From: Rich Felker <dalias@xxxxxxxx> The ELF binary loader in binfmt_elf.c requires an MMU, making it impossible to use regular ELF binaries on NOMMU archs. However, the FDPIC ELF loader in binfmt_elf_fdpic.c is fully capable as a loader for plain ELF, which requires constant displacements between LOAD segments, since it already supports FDPIC ELF files flagged as needing constant displacement. This patch adjusts the FDPIC ELF loader to accept non-FDPIC ELF files on NOMMU archs. They are treated identically to FDPIC ELF files with the constant-displacement flag bit set, except for personality, which must match the ABI of the program being loaded; the PER_LINUX_FDPIC personality controls how the kernel interprets function pointers passed to sigaction. Files that do not set a stack size requirement explicitly are given a default stack size (matching the amount of committed stack the normal ELF loader for MMU archs would give them) rather than being rejected; this is necessary because plain ELF files generally do not declare stack requirements in theit program headers. Only ET_DYN (PIE) format ELF files are supported, since loading at a fixed virtual address is not possible on NOMMU. Signed-off-by: Rich Felker <dalias@xxxxxxxx> --- This patch was developed and tested on J2 (SH2-compatible) but should be usable immediately on all archs where binfmt_elf_fdpic is available. Moreover, by providing dummy definitions of the elf_check_fdpic() and elf_check_const_displacement() macros for archs which lack an FDPIC ABI, it should be possible to enable building of binfmt_elf_fdpic on all other NOMMU archs and thereby give them ELF binary support, but I have not yet tested this. The motivation for using binfmt_elf_fdpic.c rather than adapting binfmt_elf.c to NOMMU is that the former already has all the necessary code to work properly on NOMMU and has already received widespread real-world use and testing. I hope this is not controversial. I'm not really happy with having to unset the FDPIC_FUNCPTRS personality bit when loading non-FDPIC ELF. This bit should really reset automatically on execve, since otherwise, executing non-ELF binaries (e.g. bFLT) from an FDPIC process will leave the personality in the wrong state and severely break signal handling. But that's a separate, existing bug and I don't know the right place to fix it. --- fs/binfmt_elf_fdpic.c.orig 2015-09-29 22:13:06.716412478 +0000 +++ fs/binfmt_elf_fdpic.c 2015-09-29 22:38:24.122986621 +0000 @@ -103,19 +103,36 @@ core_initcall(init_elf_fdpic_binfmt); module_exit(exit_elf_fdpic_binfmt); -static int is_elf_fdpic(struct elfhdr *hdr, struct file *file) +static int is_elf(struct elfhdr *hdr, struct file *file) { if (memcmp(hdr->e_ident, ELFMAG, SELFMAG) != 0) return 0; if (hdr->e_type != ET_EXEC && hdr->e_type != ET_DYN) return 0; - if (!elf_check_arch(hdr) || !elf_check_fdpic(hdr)) + if (!elf_check_arch(hdr)) return 0; if (!file->f_op->mmap) return 0; return 1; } +#ifndef elf_check_fdpic +#define elf_check_fdpic(x) 0 +#endif + +#ifndef elf_check_const_displacement +#define elf_check_const_displacement(x) 0 +#endif + +static int is_constdisp(struct elfhdr *hdr) +{ + if (!elf_check_fdpic(hdr)) + return 1; + if (elf_check_const_displacement(hdr)) + return 1; + return 0; +} + /*****************************************************************************/ /* * read the program headers table into memory @@ -191,8 +208,16 @@ /* check that this is a binary we know how to deal with */ retval = -ENOEXEC; - if (!is_elf_fdpic(&exec_params.hdr, bprm->file)) + if (!is_elf(&exec_params.hdr, bprm->file)) + goto error; +#ifdef CONFIG_MMU + /* binfmt_elf handles non-fdpic elf except on nommu */ + if (!elf_check_fdpic(&exec_params.hdr)) + goto error; +#else + if (exec_params.hdr.e_type != ET_DYN) goto error; +#endif /* read the program header table */ retval = elf_fdpic_fetch_phdrs(&exec_params, bprm->file); @@ -269,13 +294,13 @@ } - if (elf_check_const_displacement(&exec_params.hdr)) + if (is_constdisp(&exec_params.hdr)) exec_params.flags |= ELF_FDPIC_FLAG_CONSTDISP; /* perform insanity checks on the interpreter */ if (interpreter_name) { retval = -ELIBBAD; - if (!is_elf_fdpic(&interp_params.hdr, interpreter)) + if (!is_elf(&interp_params.hdr, interpreter)) goto error; interp_params.flags = ELF_FDPIC_FLAG_PRESENT; @@ -306,9 +331,9 @@ retval = -ENOEXEC; if (stack_size == 0) - goto error; + stack_size = 131072UL; /* same as exec.c's default commit */ - if (elf_check_const_displacement(&interp_params.hdr)) + if (is_constdisp(&interp_params.hdr)) interp_params.flags |= ELF_FDPIC_FLAG_CONSTDISP; /* flush all traces of the currently running executable */ @@ -319,7 +344,10 @@ /* there's now no turning back... the old userspace image is dead, * defunct, deceased, etc. */ - set_personality(PER_LINUX_FDPIC); + if (elf_check_fdpic(&exec_params.hdr)) + set_personality(PER_LINUX_FDPIC); + else + set_personality(PER_LINUX); if (elf_read_implies_exec(&exec_params.hdr, executable_stack)) current->personality |= READ_IMPLIES_EXEC; -- To unsubscribe from this list: send the line "unsubscribe linux-embedded" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html