[PATCH] Fix lguest bzImage loading with CONFIG_RELOCATABLE=y

[PATCH] Fix lguest bzImage loading with CONFIG_RELOCATABLE=y

[Rusty Russell]

Jason Yeh sent his crashing .config: bzImages made with
CONFIG_RELOCATABLE=y put the relocs where the BSS is expected, and we
crash with unusual results such as:

	lguest: unhandled trap 14 at 0xc0122ae1 (0xa9)

Relying on BSS being zero was merely laziness on my part.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

diff -r cf2bfb364110 drivers/lguest/lguest.c
--- a/drivers/lguest/lguest.c	Fri Jul 27 13:04:22 2007 +1000
+++ b/drivers/lguest/lguest.c	Fri Jul 27 13:15:22 2007 +1000
@@ -1039,6 +1039,11 @@ __init void lguest_init(void *boot)
 	 * the normal data segment to get through booting. */
 	asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_DS) : "memory");
 
+	/* Clear the part of the kernel data which is expected to be zero.
+	 * Normally it will be anyway, but if we're loading from a bzImage with
+	 * CONFIG_RELOCATALE=y, the relocations will be sitting here. */
+	memset(__bss_start, 0, __bss_stop - __bss_start);
+
 	/* The Host uses the top of the Guest's virtual address space for the
 	 * Host<->Guest Switcher, and it tells us how much it needs in
 	 * lguest_data.reserve_mem, set up on the LGUEST_INIT hypercall. */

Re: [PATCH] Fix lguest bzImage loading with CONFIG_RELOCATABLE=y

[Andi Kleen]

Rusty Russell <rusty@rustcorp.com.au> writes:

> Jason Yeh sent his crashing .config: bzImages made with
> CONFIG_RELOCATABLE=y put the relocs where the BSS is expected, and we
> crash with unusual results such as:

The normal kernel startup should already clear BSS. Why does
this not work here? Can it be fixed?

-Andi

Re: [PATCH] Fix lguest bzImage loading with CONFIG_RELOCATABLE=y

[Rusty Russell]

On Fri, 2007-07-27 at 12:45 +0200, Andi Kleen wrote:
> Rusty Russell <rusty@rustcorp.com.au> writes:
> 
> > Jason Yeh sent his crashing .config: bzImages made with
> > CONFIG_RELOCATABLE=y put the relocs where the BSS is expected, and we
> > crash with unusual results such as:
> 
> The normal kernel startup should already clear BSS. Why does
> this not work here? Can it be fixed?

Unfortunately, lguest doesn't go through the normal startup path (which
does this in asm).

Thanks,
Rusty.