Re: [PATCH] pc-bios/s390x: Pack ResetInfo struct

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 05.02.20 19:21, Jason J. Herne wrote:
> This fixes vfio-ccw when booting non-Linux operating systems. Without this
> struct being packed, a few extra bytes of low core memory get overwritten when
> we  assign a value to memory address 0 in jump_to_IPL_2. This is enough to
> cause some non-Linux OSes of fail when booting.
> 
> The problem was introduced by:
> 5c6f0d5f46a77d77 "pc-bios/s390x: Fix reset psw mask".
> 
> The fix is to pack the struct thereby removing the 4 bytes of padding that get
> added at the end, likely to allow an array of these structs to naturally align
> on an 8-byte boundary.
> 
> Fixes: 5c6f0d5f46a7 ("pc-bios/s390x: Fix reset psw mask")
> CC: Janosch Frank <frankja@linux.ibm.com>
> Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
> ---
>  pc-bios/s390-ccw/jump2ipl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
> index da13c43cc0..1e9eaa037f 100644
> --- a/pc-bios/s390-ccw/jump2ipl.c
> +++ b/pc-bios/s390-ccw/jump2ipl.c
> @@ -18,7 +18,7 @@
>  typedef struct ResetInfo {
>      uint64_t ipl_psw;
>      uint32_t ipl_continue;
> -} ResetInfo;
> +} __attribute__((packed)) ResetInfo;
>  
>  static ResetInfo save;

Just looked into that.

We do save the old content in "save" and restore the old memory content.

static void jump_to_IPL_2(void)
{
    ResetInfo *current = 0;

    void (*ipl)(void) = (void *) (uint64_t) current->ipl_continue;
--->*current = save;
    ipl(); /* should not return */
}

void jump_to_IPL_code(uint64_t address)
{
    /* store the subsystem information _after_ the bootmap was loaded */
    write_subsystem_identification();

    /* prevent unknown IPL types in the guest */
    if (iplb.pbt == S390_IPL_TYPE_QEMU_SCSI) {
        iplb.pbt = S390_IPL_TYPE_CCW;
        set_iplb(&iplb);
    }

    /*
     * The IPL PSW is at address 0. We also must not overwrite the
     * content of non-BIOS memory after we loaded the guest, so we
     * save the original content and restore it in jump_to_IPL_2.
     */
    ResetInfo *current = 0;

--->save = *current;



does something like


diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
index da13c43cc0..8839226803 100644
--- a/pc-bios/s390-ccw/jump2ipl.c
+++ b/pc-bios/s390-ccw/jump2ipl.c
@@ -18,6 +18,7 @@
 typedef struct ResetInfo {
     uint64_t ipl_psw;
     uint32_t ipl_continue;
+    uint32_t pad;
 } ResetInfo;
 
 static ResetInfo save;


also work? If yes, both variants are valid. Either packed or explicit padding.