musl/src/env/__libc_start_main.c

#include <elf.h>
#include <poll.h>
#include <fcntl.h>
#include <signal.h>
#include <unistd.h>
#include "syscall.h"
#include "atomic.h"
#include "libc.h"

static void dummy(void) {}
weak_alias(dummy, _init);

extern weak hidden void (*const __init_array_start)(void), (*const __init_array_end)(void);

static void dummy1(void *p) {}
weak_alias(dummy1, __init_ssp);

#define AUX_CNT 38

#ifdef __GNUC__
__attribute__((__noinline__))
#endif
void __init_libc(char **envp, char *pn)
{
	size_t i, *auxv, aux[AUX_CNT] = { 0 };
	__environ = envp;
	for (i=0; envp[i]; i++);
	libc.auxv = auxv = (void *)(envp+i+1);
	for (i=0; auxv[i]; i+=2) if (auxv[i]<AUX_CNT) aux[auxv[i]] = auxv[i+1];
	__hwcap = aux[AT_HWCAP];
	__sysinfo = aux[AT_SYSINFO];
	libc.page_size = aux[AT_PAGESZ];

	if (!pn) pn = (void*)aux[AT_EXECFN];
	if (!pn) pn = "";
	__progname = __progname_full = pn;
	for (i=0; pn[i]; i++) if (pn[i]=='/') __progname = pn+i+1;

	__init_tls(aux);
	__init_ssp((void *)aux[AT_RANDOM]);

	if (aux[AT_UID]==aux[AT_EUID] && aux[AT_GID]==aux[AT_EGID]
		&& !aux[AT_SECURE]) return;

	struct pollfd pfd[3] = { {.fd=0}, {.fd=1}, {.fd=2} };
	int r =
#ifdef SYS_poll
	__syscall(SYS_poll, pfd, 3, 0);
#else
	__syscall(SYS_ppoll, pfd, 3, &(struct timespec){0}, 0, _NSIG/8);
#endif
	if (r<0) a_crash();
	for (i=0; i<3; i++) if (pfd[i].revents&POLLNVAL)
		if (__sys_open("/dev/null", O_RDWR)<0)
			a_crash();
	libc.secure = 1;
}

static void libc_start_init(void)
{
	_init();
	uintptr_t a = (uintptr_t)&__init_array_start;
	for (; a<(uintptr_t)&__init_array_end; a+=sizeof(void(*)()))
		(*(void (**)(void))a)();
}

weak_alias(libc_start_init, __libc_start_init);

static int libc_start_main_stage2(int (*)(int,char **,char **), int, char **);

int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
{
	char **envp = argv+argc+1;

	/* External linkage, and explicit noinline attribute if available,
	 * are used to prevent the stack frame used during init from
	 * persisting for the entire process lifetime. */
	__init_libc(envp, argv[0]);

	/* Barrier against hoisting application code or anything using ssp
	 * or thread pointer prior to its initialization above. */
	int (*stage2)();
	__asm__ ( "" : "=r"(stage2) : "r"(libc_start_main_stage2) : "memory" );
	return stage2(main, argc, argv);
}

static int libc_start_main_stage2(int (*main)(int,char **,char **), int argc, char **argv)
{
	char **envp = argv+argc+1;
	__libc_start_init();

	/* Pass control to the application */
	exit(main(argc, argv, envp));
	return 0;
}