ocb.c fails to build on Ubuntu 12.04 armel and armhf

[kmcallister]

Mosh's file src/crypto/ocb.cc fails to build on Ubuntu 12.04 LTS (Precise Pangolin) for architectures armel and armhf (build logs: 1, 2). The problem is the same in both cases.

ARM processors support three different instruction sets:

• The original ARM instruction set. All instructions are 32 bits. Any instruction can be conditional.
• Thumb, a compact 16-bit encoding for a subset of ARM instructions. Only branches may be conditional. Thumb code has worse performance per instruction, and is expected to call ARM code for CPU-intensive tasks.
• Thumb-2, which adds some 32-bit instructions to Thumb, and is considered a full replacement for ARM. Any instruction can be conditional, but a block of conditional instructions must be introduced by an instruction such as IT (if-then), ITE (if-then-else), etc.

Ted Krovetz's ocb.c contains inline ARM assembly with a conditional instruction eorcs (exclusive OR if carry set). It lacks an IT instruction and so can only be assembled in ARM mode. But GCC on Ubuntu 12.04 is compiled with --with-arch=armv7-a --with-mode=thumb, so it will emit Thumb-2 code by default.

Steps to reproduce:

$ wget http://www.cs.ucdavis.edu/~rogaway/ocb/{ocb-ref/rijndael-alg-fst.h,news/code/{ae.h,ocb.c}}
$ sed -i '/^#define USE_.*$/ d' ocb.c
$ gcc -DUSE_REFERENCE_AES=1 -c ocb.c
/tmp/ccLgYjJc.s: Assembler messages:
/tmp/ccLgYjJc.s:269: Error: thumb conditional instruction should be in IT block -- `eorcs r2,r2,#135'

Adding an IT instruction solves the problem:

--- ocb.c.orig  2011-07-14 12:54:08.000000000 -0400
+++ ocb.c       2012-03-27 02:53:09.000000000 -0400
@@ -294,6 +294,7 @@
                                 "adcs %H1,%H1,%H1\n\t"
                                 "adcs %0,%0,%0\n\t"
                                 "adcs %H0,%H0,%H0\n\t"
+                                "it cs\n\t"
                                 "eorcs %1,%1,#135"
                : "+r"(b.l), "+r"(b.r) : : "cc");
                return b;

This will build in ARM mode too, but presumably not on a sufficiently old as.

Besides this patch, we have a few other options:

• Remove the inline assembly and use the C version of this function.
• Override distribution preferences and build Mosh in ARM mode, using gcc -marm.
• Build just ocb.cc in ARM mode. ARM-Thumb interworking is supposed to work, but there might be bugs down this road.
• Pass -Wa,-mimplicit-it=thumb to GCC, telling the assembler to insert IT instructions as necessary. Ubuntu warns that it "is not implemented as of gcc-4.4 4.4.2-3ubuntu1, but [sic] it's probably best to avoid manually adding this flag to packages which encounter build problems".

I've confirmed that CXXFLAGS=-marm, CXXFLAGS=-Wa,-mimplicit-it=thumb, and the patch are each sufficient to build mosh-1.1 (using ./configure; make) on Ubuntu 12.04 armel. You can set up your own armel QEMUlated chroot on Debian using roughly:

$ sudo apt-get install ubuntu-dev-tools qemu-user-static
$ mk-sbuild --arch=armel precise
$ sudo schroot -c precise-armel-source