passwd: Respect --prefix/-P options

Add prefix_getpwnam_r() and xprefix_getpwnam() and make passwd
use prefix-aware functions when handling the database.

libmisc/Makefile.am

@@ -75,6 +75,7 @@ libmisc_la_SOURCES = \
 	utmp.c \
 	valid.c \
 	xgetpwnam.c \
+	xprefix_getpwnam.c \
 	xgetpwuid.c \
 	xgetgrnam.c \
 	xgetgrgid.c \

libmisc/prefix_flag.c

@@ -238,6 +238,27 @@ extern struct passwd *prefix_getpwnam(const char* name)
 		return getpwnam(name);
 	}
 }
+extern int prefix_getpwnam_r(const char* name, struct passwd* pwd,
+                             char* buf, size_t buflen, struct passwd** result)
+{
+	if (passwd_db_file) {
+		FILE* fg;
+		int ret = 0;
+
+		fg = fopen(passwd_db_file, "rt");
+		if (!fg)
+			return errno;
+		while ((ret = fgetpwent_r(fg, pwd, buf, buflen, result)) == 0) {
+			if (!strcmp(name, pwd->pw_name))
+				break;
+		}
+		fclose(fg);
+		return ret;
+	}
+	else {
+		return getpwnam_r(name, pwd, buf, buflen, result);
+	}
+}
 extern struct spwd *prefix_getspnam(const char* name)
 {
 	if (spw_db_file) {

libmisc/xprefix_getpwnam.c

@@ -0,0 +1,41 @@
+/*
+ * SPDX-FileCopyrightText: 2007 - 2009, Nicolas François
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+/*
+ * According to the Linux-PAM documentation:
+ *
+ *  4.1. Care about standard library calls
+ *
+ *  In general, writers of authorization-granting applications should
+ *  assume that each module is likely to call any or all 'libc' functions.
+ *  For 'libc' functions that return pointers to static/dynamically
+ *  allocated structures (ie.  the library allocates the memory and the
+ *  user is not expected to 'free()' it) any module call to this function
+ *  is likely to corrupt a pointer previously obtained by the application.
+ *  The application programmer should either re-call such a 'libc'
+ *  function after a call to the Linux-PAM library, or copy the structure
+ *  contents to some safe area of memory before passing control to the
+ *  Linux-PAM library.
+ *
+ *  Two important function classes that fall into this category are
+ *  getpwnam(3) and syslog(3).
+ *
+ * This file provides wrapper to the prefix_getpwnam or prefix_getpwnam_r functions.
+ */
+
+#include <config.h>
+
+#include "pwio.h"
+
+#define LOOKUP_TYPE	struct passwd
+#define FUNCTION_NAME	prefix_getpwnam
+#define ARG_TYPE	const char *
+#define ARG_NAME	name
+#define DUP_FUNCTION	__pw_dup
+#define HAVE_FUNCTION_R 1
+
+#include "xgetXXbyYY.c"
+