Re: [Libguestfs] [PATCH supermin] bin2s: make sure the data is aligned
On Wed, Aug 23, 2017 at 01:12:13PM +0200, Pino Toscano wrote:
Not all the architectures can cope correctly with unaligned data,
and
thus the embedded init written is bogus (since it will contain random
padding at the beginning, and truncated of the same amount at the end).
To overcome that, make sure the rodata section with the embedded init
is always aligned to 8 bytes, which should work fine for both 32bit and
64bit architectures.
---
src/bin2s.pl | 1 +
1 file changed, 1 insertion(+)
diff --git a/src/bin2s.pl b/src/bin2s.pl
index 6c70446..67ff1f2 100755
--- a/src/bin2s.pl
+++ b/src/bin2s.pl
@@ -44,6 +44,7 @@ print $ofh <<"EOF";
\t.globl\t_binary_${infile_basename}_end
\t.section\t.rodata
+\t.align 8
_binary_${infile_basename}_start:
EOF
It's unfortunate that ‘.align’ doesn't work the same way on all
architectures. To quote from the gas info page:
"The way the required alignment is specified varies from system to
system. For the a29k, hppa, m68k, m88k, w65, sparc, and Hitachi SH,
and i386 using ELF format, the first expression is the alignment
request in bytes. For example `.align 8' advances the location
counter until it is a multiple of 8. If the location counter is
already a multiple of 8, no change is needed.
For other systems, including the i386 using a.out format, it is the
number of low-order zero bits the location counter must have after
advancement. For example `.align 3' advances the location counter
until it a multiple of 8. If the location counter is already a
multiple of 8, no change is needed."
[ftp://ftp.gnu.org/old-gnu/Manuals/gas/html_chapter/as_7.html]
The info page goes on to note that there is a ‘.balign’ directive
(which is gas-specific, but I don't know if that matters) which always
byte aligns.
It's possible we don't care about i386 + a.out format, and maybe this
only affects weird old architectures.
So, ACK, but if there's an easy way to fix the above mess then that
could be better.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine. Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/