Re: [PATCHv3 8/9] kexec: Integrate bpf light skeleton to load zboot image

[Philipp Rudo <prudo@redhat.com>]

Hi Pingfan,

On Thu, 29 May 2025 12:17:43 +0800
Pingfan Liu <piliu@redhat.com> wrote:

> All kexec PE bpf prog should align with the interface exposed by the
> light skeleton
>     four maps:
>                     struct bpf_map_desc ringbuf_1;
>                     struct bpf_map_desc ringbuf_2;
>                     struct bpf_map_desc ringbuf_3;
>                     struct bpf_map_desc ringbuf_4;
>     four sections:
>                     struct bpf_map_desc rodata;
>                     struct bpf_map_desc data;
>                     struct bpf_map_desc bss;
>                     struct bpf_map_desc rodata_str1_1;
>     two progs:
>             SEC("fentry.s/bpf_handle_pefile")
>             SEC("fentry.s/bpf_post_handle_pefile")
> 
> With the above presumption, the integration consists of two parts:
>   -1. Call API exposed by light skeleton from kexec
>   -2. The opts_insn[] and opts_data[] are bpf-prog dependent and
>       can be extracted and passed in from the user space. In the
>       kexec_file_load design, a PE file has a .bpf section, which data
>       content is a ELF, and the ELF contains opts_insn[] opts_data[].
>       As a bonus, BPF bytecode can be placed under the protection of the
>       entire PE signature.
>       (Note, since opts_insn[] contains the information of the ringbuf
>        size, the bpf-prog writer can change its proper size according to
>        the kernel image size without modifying the kernel code)
> 
> Signed-off-by: Pingfan Liu <piliu@redhat.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Dave Young <dyoung@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Philipp Rudo <prudo@redhat.com>
> Cc: bpf@vger.kernel.org
> To: kexec@lists.infradead.org
> ---
>  kernel/Makefile                              |   1 +
>  kernel/kexec_bpf/Makefile                    |   8 +
>  kernel/kexec_bpf/kexec_pe_parser_bpf.lskel.h | 292 +------------------
>  kernel/kexec_pe_image.c                      |  70 +++++
>  4 files changed, 83 insertions(+), 288 deletions(-)
> 
[...]

> diff --git a/kernel/kexec_pe_image.c b/kernel/kexec_pe_image.c
> index e49d6db3c329d..f47c1e46dba97 100644
> --- a/kernel/kexec_pe_image.c
> +++ b/kernel/kexec_pe_image.c
> @@ -13,6 +13,7 @@
>  #include <linux/kernel.h>
>  #include <linux/vmalloc.h>
>  #include <linux/kexec.h>
> +#include <linux/elf.h>
>  #include <linux/pe.h>
>  #include <linux/string.h>
>  #include <linux/bpf.h>
> @@ -21,6 +22,7 @@
>  #include <asm/image.h>
>  #include <asm/memory.h>
>  
> +#include "kexec_bpf/kexec_pe_parser_bpf.lskel.h"
>  
>  static LIST_HEAD(phase_head);
>  
> @@ -163,14 +165,82 @@ static bool pe_has_bpf_section(char *file_buf, unsigned long pe_sz)
>  	return true;
>  }
>  
> +static struct kexec_pe_parser_bpf *pe_parser;
> +
> +static void *get_symbol_from_elf(const char *elf_data, size_t elf_size,
> +		const char *symbol_name, unsigned int *symbol_size)
> +{
> +	Elf_Ehdr *ehdr = (Elf_Ehdr *)elf_data;
> +	Elf_Shdr *shdr, *symtab_shdr, *strtab_shdr, *dst_shdr;
> +	Elf64_Sym *sym, *symtab = NULL;
> +	char *strtab = NULL;
> +	void *symbol_data = NULL;
> +	int i;
> +
> +	symtab_shdr = strtab_shdr = NULL;
> +	if (memcmp(ehdr->e_ident, ELFMAG, SELFMAG) != 0) {
> +		pr_err("Not a valid ELF file\n");
> +		goto out;
> +	}
> +
> +	shdr = (struct elf_shdr *)(elf_data + ehdr->e_shoff);
> +	for (i = 0; i < ehdr->e_shnum; i++) {
> +		if (shdr[i].sh_type == SHT_SYMTAB)
> +			symtab_shdr = &shdr[i];
> +		else if (shdr[i].sh_type == SHT_STRTAB && i != ehdr->e_shstrndx)
> +			strtab_shdr = &shdr[i];
> +	}
> +
> +	if (!symtab_shdr || !strtab_shdr) {
> +		pr_err("Symbol table or string table not found\n");
> +		goto out;
> +	}
> +	symtab = (Elf64_Sym *)(elf_data + symtab_shdr->sh_offset);
> +	strtab = (char *)(elf_data + strtab_shdr->sh_offset);
> +	for (i = 0; i < symtab_shdr->sh_size / sizeof(Elf64_Sym); i++) {
> +		sym = &symtab[i];
> +		if (strcmp(&strtab[sym->st_name], symbol_name) == 0) {
> +			if (sym->st_shndx >= SHN_LORESERVE)
> +				return NULL; // No section data for these
> +			dst_shdr = &shdr[sym->st_shndx];
> +			symbol_data = (void *)(elf_data + dst_shdr->sh_offset + sym->st_value);
> +			*symbol_size = symtab[i].st_size;
> +			break;
> +		}
> +	}
> +
> +out:
> +	return symbol_data;
> +}

In kernel/kexec_file.c there is kexec_purgatory_find_symbol which is
basically identical to this function. With a little bit of refractoring
it should work for both cases. I prefer using
kexec_purgatory_find_symbol as your implementation cannot handle elf
files with multiple symtab and strtab sections.

Thanks
Philipp

> +
>  /* Load a ELF */
>  static int arm_bpf_prog(char *bpf_elf, unsigned long sz)
>  {
> +	opts_data = get_symbol_from_elf(bpf_elf, sz, "opts_data", &opts_data_sz);
> +	opts_insn = get_symbol_from_elf(bpf_elf, sz, "opts_insn", &opts_insn_sz);
> +	if (!opts_data || !opts_insn)
> +		return -1;
> +	/*
> +	 * When light skeleton generates opts_data[] and opts_insn[], it appends a
> +	 * NULL terminator at the end of string
> +	 */
> +	opts_data_sz = opts_data_sz - 1;
> +	opts_insn_sz = opts_insn_sz - 1;
> +
> +	pe_parser = kexec_pe_parser_bpf__open_and_load();
> +	if (!pe_parser)
> +		return -1;
> +	kexec_pe_parser_bpf__attach(pe_parser);
> +
>  	return 0;
>  }
>  
>  static void disarm_bpf_prog(void)
>  {
> +	kexec_pe_parser_bpf__destroy(pe_parser);
> +	pe_parser = NULL;
> +	opts_data = NULL;
> +	opts_insn = NULL;
>  }
>  
>  struct kexec_context {