Re: [Qemu-devel] qemu-ppc can't run static uClibc binaries.

[Rob Landley <rob@landley.net>]

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On Thursday 11 February 2010 06:32:12 Alexander Graf wrote:
> Rob Landley wrote:
> > Static binaries that run under the Linux kernel don't run under qemu-ppc.
> >  For example, the prebuilt busybox binaries here:
> >
> >   http://busybox.net/downloads/binaries/1.16.0/busybox-powerpc
> >
> > Don't run under qemu-ppc, but runs just fine under qemu-system-ppc with
> > the image at:
> >
> >  
> > http://impactlinux.com/fwl/downloads/binaries/system-image-powerpc.tar.bz
> >2
> >
> > The reason is that the "powerpc spec" that qemu was written to is for
> > AIX, not for Linux, and thus the register layout qemu application
> > emulation provides for powerpc doesn't match what the kernel is actually
> > doing.
> >
> > For dynamically linked executables, the dynamic linker reorganizes the
> > register contents to match the AIX spec from IBM, but statically linked
> > binaries get what the kernel provides directly.  Thus binaries statically
> > linked against uClibc won't run under qemu-ppc, but run under
> > qemu-system-ppc just fine.
> >
> > I tracked down this problem in 2007:
> >
> >   http://landley.net/notes-2007.html#28-03-2007
> >
> > And reported it on the list at the time:
> >
> >   http://lists.gnu.org/archive/html/qemu-devel/2007-03/msg00713.html
> >   http://lists.gnu.org/archive/html/qemu-devel/2007-03/msg00720.html
> >   http://lists.gnu.org/archive/html/qemu-devel/2007-04/msg00315.html
> >
> > However, the then-maintainer of powerpc believed nobody else ever had the
> > right to touch "her code":
> >
> >   http://lists.gnu.org/archive/html/qemu-devel/2007-04/msg00198.html
> >
> > And I was unable to convince her that insisting reality change to match a
> > spec which wasn't even for the right platform was not a useful approach. 
> > Thus the binary in the first link still won't run under qemu-ppc three
> > years later, despite running fine under a real Linux kernel.
>
> Patches are always welcome. The only thing you might want to make sure
> is that dynamically linked binaries also still continue to work :-).

Attached.

This may help explain the issue:

  http://sources.redhat.com/ml/libc-alpha/2003-03/msg00272.html

It's not a question of dynamically linked Linux binaries.  They work just fine 
with either register layout.  The dynamic linker converts the Linux layout to 
the AIX layout, and is reentrant so it won't do it a second time if it's 
already been converted.

The problem is that BSD wants the AIX layout, and hence this comment in linux-
user/elfload.c function init_thread():

    /* Note that isn't exactly what regular kernel does
     * but this is what the ABI wants and is needed to allow
     * execution of PPC BSD programs.
     */

I.E. whoever wrote this already knows it's not what the Linux kernel is 
actually doing, and they're not doing it for Linux, they're doing it for BSD.

The fix is probably to add #ifdef CONFIG_BSD around the appropriate chunk of 
code.  Attached is a patch to do that (plus tweaks to make the "you have an 
unused variable, break the build!" logic shut up about it).

(Yes, I tested that a dynamically linked hello world still worked for me.)

Rob
-- 
Latency is more important than throughput. It's that simple. - Linus Torvalds

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diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 1d5f651..eaabdac 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -513,12 +513,11 @@ do {                                                                    \
 static inline void init_thread(struct target_pt_regs *_regs, struct image_info *infop)
 {
     abi_ulong pos = infop->start_stack;
-    abi_ulong tmp;
 #if defined(TARGET_PPC64) && !defined(TARGET_ABI32)
     abi_ulong entry, toc;
 #endif
 
-    _regs->gpr[1] = infop->start_stack;
+    _regs->gpr[1] = pos;
 #if defined(TARGET_PPC64) && !defined(TARGET_ABI32)
     entry = ldq_raw(infop->entry) + infop->load_addr;
     toc = ldq_raw(infop->entry + 8) + infop->load_addr;
@@ -526,6 +525,8 @@ static inline void init_thread(struct target_pt_regs *_regs, struct image_info *
     infop->entry = entry;
 #endif
     _regs->nip = infop->entry;
+
+#if defined(CONFIG_BSD)
     /* Note that isn't exactly what regular kernel does
      * but this is what the ABI wants and is needed to allow
      * execution of PPC BSD programs.
@@ -534,9 +535,13 @@ static inline void init_thread(struct target_pt_regs *_regs, struct image_info *
     get_user_ual(_regs->gpr[3], pos);
     pos += sizeof(abi_ulong);
     _regs->gpr[4] = pos;
-    for (tmp = 1; tmp != 0; pos += sizeof(abi_ulong))
-        tmp = ldl(pos);
+    for (;;) {
+        abi_ulong tmp = pos;
+        pos += sizeof(abi_ulong);
+        if (!ldl(tmp)) break;
+    }
     _regs->gpr[5] = pos;
+#endif
 }
 
 /* See linux kernel: arch/powerpc/include/asm/elf.h.  */