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Commit 32b63776 authored by Russell King's avatar Russell King
Browse files

Merge tag 'arm-plt-optimizations-for-v4.9' of... 

Merge tag 'arm-plt-optimizations-for-v4.9' of git://git.linaro.org/people/ard.biesheuvel/linux-arm into devel-stable

This series of 4 patches optimizes the ARM PLT generation code that
is invoked at module load time, to get rid of the O(n^2) algorithm
that results in pathological load times of 10 seconds or more for
large modules on certain STB platforms
parents c6935931 66e94ba3

arch/arm/include/asm/module.h

+2 −4
Original line number Diff line number Diff line
@@ -23,10 +23,8 @@ struct mod_arch_specific { 
	struct unwind_table *unwind[ARM_SEC_MAX];

#endif

#ifdef CONFIG_ARM_MODULE_PLTS

	struct elf32_shdr   *core_plt;

	struct elf32_shdr   *init_plt;

	int		    core_plt_count;

	int		    init_plt_count;

	struct elf32_shdr   *plt;

	int		    plt_count;

#endif

};

arch/arm/kernel/module-plts.c

+144 −99
Original line number Diff line number Diff line
@@ -9,6 +9,7 @@ 
#include <linux/elf.h>

#include <linux/kernel.h>

#include <linux/module.h>

#include <linux/sort.h>



#include <asm/cache.h>

#include <asm/opcodes.h>
@@ -30,101 +31,142 @@ struct plt_entries { 
	u32	lit[PLT_ENT_COUNT];

};



static bool in_init(const struct module *mod, u32 addr)

{

	return addr - (u32)mod->init_layout.base < mod->init_layout.size;

}



u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)

{

	struct plt_entries *plt, *plt_end;

	int c, *count;



	if (in_init(mod, loc)) {

		plt = (void *)mod->arch.init_plt->sh_addr;

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

		count = &mod->arch.init_plt_count;

	} else {

		plt = (void *)mod->arch.core_plt->sh_addr;

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

		count = &mod->arch.core_plt_count;

	struct plt_entries *plt = (struct plt_entries *)mod->arch.plt->sh_addr;

	int idx = 0;



	/*

	 * Look for an existing entry pointing to 'val'. Given that the

	 * relocations are sorted, this will be the last entry we allocated.

	 * (if one exists).

	 */

	if (mod->arch.plt_count > 0) {

		plt += (mod->arch.plt_count - 1) / PLT_ENT_COUNT;

		idx = (mod->arch.plt_count - 1) % PLT_ENT_COUNT;



		if (plt->lit[idx] == val)

			return (u32)&plt->ldr[idx];



		idx = (idx + 1) % PLT_ENT_COUNT;

		if (!idx)

			plt++;

	}



	/* Look for an existing entry pointing to 'val' */

	for (c = *count; plt < plt_end; c -= PLT_ENT_COUNT, plt++) {

		int i;

	mod->arch.plt_count++;

	BUG_ON(mod->arch.plt_count * PLT_ENT_SIZE > mod->arch.plt->sh_size);



		if (!c) {

	if (!idx)

		/* Populate a new set of entries */

		*plt = (struct plt_entries){

			{ [0 ... PLT_ENT_COUNT - 1] = PLT_ENT_LDR, },

			{ val, }

		};

			++*count;

			return (u32)plt->ldr;

		}

		for (i = 0; i < PLT_ENT_COUNT; i++) {

			if (!plt->lit[i]) {

				plt->lit[i] = val;

				++*count;

	else

		plt->lit[idx] = val;



	return (u32)&plt->ldr[idx];

}

			if (plt->lit[i] == val)

				return (u32)&plt->ldr[i];



#define cmp_3way(a,b)	((a) < (b) ? -1 : (a) > (b))



static int cmp_rel(const void *a, const void *b)

{

	const Elf32_Rel *x = a, *y = b;

	int i;



	/* sort by type and symbol index */

	i = cmp_3way(ELF32_R_TYPE(x->r_info), ELF32_R_TYPE(y->r_info));

	if (i == 0)

		i = cmp_3way(ELF32_R_SYM(x->r_info), ELF32_R_SYM(y->r_info));

	return i;

}



static bool is_zero_addend_relocation(Elf32_Addr base, const Elf32_Rel *rel)

{

	u32 *tval = (u32 *)(base + rel->r_offset);



	/*

	 * Do a bitwise compare on the raw addend rather than fully decoding

	 * the offset and doing an arithmetic comparison.

	 * Note that a zero-addend jump/call relocation is encoded taking the

	 * PC bias into account, i.e., -8 for ARM and -4 for Thumb2.

	 */

	switch (ELF32_R_TYPE(rel->r_info)) {

		u16 upper, lower;



	case R_ARM_THM_CALL:

	case R_ARM_THM_JUMP24:

		upper = __mem_to_opcode_thumb16(((u16 *)tval)[0]);

		lower = __mem_to_opcode_thumb16(((u16 *)tval)[1]);



		return (upper & 0x7ff) == 0x7ff && (lower & 0x2fff) == 0x2ffe;



	case R_ARM_CALL:

	case R_ARM_PC24:

	case R_ARM_JUMP24:

		return (__mem_to_opcode_arm(*tval) & 0xffffff) == 0xfffffe;

	}

	BUG();

}



static int duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num,

			   u32 mask)

static bool duplicate_rel(Elf32_Addr base, const Elf32_Rel *rel, int num)

{

	u32 *loc1, *loc2;

	int i;



	for (i = 0; i < num; i++) {

		if (rel[i].r_info != rel[num].r_info)

			continue;

	const Elf32_Rel *prev;



	/*

		 * 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.

	 * Entries are sorted by type and symbol index. That means that,

	 * if a duplicate entry exists, it must be in the preceding

	 * slot.

	 */

		loc1 = (u32 *)(base + rel[i].r_offset);

		loc2 = (u32 *)(base + rel[num].r_offset);

		if (((*loc1 ^ *loc2) & mask) == 0)

			return 1;

	}

	return 0;

	if (!num)

		return false;



	prev = rel + num - 1;

	return cmp_rel(rel + num, prev) == 0 &&

	       is_zero_addend_relocation(base, prev);

}



/* Count how many PLT entries we may need */

static unsigned int count_plts(Elf32_Addr base, const Elf32_Rel *rel, int num)

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;

	int i;



	/*

	 * Sure, this is order(n^2), but it's usually short, and not

	 * time critical

	 */

	for (i = 0; i < num; i++)

	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:

			if (!duplicate_rel(base, rel, i,

					   __opcode_to_mem_arm(0x00ffffff)))

				ret++;

			break;

#ifdef CONFIG_THUMB2_KERNEL

		case R_ARM_THM_CALL:

		case R_ARM_THM_JUMP24:

			if (!duplicate_rel(base, rel, i,

					   __opcode_to_mem_thumb32(0x07ff2fff)))

			/*

			 * 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;



			/*

			 * 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.

			 */

			if (!is_zero_addend_relocation(base, rel + i) ||

			    !duplicate_rel(base, rel, i))

				ret++;

#endif

		}

	}

	return ret;

}
@@ -132,52 +174,55 @@ static unsigned int count_plts(Elf32_Addr base, const Elf32_Rel *rel, int num) 
int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,

			      char *secstrings, struct module *mod)

{

	unsigned long core_plts = 0, init_plts = 0;

	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 the .init.text section for initialization code.

	 * and for initialization code.

	 */

	for (s = sechdrs; s < sechdrs_end; ++s)

		if (strcmp(".core.plt", secstrings + s->sh_name) == 0)

			mod->arch.core_plt = s;

		else if (strcmp(".init.plt", secstrings + s->sh_name) == 0)

			mod->arch.init_plt = s;



	if (!mod->arch.core_plt || !mod->arch.init_plt) {

		pr_err("%s: sections missing\n", mod->name);

	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;

		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;



		if (strstr(secstrings + s->sh_name, ".init"))

			init_plts += count_plts(dstsec->sh_addr, rels, numrels);

		else

			core_plts += count_plts(dstsec->sh_addr, rels, numrels);

		/* ignore relocations that operate on non-exec sections */

		if (!(dstsec->sh_flags & SHF_EXECINSTR))

			continue;



		/* sort by type and symbol index */

		sort(rels, numrels, sizeof(Elf32_Rel), cmp_rel, NULL);



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

	}



	mod->arch.core_plt->sh_type = SHT_NOBITS;

	mod->arch.core_plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;

	mod->arch.core_plt->sh_addralign = L1_CACHE_BYTES;

	mod->arch.core_plt->sh_size = round_up(core_plts * PLT_ENT_SIZE,

	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.core_plt_count = 0;

	mod->arch.plt_count = 0;



	mod->arch.init_plt->sh_type = SHT_NOBITS;

	mod->arch.init_plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;

	mod->arch.init_plt->sh_addralign = L1_CACHE_BYTES;

	mod->arch.init_plt->sh_size = round_up(init_plts * PLT_ENT_SIZE,

					       sizeof(struct plt_entries));

	mod->arch.init_plt_count = 0;

	pr_debug("%s: core.plt=%x, init.plt=%x\n", __func__,

		 mod->arch.core_plt->sh_size, mod->arch.init_plt->sh_size);

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

	return 0;

}

arch/arm/kernel/module.lds

+1 −2
Original line number Diff line number Diff line 
SECTIONS {

        .core.plt : { BYTE(0) }

        .init.plt : { BYTE(0) }

	.plt : { BYTE(0) }

}