +	size_t jt_cnt;
> +
>  	struct bpf_gen *gen_loader;
>  
>  	/* Information when doing ELF related work. Only valid if efile.elf is not NULL */

[...]