path: root/arch/arm/kernel/module-plts.c

/*
 * Copyright (C) 2014 Linaro Ltd. <ard.biesheuvel@linaro.org>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as
 * published by the Free Software Foundation.
 */

#include <linux/elf.h>
#include <linux/kernel.h>
#include <linux/module.h>

#include <asm/cache.h>
#include <asm/opcodes.h>

#define PLT_ENT_STRIDE		L1_CACHE_BYTES
#define PLT_ENT_COUNT		(PLT_ENT_STRIDE / sizeof(u32))
#define PLT_ENT_SIZE		(sizeof(struct plt_entries) / PLT_ENT_COUNT)

#ifdef CONFIG_THUMB2_KERNEL
#define PLT_ENT_LDR		__opcode_to_mem_thumb32(0xf8dff000 | \
							(PLT_ENT_STRIDE - 4))
#else
#define PLT_ENT_LDR		__opcode_to_mem_arm(0xe59ff000 | \
						    (PLT_ENT_STRIDE - 8))
#endif

struct plt_entries {
	u32	ldr[PLT_ENT_COUNT];
	u32	lit[PLT_ENT_COUNT];
};

u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
{
	struct plt_entries *plt, *plt_end;
	int c;

	plt = (void *)mod->arch.plt->sh_addr;
	plt_end = (void *)plt + mod->arch.plt->sh_size;

	/* Look for an existing entry pointing to 'val' */
	for (c = mod->arch.plt_count; plt < plt_end; c -= PLT_ENT_COUNT, plt++) {
		int i;

		if (!c) {
			/* Populate a new set of entries */
			*plt = (struct plt_entries){
				{ [0 ... PLT_ENT_COUNT - 1] = PLT_ENT_LDR, },
				{ val, }
			};
			mod->arch.plt_count++;
			return (u32)plt->ldr;
		}
		for (i = 0; i < PLT_ENT_COUNT; i++) {
			if (!plt->lit[i]) {
				plt->lit[i] = val;
				mod->arch.plt_count++;
			}
			if (plt->lit[i] == val)
				return (u32)&plt->ldr[i];
		}
	}
	BUG();
}

static int duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num,
			   u32 mask)
{
	u32 *loc1, *loc2;
	int i;

	for (i = 0; i < num; i++) {
		if (rel[i].r_info != rel[num].r_info)
			continue;

		/*
		 * Identical relocation types against identical symbols can
		 * still result in different PLT entries if the addend in the
		 * place is different. So resolve the target of the relocation
		 * to compare the values.
		 */
		loc1 = (u32 *)(base + rel[i].r_offset);
		loc2 = (u32 *)(base + rel[num].r_offset);
		if (((*loc1 ^ *loc2) & mask) == 0)
			return 1;
	}
	return 0;
}

/* Count how many PLT entries we may need */
static unsigned int count_plts(const Elf32_Sym *syms, Elf32_Addr base,
			       const Elf32_Rel *rel, int num)
{
	unsigned int ret = 0;
	const Elf32_Sym *s;
	u32 mask;
	int i;

	if (IS_ENABLED(CONFIG_THUMB2_KERNEL))
		mask = __opcode_to_mem_thumb32(0x07ff2fff);
	else
		mask = __opcode_to_mem_arm(0x00ffffff);

	/*
	 * Sure, this is order(n^2), but it's usually short, and not
	 * time critical
	 */
	for (i = 0; i < num; i++) {
		switch (ELF32_R_TYPE(rel[i].r_info)) {
		case R_ARM_CALL:
		case R_ARM_PC24:
		case R_ARM_JUMP24:
		case R_ARM_THM_CALL:
		case R_ARM_THM_JUMP24:
			/*
			 * We only have to consider branch targets that resolve
			 * to undefined symbols. This is not simply a heuristic,
			 * it is a fundamental limitation, since the PLT itself
			 * is part of the module, and needs to be within range
			 * as well, so modules can never grow beyond that limit.
			 */
			s = syms + ELF32_R_SYM(rel[i].r_info);
			if (s->st_shndx != SHN_UNDEF)
				break;

			if (!duplicate_rel(base, rel, i, mask))
				ret++;
		}
	}
	return ret;
}

int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
			      char *secstrings, struct module *mod)
{
	unsigned long plts = 0;
	Elf32_Shdr *s, *sechdrs_end = sechdrs + ehdr->e_shnum;
	Elf32_Sym *syms = NULL;

	/*
	 * To store the PLTs, we expand the .text section for core module code
	 * and for initialization code.
	 */
	for (s = sechdrs; s < sechdrs_end; ++s) {
		if (strcmp(".plt", secstrings + s->sh_name) == 0)
			mod->arch.plt = s;
		else if (s->sh_type == SHT_SYMTAB)
			syms = (Elf32_Sym *)s->sh_addr;
	}

	if (!mod->arch.plt) {
		pr_err("%s: module PLT section missing\n", mod->name);
		return -ENOEXEC;
	}
	if (!syms) {
		pr_err("%s: module symtab section missing\n", mod->name);
		return -ENOEXEC;
	}

	for (s = sechdrs + 1; s < sechdrs_end; ++s) {
		const Elf32_Rel *rels = (void *)ehdr + s->sh_offset;
		int numrels = s->sh_size / sizeof(Elf32_Rel);
		Elf32_Shdr *dstsec = sechdrs + s->sh_info;

		if (s->sh_type != SHT_REL)
			continue;

		/* ignore relocations that operate on non-exec sections */
		if (!(dstsec->sh_flags & SHF_EXECINSTR))
			continue;

		plts += count_plts(syms, dstsec->sh_addr, rels, numrels);
	}

	mod->arch.plt->sh_type = SHT_NOBITS;
	mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
	mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
	mod->arch.plt->sh_size = round_up(plts * PLT_ENT_SIZE,
					  sizeof(struct plt_entries));
	mod->arch.plt_count = 0;

	pr_debug("%s: plt=%x\n", __func__, mod->arch.plt->sh_size);
	return 0;
}