git-compat-util: work around for access(X_OK) under root

On AIX, access(X_OK) may succeed when run as root even if the
execution isn't possible. This behavior is allowed by POSIX
which says:

  ... for a process with appropriate privileges, an implementation
  may indicate success for X_OK even if execute permission is not
  granted to any user.

It can lead hook programs to have their execution refused:

   git commit -m content
   fatal: cannot exec '.git/hooks/pre-commit': Permission denied

Add NEED_ACCESS_ROOT_HANDLER in order to use an access helper function.
It checks with stat if any executable flags is set when the current user
is root.

Signed-off-by: Clément Chigot <clement.chigot@atos.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Clément Chigot

Makefile

@@ -439,6 +439,9 @@ all::
 #
 # Define FILENO_IS_A_MACRO if fileno() is a macro, not a real function.
 #
+# Define NEED_ACCESS_ROOT_HANDLER if access() under root may success for X_OK
+# even if execution permission isn't granted for any user.
+#
 # Define PAGER_ENV to a SP separated VAR=VAL pairs to define
 # default environment variables to be passed when a pager is spawned, e.g.
 #
@@ -1815,6 +1818,11 @@ ifdef FILENO_IS_A_MACRO
 	COMPAT_OBJS += compat/fileno.o
 endif
 
+ifdef NEED_ACCESS_ROOT_HANDLER
+	COMPAT_CFLAGS += -DNEED_ACCESS_ROOT_HANDLER
+	COMPAT_OBJS += compat/access.o
+endif
+
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK = NoThanks
 endif

compat/access.c

@@ -0,0 +1,31 @@
+#define COMPAT_CODE_ACCESS
+#include "../git-compat-util.h"
+
+/* Do the same thing access(2) does, but use the effective uid,
+ * and don't make the mistake of telling root that any file is
+ * executable.  This version uses stat(2).
+ */
+int git_access(const char *path, int mode)
+{
+	struct stat st;
+
+	/* do not interfere a normal user */
+	if (geteuid())
+		return access(path, mode);
+
+	if (stat(path, &st) < 0)
+		return -1;
+
+	/* Root can read or write any file. */
+	if (!(mode & X_OK))
+		return 0;
+
+	/* Root can execute any file that has any one of the execute
+	 * bits set.
+	 */
+	if (st.st_mode & (S_IXUSR | S_IXGRP | S_IXOTH))
+		return 0;
+
+	errno = EACCES;
+	return -1;
+}

compat/fileno.c

@@ -1,4 +1,4 @@
-#define COMPAT_CODE
+#define COMPAT_CODE_FILENO
 #include "../git-compat-util.h"
 
 int git_fileno(FILE *stream)

config.mak.uname

@@ -270,6 +270,7 @@ ifeq ($(uname_S),AIX)
 	NEEDS_LIBICONV = YesPlease
 	BASIC_CFLAGS += -D_LARGE_FILES
 	FILENO_IS_A_MACRO = UnfortunatelyYes
+	NEED_ACCESS_ROOT_HANDLER = UnfortunatelyYes
 	ifeq ($(shell expr "$(uname_V)" : '[1234]'),1)
 		NO_PTHREADS = YesPlease
 	else

git-compat-util.h

@@ -1236,12 +1236,22 @@ struct tm *git_gmtime_r(const time_t *, struct tm *);
 
 #ifdef FILENO_IS_A_MACRO
 int git_fileno(FILE *stream);
-# ifndef COMPAT_CODE
+# ifndef COMPAT_CODE_FILENO
 #  undef fileno
 #  define fileno(p) git_fileno(p)
 # endif
 #endif
 
+#ifdef NEED_ACCESS_ROOT_HANDLER
+int git_access(const char *path, int mode);
+# ifndef COMPAT_CODE_ACCESS
+#  ifdef access
+#  undef access
+#  endif
+#  define access(path, mode) git_access(path, mode)
+# endif
+#endif
+
 /*
  * Our code often opens a path to an optional file, to work on its
  * contents when we can successfully open it.  We can ignore a failure