util: add virGetGroupList

https://bugzilla.redhat.com/show_bug.cgi?id=964358

Since neither getpwuid_r() nor initgroups() are safe to call in
between fork and exec (they obtain a mutex, but if some other
thread in the parent also held the mutex at the time of the fork,
the child will deadlock), we have to split out the functionality
that is unsafe.  At least glibc's initgroups() uses getgrouplist
under the hood, so the ideal split is to expose getgrouplist for
use before a fork.  Gnulib already gives us a nice wrapper via
mgetgroups; we wrap it once more to look up by uid instead of name.

* bootstrap.conf (gnulib_modules): Add mgetgroups.
* src/util/virutil.h (virGetGroupList): New declaration.
* src/util/virutil.c (virGetGroupList): New function.
* src/libvirt_private.syms (virutil.h): Export it.
Signed-off-by: NEric Blake <eblake@redhat.com>
(cherry picked from commit 75c12564)

Conflicts:
	bootstrap.conf - not updating gnulib submodule...
	configure.ac - ...so checking for getgrouplist by hand...
	src/util/virutil.c - ...and copying only the getgrouplist implementation rather than calling the gnulib function; also, file still named util.c
	src/libvirt_private.syms - context

configure.ac

@@ -171,7 +171,7 @@ AC_CHECK_SIZEOF([long])
 
 dnl Availability of various common functions (non-fatal if missing),
 dnl and various less common threadsafe functions
-AC_CHECK_FUNCS_ONCE([cfmakeraw geteuid getgid getgrnam_r getmntent_r \
+AC_CHECK_FUNCS_ONCE([cfmakeraw geteuid getgid getgrnam_r getgrouplist getmntent_r \
   getpwuid_r getuid initgroups kill mmap newlocale posix_fallocate \
   posix_memalign regexec sched_getaffinity])
 

src/libvirt_private.syms

@@ -1243,6 +1243,7 @@ virFileWaitForDevices;
 virFileWriteStr;
 virFindFileInPath;
 virGetGroupID;
+virGetGroupList;
 virGetGroupName;
 virGetHostname;
 virGetUserDirectory;

src/util/util.c

@@ -2575,6 +2575,66 @@ int virGetGroupID(const char *name,
 }
 
 
+/* Compute the list of supplementary groups associated with @uid, and
+ * including @gid in the list (unless it is -1), storing a malloc'd
+ * result into @list.  Return the size of the list on success, or -1
+ * on failure with error reported and errno set. May not be called
+ * between fork and exec. */
+int
+virGetGroupList(uid_t uid, gid_t gid, gid_t **list)
+{
+    int ret = -1;
+    char *user = NULL;
+
+    *list = NULL;
+    if (uid == (uid_t)-1)
+        return 0;
+
+    if (virGetUserEnt(uid, &user,
+                      gid == (gid_t)-1 ? &gid : NULL, NULL) < 0)
+        return -1;
+
+# if HAVE_GETGROUPLIST
+    /* Borrowing from gnulib's LGPLv2+ mgetgroups.c as of July 2013. */
+    /* Avoid a bug in older glibc with size 0, by pre-allocating a
+     * list size and then enlarging if needed.  */
+    int max = 10;
+    if (VIR_ALLOC_N(*list, max) < 0)
+        goto no_memory;
+    while (1)
+    {
+        int ngroups;
+        int last = max;
+
+        ngroups = getgrouplist(user, gid, *list, &max);
+
+        /* Avoid a bug in Darwin where max is not increased.  */
+        if (ngroups < 0 && last == max)
+            max *= 2;
+        if (VIR_REALLOC_N(*list, max) < 0) {
+            VIR_FREE(*list);
+            goto no_memory;
+        }
+        if (0 <= ngroups) {
+            ret = ngroups;
+            break;
+        }
+    }
+# else
+    if (VIR_ALLOC_N(*list, 1) < 0)
+        goto no_memory;
+    (*list)[0] = gid;
+    ret = 1;
+# endif
+
+cleanup:
+    VIR_FREE(user);
+    return ret;
+no_memory:
+    virReportOOMError();
+    goto cleanup;
+}
+
 /* Set the real and effective uid and gid to the given values, and call
  * initgroups so that the process has all the assumed group membership of
  * that uid. return 0 on success, -1 on failure (the original system error

src/util/util.h

@@ -261,6 +261,8 @@ char *virGetUserCacheDirectory(void);
 char *virGetUserRuntimeDirectory(void);
 char *virGetUserName(uid_t uid);
 char *virGetGroupName(gid_t gid);
+int virGetGroupList(uid_t uid, gid_t group, gid_t **groups)
+    ATTRIBUTE_NONNULL(3);
 int virGetUserID(const char *name,
                  uid_t *uid) ATTRIBUTE_RETURN_CHECK;
 int virGetGroupID(const char *name,