Re: [SeaBIOS] [PATCH] ld: fix .text section address alignment

[Kevin O'Connor]

On Tue, Feb 16, 2016 at 06:21:10PM +0100, Roger Pau Monné wrote:
> El 16/2/16 a les 17:33, Kevin O'Connor ha escrit:
> > On Tue, Feb 16, 2016 at 01:56:26PM +0100, Roger Pau Monne wrote:
> > > According to the output from readelf, the .text section should be aligned to
> > > 16:
> > >
> > > Section Headers:
> > >   [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
> > >   [ 0] (null)            NULL            00000000 000000 000000 00      0   0  0
> > >   [ 1] .text             PROGBITS        000de300 000300 021d00 00  AX  0   0 16
> > > [...]
> > >
> > > This however doesn't seem to be enforced when the relocations are generated.
> > > The following patch tries to address this by making sure the space used for
> > > the relocations it also aligned to the same value as the .text section.
> >
> > Thanks.  What goes wrong if the .text section is not aligned?  The
> > code has already been assigned physical addresses by this point, so it
> > should not impact the runtime code.
>
> It seems like ELF toolchain objcopy chokes if a section address is not
> aligned to the alignment specified by the section, see:
>
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207170
>
> The snippet shown above has addr aligned to 16 (which matches latest
> upstream), so it's not a problem, but the current SeaBIOS version
> shipped in Xen 4.5 (1.7.5 IIRC) ends up with an addr that's not a
> multiple of 16, as shown in the bug report, and objcopy complains with:
>
> objcopy: elf_update() failed: Layout constraint violation

Thanks.  I agree it should be fixed.  However, I think there are a few
other cases that could cause the ".text" section alignment to be off.
Are you okay with the patch below instead?

-Kevin

commit 3910de0dee216d5b5bf23cfa29bfc80d082b2ee7
Author: Kevin O'Connor 
Date:   Fri Feb 19 21:34:16 2016 -0500

build: fix .text section address alignment

Some linkers verify that sections have a start address that is aligned
    with the minimum alignment of that section.  Add extra padding to the
    ".text" section to ensure it is always aligned with the maximum
    alignment of any section placed in ".text".

Signed-off-by: Kevin O'Connor 
    Signed-off-by: Roger Pau Monné 
    Reported by: Ed Maste

diff --git a/scripts/layoutrom.py b/scripts/layoutrom.py
index b976fb0..6616721 100755
--- a/scripts/layoutrom.py
+++ b/scripts/layoutrom.py
@@ -34,18 +34,22 @@ COMMONTRAILER = """
 # Determine section locations
 ######################################################################

-# Align 'pos' to 'alignbytes' offset
+# Align 'pos' up to 'alignbytes' offset
 def alignpos(pos, alignbytes):
     mask = alignbytes - 1
     return (pos + mask) & ~mask

+# Align 'pos' down to 'alignbytes' offset
+def aligndown(pos, alignbytes):
+    mask = alignbytes - 1
+    return pos & ~mask
+
 # Determine the final addresses for a list of sections that end at an
 # address.
 def setSectionsStart(sections, endaddr, minalign=1, segoffset=0):
     totspace = 0
     for section in sections:
-        if section.align > minalign:
-            minalign = section.align
+        minalign = max(minalign, section.align)
         totspace = alignpos(totspace, section.align) + section.size
     startaddr = int((endaddr - totspace) / minalign) * minalign
     curaddr = startaddr
@@ -269,7 +273,7 @@ def doLayout(sections, config, genreloc):
         final_sec32low_end = BUILD_LOWRAM_END
         zonelow_base = final_sec32low_end - 64*1024
     relocdelta = final_sec32low_end - sec32low_end
-    li.sec32low_start, li.sec32low_align = setSectionsStart(
+    li.sec32low_start, sec32low_align = setSectionsStart(
         sections32low, sec32low_end, 16
         , segoffset=zonelow_base - relocdelta)
     li.sec32low_end = sec32low_end
@@ -405,6 +409,8 @@ def writeLinkerScripts(li, out16, out32seg, out32flat):
     if li.config.get('CONFIG_MULTIBOOT'):
         multiboot_header = "LONG(0x1BADB002) LONG(0) LONG(-0x1BADB002)"
         sec32all_start -= 3 * 4
+    sec32all_align = max([section.align for section in li.sections])
+    sec32all_start = aligndown(sec32all_start, sec32all_align)
     out += outXRefs(filesections32flat, exportsyms=[li.entrysym]) + """
     _reloc_min_align = 0x%x ;
     zonefseg_start = 0x%x ;