[PATCH] tilegx: provide a setxid.h to handle negative uids

[Chris Metcalf <cmetcalf at tilera dot com>]

We observed a bug where setreuid(-1, -1) was becoming
setreuid(4294967295, 4294967295) instead, and therefore failing,
instead of doing nothing and returning success.

The problem is that normally, tilegx64 converts uint32_t to 64 bits
by sign extending.  This is counter-intuitive, but does allow for some
simplifications of the comparison instructions in the generated code.

So for a normal setreuid() call, the compiler would see the uint32_t
(aka uid_t) type for the function, and sign-extend for the SYSCALL
macro.  However, with the pthread setxid mechanism, we store the
arguments in an array of int64_t.  When we put the values into the
array, the compiler zero-extends them, and then they would be
passed to the kernel unchanged since they were already 64-bit types.
The result is the value 4294967295L, which is different from -1U.

To make this work properly on tilegx, we have to add a cast to
(int32_t) on the path so that whatever 32-bit values are put in the
array of longs get properly sign-extended.
---
This change is obviously accompanied by a change in the core
nptl/sysdeps/pthread/setxid.h file; we put an #ifndef __SETXID_1
around the chunk of code that #defines __SETXID_1 through __SETXID_3.
If the idea, and this change, seem acceptable, I'll send the patch
for the core file as well.

 ChangeLog.tile                             |    2 +
 sysdeps/unix/sysv/linux/tile/nptl/setxid.h |   43 ++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+), 0 deletions(-)
 create mode 100644 sysdeps/unix/sysv/linux/tile/nptl/setxid.h

diff --git a/ChangeLog.tile b/ChangeLog.tile
index 0befa86..baebcce 100644
--- a/ChangeLog.tile
+++ b/ChangeLog.tile
@@ -1,5 +1,7 @@
 2012-05-17  Chris Metcalf  <cmetcalf@tilera.com>
 
+	* sysdeps/unix/sysv/linux/tile/nptl/setxid.h: New file.
+
 	* sysdeps/tile/fegetenv.c: Version fegetenv() like fesetenv().
 	* sysdeps/unix/sysv/linux/tile/tilegx/tilegx32/nptl/libm.abilist:
 	Add fegetenv.
diff --git a/sysdeps/unix/sysv/linux/tile/nptl/setxid.h b/sysdeps/unix/sysv/linux/tile/nptl/setxid.h
new file mode 100644
index 0000000..8a4902b
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tile/nptl/setxid.h
@@ -0,0 +1,43 @@
+/* Override nptl/sysdeps/pthread/setxid.h for tilegx64.
+   Copyright (C) 2012 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <bits/wordsize.h>
+
+/* On tilegx, we sign-extend 32-bit unsigned values to set the high 32
+   bits in a 64-bit register.  Thus, a -1U value and a -1UL value are
+   represented the same way in a register.  This allows some compiler
+   optimizations where we don't have to explicitly clear the high bits
+   and can use native 64-bit instructions for 32-bit values.  However,
+   this means that if we are trying to pass a -1U value into the "long
+   int" fields of the xid_command.id[] fields, we need to explicitly
+   sign-extend the values before placing them in the fields, since we
+   eventually make the syscall using INTERNAL_SYSCALL_NCS(), at which
+   point the register values need to be in the correct ABI format.  */
+
+#if __WORDSIZE == 64
+# define __SETXID_1(cmd, arg1) \
+  cmd.id[0] = (sizeof (arg1) == 4) ? (int) arg1 : (long int) arg1
+# define __SETXID_2(cmd, arg1, arg2) \
+  __SETXID_1 (cmd, arg1); \
+  cmd.id[1] = (sizeof (arg2) == 4) ? (int) arg2 : (long int) arg2
+# define __SETXID_3(cmd, arg1, arg2, arg3) \
+  __SETXID_2 (cmd, arg1, arg2); \
+  cmd.id[2] = (sizeof (arg3) == 4) ? (int) arg3 : (long int) arg3
+#endif
+
+#include_next <setxid.h>
-- 
1.7.1