[PATCH] powerpc/boot: Fix the initrd being overwritten under qemu

[Alexey Kardashevskiy]

On 23/10/2019 22:21, Segher Boessenkool wrote:
> On Wed, Oct 23, 2019 at 12:36:35PM +1100, Oliver O'Halloran wrote:
>> When booting under OF the zImage expects the initrd address and size to be
>> passed to it using registers r3 and r4. SLOF (guest firmware used by QEMU)
>> currently doesn't do this so the zImage is not aware of the initrd
>> location.  This can result in initrd corruption either though the zImage
>> extracting the vmlinux over the initrd, or by the vmlinux overwriting the
>> initrd when relocating itself.
>>
>> QEMU does put the linux,initrd-start and linux,initrd-end properties into
>> the devicetree to vmlinux to find the initrd. We can work around the SLOF
>> bug by also looking those properties in the zImage.
> 
> This is not a bug.  What boot protocol requires passing the initrd start
> and size in GPR3, GPR4?

So far I was unable to identify it...

> The CHRP binding (what SLOF implements) requires passing two zeroes here.
> And ePAPR requires passing the address of a device tree and a zero, plus
> something in GPR6 to allow distinguishing what it does.
> 
> As Alexey says, initramfs works just fine, so please use that?  initrd was
> deprecated when this code was written already.

I did not say about anything working fine :)

In my case I was using a new QEMU which does full FDT on client-arch-support and that thing would put the original
linux,initrd-start/end to the FDT even though the initrd was unpacked and the properties were changes in SLOF. With that
fixed, this is an alternative fix for SLOF but I am not pushing it out as I have no idea about the bindings and this
also breaks "vmlinux".


diff --git a/slof/fs/client.fs b/slof/fs/client.fs
index 8a7f6ac4326d..138177e4c2a3 100644
--- a/slof/fs/client.fs
+++ b/slof/fs/client.fs
@@ -45,6 +45,17 @@ VARIABLE  client-callback \ Address of client's callback function
   >r  ciregs >r7 !  ciregs >r6 !  client-entry-point @ ciregs >r5 !
   \ Initialise client-stack-pointer
   cistack ciregs >r1 !
+
+  s" linux,initrd-end" get-chosen IF decode-int -rot 2drop ELSE 0 THEN
+  s" linux,initrd-start" get-chosen IF decode-int -rot 2drop ELSE 0 THEN
+  2dup - dup IF
+    ciregs >r4 !
+    ciregs >r3 !
+    drop
+  ELSE
+    3drop
+  THEN
+


-- 
Alexey