Proposed getlogin[_r] replacement

Proposed getlogin[_r] replacement

[Rich Felker]

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The attached getlogin[_r] implementation (with simple test program)
fixes conformance issues with the current one, which uses the
environment rather than (as required by POSIX) the controlling tty to
determine the identity of the logged-in user. It's missing protection
against cancellation points but otherwise should be doing correct
error-checking. If no problems are seen I'd like to go ahead and
commit it in this release cycle.

Rich

[-- Attachment #2: new_getlogin_3.c --]
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#include <stdio.h>
#include <pwd.h>
#include <limits.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <ctype.h>
#include <sys/stat.h>
#include <sys/sysmacros.h>

int getlogin_r(char *buf, size_t size)
{
	struct passwd pw, *ppw;
	char pw_buf[1024];
	struct stat st;
	char tty_buf[TTY_NAME_MAX];
	int i;
	dev_t dev;

	char procbuf[1024];
	int fd = open("/proc/self/stat", O_RDONLY|O_CLOEXEC);
	if (fd < 0) switch (errno) {
	case EMFILE:
	case ENFILE:
		return errno;
	default:
		return ENOSYS;
	}
	int n = read(fd, procbuf, sizeof procbuf - 1);
	close(fd);
	if (n < 0) return ENOSYS;
	procbuf[n] = 0;

	char *s = strrchr(procbuf, ')');
	if (!s) return ENOSYS;

	for (i=0; i<5 && *s; i+=(*s++==' '));
	if (!*s) return ENOSYS;

	for (dev=0; isdigit(*s); s++)
		dev = 10*dev + (*s-'0');

	if (!dev) return ENXIO;

	for (i=0; i<3; i++)
		if (!fstat(i, &st) && S_ISCHR(st.st_mode) && st.st_rdev == dev)
			break;

	snprintf(tty_buf, sizeof tty_buf, "/dev/pts/%u", (unsigned)minor(dev));
	if (i<3 || (!stat(tty_buf, &st) && S_ISCHR(st.st_mode) && st.st_rdev == dev)) {
		getpwuid_r(st.st_uid, &pw, pw_buf, sizeof pw_buf, &ppw);
		size_t l = strlen(pw.pw_name);
		if (l >= LOGIN_NAME_MAX) return EOVERFLOW;
		if (l >= size) return ERANGE;
		memcpy(buf, pw.pw_name, l+1);
		return 0;
	}

	return ENOTTY;
}

char *getlogin()
{
	static char buf[LOGIN_NAME_MAX];
	int r = getlogin_r(buf, sizeof buf);
	if (r) {
		errno = r;
		return 0;
	}
	return buf;
}

int main()
{
	printf("%s %m\n", getlogin());
	return 0;
}