1/*
  2 * Copyright (C) 2014-2016 Linaro Ltd. <ard.biesheuvel@linaro.org>
  3 *
  4 * This program is free software; you can redistribute it and/or modify
  5 * it under the terms of the GNU General Public License version 2 as
  6 * published by the Free Software Foundation.
  7 */
  8
  9#include <linux/elf.h>
 10#include <linux/kernel.h>
 11#include <linux/module.h>
 12#include <linux/sort.h>
 13
 14struct plt_entry {
 15	/*
 16	 * A program that conforms to the AArch64 Procedure Call Standard
 17	 * (AAPCS64) must assume that a veneer that alters IP0 (x16) and/or
 18	 * IP1 (x17) may be inserted at any branch instruction that is
 19	 * exposed to a relocation that supports long branches. Since that
 20	 * is exactly what we are dealing with here, we are free to use x16
 21	 * as a scratch register in the PLT veneers.
 22	 */
 23	__le32	mov0;	/* movn	x16, #0x....			*/
 24	__le32	mov1;	/* movk	x16, #0x...., lsl #16		*/
 25	__le32	mov2;	/* movk	x16, #0x...., lsl #32		*/
 26	__le32	br;	/* br	x16				*/
 27};
 28
 29u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
 30			  Elf64_Sym *sym)
 31{
 32	struct plt_entry *plt = (struct plt_entry *)mod->arch.plt->sh_addr;
 33	int i = mod->arch.plt_num_entries;
 34	u64 val = sym->st_value + rela->r_addend;
 35
 36	/*
 37	 * We only emit PLT entries against undefined (SHN_UNDEF) symbols,
 38	 * which are listed in the ELF symtab section, but without a type
 39	 * or a size.
 40	 * So, similar to how the module loader uses the Elf64_Sym::st_value
 41	 * field to store the resolved addresses of undefined symbols, let's
 42	 * borrow the Elf64_Sym::st_size field (whose value is never used by
 43	 * the module loader, even for symbols that are defined) to record
 44	 * the address of a symbol's associated PLT entry as we emit it for a
 45	 * zero addend relocation (which is the only kind we have to deal with
 46	 * in practice). This allows us to find duplicates without having to
 47	 * go through the table every time.
 48	 */
 49	if (rela->r_addend == 0 && sym->st_size != 0) {
 50		BUG_ON(sym->st_size < (u64)plt || sym->st_size >= (u64)&plt[i]);
 51		return sym->st_size;
 52	}
 53
 54	mod->arch.plt_num_entries++;
 55	BUG_ON(mod->arch.plt_num_entries > mod->arch.plt_max_entries);
 56
 57	/*
 58	 * MOVK/MOVN/MOVZ opcode:
 59	 * +--------+------------+--------+-----------+-------------+---------+
 60	 * | sf[31] | opc[30:29] | 100101 | hw[22:21] | imm16[20:5] | Rd[4:0] |
 61	 * +--------+------------+--------+-----------+-------------+---------+
 62	 *
 63	 * Rd     := 0x10 (x16)
 64	 * hw     := 0b00 (no shift), 0b01 (lsl #16), 0b10 (lsl #32)
 65	 * opc    := 0b11 (MOVK), 0b00 (MOVN), 0b10 (MOVZ)
 66	 * sf     := 1 (64-bit variant)
 67	 */
 68	plt[i] = (struct plt_entry){
 69		cpu_to_le32(0x92800010 | (((~val      ) & 0xffff)) << 5),
 70		cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
 71		cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
 72		cpu_to_le32(0xd61f0200)
 73	};
 74
 75	if (rela->r_addend == 0)
 76		sym->st_size = (u64)&plt[i];
 77
 78	return (u64)&plt[i];
 79}
 80
 81#define cmp_3way(a,b)	((a) < (b) ? -1 : (a) > (b))
 82
 83static int cmp_rela(const void *a, const void *b)
 84{
 85	const Elf64_Rela *x = a, *y = b;
 86	int i;
 87
 88	/* sort by type, symbol index and addend */
 89	i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
 90	if (i == 0)
 91		i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
 92	if (i == 0)
 93		i = cmp_3way(x->r_addend, y->r_addend);
 94	return i;
 95}
 96
 97static bool duplicate_rel(const Elf64_Rela *rela, int num)
 98{
 99	/*
100	 * Entries are sorted by type, symbol index and addend. That means
101	 * that, if a duplicate entry exists, it must be in the preceding
102	 * slot.
103	 */
104	return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
105}
106
107static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num)
108{
109	unsigned int ret = 0;
110	Elf64_Sym *s;
111	int i;
112
113	for (i = 0; i < num; i++) {
114		switch (ELF64_R_TYPE(rela[i].r_info)) {
115		case R_AARCH64_JUMP26:
116		case R_AARCH64_CALL26:
117			/*
118			 * We only have to consider branch targets that resolve
119			 * to undefined symbols. This is not simply a heuristic,
120			 * it is a fundamental limitation, since the PLT itself
121			 * is part of the module, and needs to be within 128 MB
122			 * as well, so modules can never grow beyond that limit.
123			 */
124			s = syms + ELF64_R_SYM(rela[i].r_info);
125			if (s->st_shndx != SHN_UNDEF)
126				break;
127
128			/*
129			 * Jump relocations with non-zero addends against
130			 * undefined symbols are supported by the ELF spec, but
131			 * do not occur in practice (e.g., 'jump n bytes past
132			 * the entry point of undefined function symbol f').
133			 * So we need to support them, but there is no need to
134			 * take them into consideration when trying to optimize
135			 * this code. So let's only check for duplicates when
136			 * the addend is zero: this allows us to record the PLT
137			 * entry address in the symbol table itself, rather than
138			 * having to search the list for duplicates each time we
139			 * emit one.
140			 */
141			if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
142				ret++;
143			break;
144		}
145	}
146	return ret;
147}
148
149int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
150			      char *secstrings, struct module *mod)
151{
152	unsigned long plt_max_entries = 0;
153	Elf64_Sym *syms = NULL;
154	int i;
155
156	/*
157	 * Find the empty .plt section so we can expand it to store the PLT
158	 * entries. Record the symtab address as well.
159	 */
160	for (i = 0; i < ehdr->e_shnum; i++) {
161		if (strcmp(".plt", secstrings + sechdrs[i].sh_name) == 0)
162			mod->arch.plt = sechdrs + i;
163		else if (sechdrs[i].sh_type == SHT_SYMTAB)
164			syms = (Elf64_Sym *)sechdrs[i].sh_addr;
165	}
166
167	if (!mod->arch.plt) {
168		pr_err("%s: module PLT section missing\n", mod->name);
169		return -ENOEXEC;
170	}
171	if (!syms) {
172		pr_err("%s: module symtab section missing\n", mod->name);
173		return -ENOEXEC;
174	}
175
176	for (i = 0; i < ehdr->e_shnum; i++) {
177		Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
178		int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
179		Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
180
181		if (sechdrs[i].sh_type != SHT_RELA)
182			continue;
183
184		/* ignore relocations that operate on non-exec sections */
185		if (!(dstsec->sh_flags & SHF_EXECINSTR))
186			continue;
187
188		/* sort by type, symbol index and addend */
189		sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
190
191		plt_max_entries += count_plts(syms, rels, numrels);
192	}
193
194	mod->arch.plt->sh_type = SHT_NOBITS;
195	mod->arch.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
196	mod->arch.plt->sh_addralign = L1_CACHE_BYTES;
197	mod->arch.plt->sh_size = plt_max_entries * sizeof(struct plt_entry);
198	mod->arch.plt_num_entries = 0;
199	mod->arch.plt_max_entries = plt_max_entries;
200	return 0;
201}