On Fri, Sep 20, 2019 at 12:05:21AM +0800, Kairui Song wrote: > Currently, kernel fails to boot on some HyperV VMs when using EFI. > And it's a potential issue on all platforms. > > It's caused a broken kernel relocation on EFI systems, when below three > conditions are met: > > 1. Kernel image is not loaded to the default address (LOAD_PHYSICAL_ADDR) > by the loader. > 2. There isn't enough room to contain the kernel, starting from the > default load address (eg. something else occupied part the region). > 3. In the memmap provided by EFI firmware, there is a memory region > starts below LOAD_PHYSICAL_ADDR, and suitable for containing the > kernel. > > Efi stub will perform a kernel relocation when condition 1 is met. But > due to condition 2, efi stub can't relocate kernel to the preferred > address, so it fallback to query and alloc from EFI firmware for lowest > usable memory region. > > It's incorrect to use the lowest memory address. In later stage, kernel > will assume LOAD_PHYSICAL_ADDR as the minimal acceptable relocate address, > but efi stub will end up relocating kernel below it. > > Then before the kernel decompressing. Kernel will do another relocation > to address not lower than LOAD_PHYSICAL_ADDR, this time the relocate will > over write the blockage at the default load address, which efi stub tried > to avoid, and lead to unexpected behavior. Beside, the memory region it > writes to is not allocated from EFI firmware, which is also wrong. > > To fix it, just don't let efi stub relocate the kernel to any address > lower than lowest acceptable address. > > Signed-off-by: Kairui Song <kasong@xxxxxxxxxx> Acked-by: <jarkko.sakkinen@xxxxxxxxxxxxxxx> /Jarkko
