Anatomy of the loadable kernel module (lkm)
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Talk about how Linux kernel invokes your module's init function. Note: When you view the the slide deck via web browser, the screenshots may be blurred. You can download and view them offline (Screenshots are clear).
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Anatomy of the loadable kernel module (lkm)
- 1. Anatomy of the Loadable Kernel Module (LKM) Adrian Huang
- 2. Agenda • From ‘insmod’ command • Call path for LKM’s init function • ‘.gnu.linkonce.this_module’ section • Deep Dive into call path • modinfo Note • Kernel source: 5.10 • Mainly focus on the ‘init’ function call path
- 3. From `insmod` command Hello World Kernel Module strace finit_module() system call loads an ELF image into kernel space
- 4. From `insmod` command Hello World Kernel Module strace finit_module() system call loads an ELF image into kernel space finit_module() • Load an ELF image into kernel space • Perform symbol relocations • Initialize module parameters to values • Run the module’s init function
- 5. Call path for LKM’s init function finit_module load_module do_init_module do_one_initcall(mod->init) mod->init(); Analysis • Key: mod->init() • How to assign the address of mod->init()? kernel_read_file_from_fd elf_header_check setup_load_info blacklisted(info->name)? cleanup & return module_sig_check Y N mod = layout_and_allocate(info, ..) rewrite_section_headers apply_relocations
- 9. ‘.gnu.linkonce.this_module’ section (4/6) User Space Tool – modpost: Generate a file ‘module_name.mod.c’ when compiling your kernel module
- 10. ‘.gnu.linkonce.this_module’ section - Where is ‘init_module()’ definition? (5/6) Hello World Kernel Module module_init() macro __init macro
- 12. Deep Dive into call path (1/7) finit_module load_module do_init_module do_one_initcall(mod->init) mod->init(); kernel_read_file_from_fd elf_header_check setup_load_info blacklisted(info->name)? cleanup & return module_sig_check Y N mod = layout_and_allocate(info, ..) rewrite_section_headers apply_relocations ELF Header Section 1 .init.text .exit.text .gnu.linkonce.this_module … Section n Section Header 1 .init.text .exit.text .gnu.linkonce.this_module … Section Header n load_info name mod hdr len sechdrs secstrings strtab A (kernel addr) A + e_shoff
- 13. finit_module load_module do_init_module do_one_initcall(mod->init) mod->init(); kernel_read_file_from_fd elf_header_check setup_load_info blacklisted(info->name)? cleanup & return module_sig_check Y N mod = layout_and_allocate(info, ..) rewrite_section_headers apply_relocations ELF Header Section 1 .init.text .exit.text .gnu.linkonce.this_module … Section n Section Header 1 .init.text .exit.text .gnu.linkonce.this_module … Section Header n load_info name mod hdr len sechdrs secstrings strtab A (kernel addr) A + e_shoff Deep Dive into call path (2/7)
- 14. finit_module load_module kernel_read_file_from_fd elf_header_check setup_load_info blacklisted(info->name)? cleanup & return module_sig_check Y N rewrite_section_headers ELF Header Section 1 .init.text .exit.text .gnu.linkonce.this_module … Section n Section Header 1 .init.text .exit.text .gnu.linkonce.this_module … Section Header n load_info name mod hdr len sechdrs secstrings strtab A (kernel addr) A + e_shoff for (i = 1; i < info->hdr->e_shnum; i++) Elf_Shdr *shdr = &info->sechdrs[i]; shdr->sh_addr = (size_t)info->hdr + shdr->sh_offset; Deep Dive into call path (3/7) Update sh_addr (virtual address) of each section header table based on address ‘A’
- 15. Update sh_addr (virtual address) of each section header table based on core/init section memory allocation module … init core_layout init_layout exit … module_layout base size text_size ro_size ro_after_init_size .init.text .symtab .strtab Init section memory allocation 0xffffffffc0819000 0xffffffffc081a000 0xffffffffc081a000 module_layout base size text_size ro_size ro_after_init_size Deep Dive into call path (4/7)
- 16. module … init core_layout init_layout exit … module_layout base size text_size ro_size ro_after_init_size .symtab .strtab Init section memory allocation 0xffffffffc0819000 0xffffffffc081a000 0xffffffffc081a000 module_layout base size text_size ro_size ro_after_init_size init_module .init.text Deep Dive into call path (5/7)
- 17. module … init core_layout init_layout exit … module_layout base size text_size ro_size ro_after_init_size .symtab .strtab Init section memory allocation 0xffffffffc0819000 0xffffffffc081a000 0xffffffffc081a000 module_layout base size text_size ro_size ro_after_init_size init_module .init.text Deep Dive into call path (6/7)
- 18. Deep Dive into call path (7/7) finit_module load_module do_init_module do_one_initcall(mod->init) mod->init(); kernel_read_file_from_fd elf_header_check setup_load_info blacklisted(info->name)? cleanup & return module_sig_check Y N mod = layout_and_allocate(info, ..) rewrite_section_headers apply_relocations free & cleanup init_layout Free memory space of init_layout after calling mod->init()
- 19. modinfo Key=Value format in .modinfo section