1 files changed, 201 insertions, 0 deletions
diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
new file mode 100644
index 000000000000..1ce90d8450ae
--- /dev/null
+++ b/arch/arm64/kernel/module-plts.c
@@ -0,0 +1,201 @@
+/*
+ * Copyright (C) 2014-2016 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 <linux/sort.h>
+
+struct plt_entry {
+	/*
+	 * A program that conforms to the AArch64 Procedure Call Standard
+	 * (AAPCS64) must assume that a veneer that alters IP0 (x16) and/or
+	 * IP1 (x17) may be inserted at any branch instruction that is
+	 * exposed to a relocation that supports long branches. Since that
+	 * is exactly what we are dealing with here, we are free to use x16
+	 * as a scratch register in the PLT veneers.
+	 */
+	__le32	mov0;	/* movn	x16, #0x....			*/
+	__le32	mov1;	/* movk	x16, #0x...., lsl #16		*/
+	__le32	mov2;	/* movk	x16, #0x...., lsl #32		*/
+	__le32	br;	/* br	x16				*/
+};
+
+u64 module_emit_plt_entry(struct module *mod, const Elf64_Rela *rela,
+			  Elf64_Sym *sym)
+{
+	struct plt_entry *plt = (struct plt_entry *)mod->arch.plt->sh_addr;
+	int i = mod->arch.plt_num_entries;
+	u64 val = sym->st_value + rela->r_addend;
+
+	/*
+	 * We only emit PLT entries against undefined (SHN_UNDEF) symbols,
+	 * which are listed in the ELF symtab section, but without a type
+	 * or a size.
+	 * So, similar to how the module loader uses the Elf64_Sym::st_value
+	 * field to store the resolved addresses of undefined symbols, let's
+	 * borrow the Elf64_Sym::st_size field (whose value is never used by
+	 * the module loader, even for symbols that are defined) to record
+	 * the address of a symbol's associated PLT entry as we emit it for a
+	 * zero addend relocation (which is the only kind we have to deal with
+	 * in practice). This allows us to find duplicates without having to
+	 * go through the table every time.
+	 */
+	if (rela->r_addend == 0 && sym->st_size != 0) {
+		BUG_ON(sym->st_size < (u64)plt || sym->st_size >= (u64)&plt[i]);
+		return sym->st_size;
+	}
+
+	mod->arch.plt_num_entries++;
+	BUG_ON(mod->arch.plt_num_entries > mod->arch.plt_max_entries);
+
+	/*
+	 * MOVK/MOVN/MOVZ opcode:
+	 * +--------+------------+--------+-----------+-------------+---------+
+	 * | sf[31] | opc[30:29] | 100101 | hw[22:21] | imm16[20:5] | Rd[4:0] |
+	 * +--------+------------+--------+-----------+-------------+---------+
+	 *
+	 * Rd     := 0x10 (x16)
+	 * hw     := 0b00 (no shift), 0b01 (lsl #16), 0b10 (lsl #32)
+	 * opc    := 0b11 (MOVK), 0b00 (MOVN), 0b10 (MOVZ)
+	 * sf     := 1 (64-bit variant)
+	 */
+	plt[i] = (struct plt_entry){
+		cpu_to_le32(0x92800010 | (((~val      ) & 0xffff)) << 5),
+		cpu_to_le32(0xf2a00010 | ((( val >> 16) & 0xffff)) << 5),
+		cpu_to_le32(0xf2c00010 | ((( val >> 32) & 0xffff)) << 5),
+		cpu_to_le32(0xd61f0200)
+	};
+
+	if (rela->r_addend == 0)
+		sym->st_size = (u64)&plt[i];
+
+	return (u64)&plt[i];
+}
+
+#define cmp_3way(a,b)	((a) < (b) ? -1 : (a) > (b))
+
+static int cmp_rela(const void *a, const void *b)
+{
+	const Elf64_Rela *x = a, *y = b;
+	int i;
+
+	/* sort by type, symbol index and addend */
+	i = cmp_3way(ELF64_R_TYPE(x->r_info), ELF64_R_TYPE(y->r_info));
+	if (i == 0)
+		i = cmp_3way(ELF64_R_SYM(x->r_info), ELF64_R_SYM(y->r_info));
+	if (i == 0)
+		i = cmp_3way(x->r_addend, y->r_addend);
+	return i;
+}
+
+static bool duplicate_rel(const Elf64_Rela *rela, int num)
+{
+	/*
+	 * Entries are sorted by type, symbol index and addend. That means
+	 * that, if a duplicate entry exists, it must be in the preceding
+	 * slot.
+	 */
+	return num > 0 && cmp_rela(rela + num, rela + num - 1) == 0;
+}
+
+static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num)
+{
+	unsigned int ret = 0;
+	Elf64_Sym *s;
+	int i;
+
+	for (i = 0; i < num; i++) {
+		switch (ELF64_R_TYPE(rela[i].r_info)) {
+		case R_AARCH64_JUMP26:
+		case R_AARCH64_CALL26:
+			/*
+			 * 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 128 MB
+			 * as well, so modules can never grow beyond that limit.
+			 */
+			s = syms + ELF64_R_SYM(rela[i].r_info);
+			if (s->st_shndx != SHN_UNDEF)
+				break;
+
+			/*
+			 * Jump relocations with non-zero addends against
+			 * undefined symbols are supported by the ELF spec, but
+			 * do not occur in practice (e.g., 'jump n bytes past
+			 * the entry point of undefined function symbol f').
+			 * So we need to support them, but there is no need to
+			 * take them into consideration when trying to optimize
+			 * this code. So let's only check for duplicates when
+			 * the addend is zero: this allows us to record the PLT
+			 * entry address in the symbol table itself, rather than
+			 * having to search the list for duplicates each time we
+			 * emit one.
+			 */
+			if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
+				ret++;
+			break;
+		}
+	}
+	return ret;
+}
+
+int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
+			      char *secstrings, struct module *mod)
+{
+	unsigned long plt_max_entries = 0;
+	Elf64_Sym *syms = NULL;
+	int i;
+
+	/*
+	 * Find the empty .plt section so we can expand it to store the PLT
+	 * entries. Record the symtab address as well.
+	 */
+	for (i = 0; i < ehdr->e_shnum; i++) {
+		if (strcmp(".plt", secstrings + sechdrs[i].sh_name) == 0)
+			mod->arch.plt = sechdrs + i;
+		else if (sechdrs[i].sh_type == SHT_SYMTAB)
+			syms = (Elf64_Sym *)sechdrs[i].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 (i = 0; i < ehdr->e_shnum; i++) {
+		Elf64_Rela *rels = (void *)ehdr + sechdrs[i].sh_offset;
+		int numrels = sechdrs[i].sh_size / sizeof(Elf64_Rela);
+		Elf64_Shdr *dstsec = sechdrs + sechdrs[i].sh_info;
+
+		if (sechdrs[i].sh_type != SHT_RELA)
+			continue;
+
+		/* ignore relocations that operate on non-exec sections */
+		if (!(dstsec->sh_flags & SHF_EXECINSTR))
+			continue;
+
+		/* sort by type, symbol index and addend */
+		sort(rels, numrels, sizeof(Elf64_Rela), cmp_rela, NULL);
+
+		plt_max_entries += count_plts(syms, 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 = plt_max_entries * sizeof(struct plt_entry);
+	mod->arch.plt_num_entries = 0;
+	mod->arch.plt_max_entries = plt_max_entries;
+	return 0;
+}