[SRU B/C/D/E][bionic/master-next 1/1] LP#1836079 efi: efi_get_memory_map -- increase map headroom

[Andy Whitcroft]

We are seeing some EFI based machines failing to boot hard in the EFI
stub:

    exit_boot() failed!
    efi_main() failed!

This seems to occur when the bootloader (grub2 in this case) has had to
manipulate some additional files.  We tracked this down to the memory map
dance efi_get_memory_map().  Basically we attempt to close boot services
and it informs us it cannot do so because it failed to record the updated
memory map.  This occurs when there is insufficient space in the passed
memory map buffer to record changes during the operation.  At the point
when this occurs we are unable to reallocate the buffer so we panic.

To avoid this we allocate some additional entries in the buffer to cover
any additional entries.  These are currently insufficient for these
machines in this use case.  Increase EFI_MMAP_NR_SLACK_SLOTS to provide
space for more map modifications.

BugLink: http://bugs.launchpad.net/bugs/1836079
Signed-off-by: Andy Whitcroft <apw at canonical.com>
---
 drivers/firmware/efi/libstub/efi-stub-helper.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/efi-stub-helper.c b/drivers/firmware/efi/libstub/efi-stub-helper.c
index 39f87e6dac5c..957dff5a26a2 100644
--- a/drivers/firmware/efi/libstub/efi-stub-helper.c
+++ b/drivers/firmware/efi/libstub/efi-stub-helper.c
@@ -49,7 +49,7 @@ int __pure novamap(void)
 	return __novamap;
 }
 
-#define EFI_MMAP_NR_SLACK_SLOTS	8
+#define EFI_MMAP_NR_SLACK_SLOTS	16
 
 struct file_info {
 	efi_file_handle_t *handle;
-- 
2.20.1