libbb: new function bb_getgroups() - allocating wrapper around getgroups()

function                                             old     new   delta
bb_getgroups                                           -     111    +111
nexpr                                                843     757     -86
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/1 up/down: 111/-86)            Total: 25 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

coreutils/test.c

@@ -563,26 +563,11 @@ static int binop(void)
 	/*return 1; - NOTREACHED */
 }
 
-
 static void initialize_group_array(void)
 {
-	int n;
-
-	/* getgroups may be expensive, try to use it only once */
-	ngroups = 32;
-	do {
-		/* FIXME: ash tries so hard to not die on OOM,
-		 * and we spoil it with just one xrealloc here */
-		/* We realloc, because test_main can be entered repeatedly by shell.
-		 * Testcase (ash): 'while true; do test -x some_file; done'
-		 * and watch top. (some_file must have owner != you) */
-		n = ngroups;
-		group_array = xrealloc(group_array, n * sizeof(gid_t));
-		ngroups = getgroups(n, group_array);
-	} while (ngroups > n);
+	group_array = bb_getgroups(&ngroups, NULL);
 }
 
-
 /* Return non-zero if GID is one that we have in our groups list. */
 //XXX: FIXME: duplicate of existing libbb function?
 // see toplevel TODO file:
@@ -610,14 +595,10 @@ static int is_a_group_member(gid_t gid)
 /* Do the same thing access(2) does, but use the effective uid and gid,
    and don't make the mistake of telling root that any file is
    executable. */
-static int test_eaccess(char *path, int mode)
+static int test_eaccess(struct stat *st, int mode)
 {
-	struct stat st;
 	unsigned int euid = geteuid();
 
-	if (stat(path, &st) < 0)
-		return -1;
-
 	if (euid == 0) {
 		/* Root can read or write any file. */
 		if (mode != X_OK)
@@ -625,16 +606,16 @@ static int test_eaccess(char *path, int mode)
 
 		/* Root can execute any file that has any one of the execute
 		 * bits set. */
-		if (st.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))
+		if (st->st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))
 			return 0;
 	}
 
-	if (st.st_uid == euid)  /* owner */
+	if (st->st_uid == euid)  /* owner */
 		mode <<= 6;
-	else if (is_a_group_member(st.st_gid))
+	else if (is_a_group_member(st->st_gid))
 		mode <<= 3;
 
-	if (st.st_mode & mode)
+	if (st->st_mode & mode)
 		return 0;
 
 	return -1;
@@ -667,7 +648,7 @@ static int filstat(char *nm, enum token mode)
 			i = W_OK;
 		if (mode == FILEX)
 			i = X_OK;
-		return test_eaccess(nm, i) == 0;
+		return test_eaccess(&s, i) == 0;
 	}
 	if (is_file_type(mode)) {
 		if (mode == FILREG)

include/libbb.h

@@ -1033,6 +1033,15 @@ void die_if_bad_username(const char* name) FAST_FUNC;
 #else
 #define die_if_bad_username(name) ((void)(name))
 #endif
+/*
+ * Returns (-1) terminated malloced result of getgroups().
+ * Reallocs group_array (useful for repeated calls).
+ * ngroups is an initial size of array. It is rounded up to 32 for realloc.
+ * ngroups is updated on return.
+ * ngroups can be NULL: bb_getgroups(NULL, NULL) is valid usage.
+ * Dies on errors (on Linux, only xrealloc can cause this, not internal getgroups call).
+ */
+gid_t *bb_getgroups(int *ngroups, gid_t *group_array) FAST_FUNC;
 
 #if ENABLE_FEATURE_UTMP
 void FAST_FUNC write_new_utmp(pid_t pid, int new_type, const char *tty_name, const char *username, const char *hostname);

libbb/bb_getgroups.c

@@ -0,0 +1,47 @@
+/*
+ * Utility routines.
+ *
+ * Copyright (C) 2017 Denys Vlasenko
+ *
+ * Licensed under GPLv2, see file LICENSE in this source tree.
+ */
+
+//kbuild:lib-y += bb_getgroups.o
+
+#include "libbb.h"
+
+gid_t* FAST_FUNC bb_getgroups(int *ngroups, gid_t *group_array)
+{
+	int n = ngroups ? *ngroups : 0;
+
+	/* getgroups may be a bit expensive, try to use it only once */
+	if (n < 32)
+		n = 32;
+
+	for (;;) {
+// FIXME: ash tries so hard to not die on OOM (when we are called from test),
+// and we spoil it with just one xrealloc here
+		group_array = xrealloc(group_array, (n+1) * sizeof(group_array[0]));
+		n = getgroups(n, group_array);
+		/*
+		 * If buffer is too small, kernel does not return new_n > n.
+		 * It returns -1 and EINVAL:
+		 */
+		if (n >= 0) {
+			/* Terminator for bb_getgroups(NULL, NULL) usage */
+			group_array[n] = (gid_t) -1;
+			break;
+		}
+		if (errno == EINVAL) { /* too small? */
+			/* This is the way to ask kernel how big the array is */
+			n = getgroups(0, group_array);
+			continue;
+		}
+		/* Some other error (should never happen on Linux) */
+		bb_perror_msg_and_die("getgroups");
+	}
+
+	if (ngroups)
+		*ngroups = n;
+	return group_array;
+}