musl/src/ldso/dlstart.c

#include <stddef.h>
#include "dynlink.h"

#ifdef SHARED

#ifndef START
#define START "_dlstart"
#endif

#include "crt_arch.h"

__attribute__((__visibility__("hidden")))
void _dlstart_c(size_t *sp, size_t *dynv)
{
	size_t i, aux[AUX_CNT], dyn[DYN_CNT];

	int argc = *sp;
	char **argv = (void *)(sp+1);

	for (i=argc+1; argv[i]; i++);
	size_t *auxv = (void *)(argv+i+1);

	for (i=0; i<AUX_CNT; i++) aux[i] = 0;
	for (i=0; auxv[i]; i+=2) if (auxv[i]<AUX_CNT)
		aux[auxv[i]] = auxv[i+1];

	for (i=0; i<DYN_CNT; i++) dyn[i] = 0;
	for (i=0; dynv[i]; i+=2) if (dynv[i]<DYN_CNT)
		dyn[dynv[i]] = dynv[i+1];

	/* If the dynamic linker is invoked as a command, its load
	 * address is not available in the aux vector. Instead, compute
	 * the load address as the difference between &_DYNAMIC and the
	 * virtual address in the PT_DYNAMIC program header. */
	unsigned char *base = (void *)aux[AT_BASE];
	if (!base) {
		size_t phnum = aux[AT_PHNUM];
		size_t phentsize = aux[AT_PHENT];
		Phdr *ph = (void *)aux[AT_PHDR];
		for (i=phnum; i--; ph = (void *)((char *)ph + phentsize)) {
			if (ph->p_type == PT_DYNAMIC) {
				base = (void *)((size_t)dynv - ph->p_vaddr);
				break;
			}
		}
	}

	/* MIPS uses an ugly packed form for GOT relocations. Since we
	 * can't make function calls yet and the code is tiny anyway,
	 * it's simply inlined here. */
	if (NEED_MIPS_GOT_RELOCS) {
		size_t local_cnt = 0;
		size_t *got = (void *)(base + dyn[DT_PLTGOT]);
		for (i=0; dynv[i]; i+=2) if (dynv[i]==DT_MIPS_LOCAL_GOTNO)
			local_cnt = dynv[i+1];
		for (i=0; i<local_cnt; i++) got[i] += (size_t)base;
	}

	/* The use of the reloc_info structure and nested loops is a trick
	 * to work around the fact that we can't necessarily make function
	 * calls yet. Each struct in the array serves like the arguments
	 * to a function call. */
	struct {
		void *rel;
		size_t size;
		size_t stride;
	} reloc_info[] = {
		{ base+dyn[DT_REL], dyn[DT_RELSZ], 2 },
		{ base+dyn[DT_RELA], dyn[DT_RELASZ], 3 },
		{ 0, 0, 0 }
	};

	for (i=0; reloc_info[i].stride; i++) {
		size_t *rel = reloc_info[i].rel;
		size_t rel_size = reloc_info[i].size;
		size_t stride = reloc_info[i].stride;
		for (; rel_size; rel+=stride, rel_size-=stride*sizeof(size_t)) {
			if (!IS_RELATIVE(rel[1])) continue;
			size_t *rel_addr = (void *)(base + rel[0]);
			size_t addend = stride==3 ? rel[2] : *rel_addr;
			*rel_addr = (size_t)base + addend;
		}
	}

	const char *strings = (void *)(base + dyn[DT_STRTAB]);
	const Sym *syms = (void *)(base + dyn[DT_SYMTAB]);

	/* Call dynamic linker stage-2, __dls2 */
	for (i=0; ;i++) {
		const char *s = strings + syms[i].st_name;
		if (s[0]=='_' && s[1]=='_' && s[2]=='d'
		 && s[3]=='l' && s[4]=='s' && s[5]=='2' && !s[6])
			break;
	}
	((stage2_func)(base + syms[i].st_value))(base);

	/* Call dynamic linker stage-3, __dls3 */
	for (i=0; ;i++) {
		const char *s = strings + syms[i].st_name;
		if (s[0]=='_' && s[1]=='_' && s[2]=='d'
		 && s[3]=='l' && s[4]=='s' && s[5]=='3' && !s[6])
			break;
	}
	((stage3_func)(base + syms[i].st_value))(sp);
}

#endif