Re: Misaligned stack on 32-bit s390?

["Carlos O'Donell" <carlos at redhat dot com>]

On 11/11/2014 04:28 AM, Richard Henderson wrote:
> On 11/11/2014 06:31 AM, Carlos O'Donell wrote:
>> Any clever ideas on how to fix this without copying up a large
>> portion of the stack?
> 
> Nope, because other targets do in fact have to do just that.

Right.
 
> I'm actually surprised that almost all of them don't.  I suppose
> that just depends on how the ABI is set up to pass parameters to
> the user _start...

Yes, on hppa the ABI was such that I didn't depend on the layout,
but s390 does, thus I think it is non-trivial.

The argv might be trivial to copy, but the size of envp is unknown,
and _start from the application side in start.S expects argv and
envp to be right up against eachother AFAICT. Worse it wants to
scan past them in order to find auxv. The saving grace is that
the auxv scan parses an arbitrary number of zeros between envp
and auxv. So I can move the gap to be between envp and auxv.
 
> Fortunately, s390 has a block copy instruction, so the move
> should be trivial to implement.

For argv only. What instruction is the block copy? Are you
talking about lm/stm?

The most naive fix I have working is as follows (I've handed
off to Siddhesh to have a look since I'm out of time this
evening/morning).

diff --git a/sysdeps/s390/s390-32/dl-machine.h b/sysdeps/s390/s390-32/dl-machine.h
index c56185c..b189552 100644
--- a/sysdeps/s390/s390-32/dl-machine.h
+++ b/sysdeps/s390/s390-32/dl-machine.h
@@ -166,18 +166,47 @@ _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\
+	# Set the back chain to zero again\n\
+.L4:	xc    0(4,%r15),0(%r15)\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\
---

Cheers,
Carlos.