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>

3 files changed, 63 insertions, 26 deletions

diff --git a/coreutils/test.c b/coreutils/test.c
index edc625f57..edcf2a2d8 100644
--- a/coreutils/test.c
+++ b/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)
diff --git a/include/libbb.h b/include/libbb.h
index 557978e66..1c9de3af0 100644
--- a/include/libbb.h
+++ b/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);
diff --git a/libbb/bb_getgroups.c b/libbb/bb_getgroups.c
new file mode 100644
index 000000000..59ae53738
--- /dev/null
+++ b/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;
+}