Re: Misaligned stack on 32-bit s390?

[Siddhesh Poyarekar <siddhesh at redhat dot com>]

On Wed, Nov 12, 2014 at 08:18:48AM +0100, Richard Henderson wrote:
> On 11/12/2014 08:09 AM, Siddhesh Poyarekar wrote:
> > +	l     %r1,0(%r1)	# load _dl_skip_args\n\
> > +	ltr   %r1,%r1\n\
> 
> 	lt	%r1, 0(%r1)
> 
> > +	l     %r1,_dl_argv@GOT12(0,%r12)\n\
> 
> Why are you using got12?  Drop the indexed form and just use got.
> Hmm.  I do see there's a mix in this file, but I don't know why...

I got an invalid reference when I used GOT and my first thought was
that the binutils manual was lying.  I think I later actually fixed it
by changing something else and I just forgot to verify the manual's
honesty again.  I'll change the remaining GOT12 in a separate patch.

Updated patch, generated code is identical.

Siddhesh

commit 5767d20b332eda3501a2b559bfdbdac943c524fd
Author: Siddhesh Poyarekar <siddhesh@redhat.com>
Date:   Wed Nov 12 13:17:15 2014 +0530

    Fix stack alignment when loader is invoked directly
    
    The s390 ABI requires the stack pointer to be aligned at 8-bytes.
    When a program is invoked as an argument to the dynamic linker,
    _dl_start_user adjusts the stack to remove the dynamic linker
    arguments so that the program sees only its name and arguments.  This
    may result in the stack being misaligned since each argument shift is
    only a word and not a double-word.
    
    This is now fixed shifting argv and envp down instead of shifting argc
    up and reclaiming the stack.  This requires _dl_argv to be adjusted
    and hence, is no longer relro.

diff --git a/sysdeps/s390/s390-32/dl-machine.h b/sysdeps/s390/s390-32/dl-machine.h
index c56185c..0fd5a4f 100644
--- a/sysdeps/s390/s390-32/dl-machine.h
+++ b/sysdeps/s390/s390-32/dl-machine.h
@@ -166,18 +166,49 @@ _dl_start_user:\n\
 	# See if we were run as a command with the executable file\n\
 	# name as an extra leading argument.\n\
 	l     %r1,_dl_skip_args@GOT12(0,%r12)\n\
-	l     %r1,0(%r1)          # load _dl_skip_args\n\
+	l     %r1,0(%r1)	# load _dl_skip_args\n\
+	ltr   %r1,%r1\n\
+	je    .L4		# Skip the arg adjustment if there were none.\n\
 	# Get the original argument count.\n\
 	l     %r0,96(%r15)\n\
 	# Subtract _dl_skip_args from it.\n\
 	sr    %r0,%r1\n\
-	# Adjust the stack pointer to skip _dl_skip_args words.\n\
-	sll   %r1,2\n\
-	ar    %r15,%r1\n\
-	# Set the back chain to zero again\n\
-	xc    0(4,%r15),0(%r15)\n\
 	# Store back the modified argument count.\n\
 	st    %r0,96(%r15)\n\
+	# Copy argv and envp forward to account for skipped argv entries.\n\
+	# We skipped at least one argument or we would not get here.\n\
+	la    %r6,100(%r15)	# Destination pointer i.e. &argv[0]\n\
+	lr    %r5,%r6\n\
+	lr    %r0,%r1\n\
+	sll   %r0,2\n		# Number of skipped bytes.\n\
+	ar    %r5,%r0		# Source pointer = Dest + Skipped args.\n\
+	# argv copy loop:\n\
+.L1:	l     %r7,0(%r5)	# Load a word from the source.\n\
+	st    %r7,0(%r6)	# Store the word in the destination.\n\
+	ahi   %r5,4\n\
+	ahi   %r6,4\n\
+	ltr   %r7,%r7\n\
+	jne   .L1		# Stop after copying the NULL.\n\
+	# envp copy loop:\n\
+.L2:	l     %r7,0(%r5)	# Load a word from the source.\n\
+	st    %r7,0(%r6)	# Store the word in the destination.\n\
+	ahi   %r5,4\n\
+	ahi   %r6,4\n\
+	ltr   %r7,%r7\n\
+	jne   .L2		# Stop after copying the NULL.\n\
+	# Now we have to zero out the envp entries after NULL to allow\n\
+	# start.S to properly find auxv by skipping zeroes.\n\
+	# zero out loop:\n\
+	lhi   %r7,0\n\
+.L3:	st    %r7,0(%r6)	# Store zero.\n\
+	ahi   %r6,4		# Advance dest pointer.\n\
+	ahi   %r1,-1		# Subtract one from the word count.\n\
+	ltr   %r1,%r1\n\
+	jne    .L3		# Keep copying if the word count is non-zero.\n\
+	# Adjust _dl_argv\n\
+	la    %r6,100(%r15)\n\
+	l     %r1,_dl_argv@GOT(%r12)\n\
+	st    %r6,0(%r1)\n\
 	# The special initializer gets called with the stack just\n\
 	# as the application's entry point will see it; it can\n\
 	# switch stacks if it moves these contents over.\n\
@@ -185,7 +216,7 @@ _dl_start_user:\n\
 	# Call the function to run the initializers.\n\
 	# Load the parameters:\n\
 	# (%r2, %r3, %r4, %r5) = (_dl_loaded, argc, argv, envp)\n\
-	l     %r2,_rtld_local@GOT(%r12)\n\
+.L4:	l     %r2,_rtld_local@GOT(%r12)\n\
 	l     %r2,0(%r2)\n\
 	l     %r3,96(%r15)\n\
 	la    %r4,100(%r15)\n\
diff --git a/sysdeps/s390/s390-32/dl-sysdep.h b/sysdeps/s390/s390-32/dl-sysdep.h
new file mode 100644
index 0000000..b992778
--- /dev/null
+++ b/sysdeps/s390/s390-32/dl-sysdep.h
@@ -0,0 +1,23 @@
+/* System-specific settings for dynamic linker code.  S/390 version.
+   Copyright (C) 2014 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_next <dl-sysdep.h>
+
+/* _dl_argv cannot be attribute_relro, because _dl_start_user
+   might write into it after _dl_start returns.  */
+#define DL_ARGV_NOT_RELRO 1

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