On 09/11/2018 23:23, H. Peter Anvin wrote: > I just noticed this patch -- I missed it because the cover message > seemed far more harmless so I didn't notice this change. > > THIS PATCH IS FATALLY WRONG AND NEEDS TO BE IMMEDIATELY REVERTED BEFORE > ANYONE STARTS RELYING ON IT; IT HAS THE POTENTIAL OF BREAKING THE > BOOTLOADER PROTOCOL FOR ALL FUTURE. > > It seems to be based on fundamental misconceptions about the various > data structures in the protocol, and does so in a way that completely > breaks the way the protocol is designed to work. > > The protocol is specifically designed such that fields are not version > dependencies. The version number is strictly to inform the boot loader > about which capabilities the kernel has, so that the boot loader can > know if a certain data field is meaningful and/or honored. > >> +Protocol 2.14: (Kernel 4.20) Added acpi_rsdp_addr holding the physical >> + address of the ACPI RSDP table. >> + The bootloader updates version with: >> + 0x8000 | min(kernel-version, bootloader-version) >> + kernel-version being the protocol version supported by >> + the kernel and bootloader-version the protocol version >> + supported by the bootloader. > > [...] > >> **** MEMORY LAYOUT >> >> The traditional memory map for the kernel loader, used for Image or >> @@ -197,6 +209,7 @@ Offset Proto Name Meaning >> 0258/8 2.10+ pref_address Preferred loading address >> 0260/4 2.10+ init_size Linear memory required during initialization >> 0264/4 2.11+ handover_offset Offset of handover entry point >> +0268/8 2.14+ acpi_rsdp_addr Physical address of RSDP table > > NO. > > That is not how struct setup_header works, nor does this belong here. > > struct setup_header contains *initialized data*, and has a length byte > at offset 0x201. The bootloader is responsible for copying the full > structure into the appropriate offset (0x1f1) in struct boot_params. > > The length byte isn't actually a requirement, since the maximum possible > size of this structure is 144 bytes, and the kernel will (obviously) not > look at the older fields anyway, but it is good practice. The kernel or > any other entity is free to zero out the bytes past this length pointer. > > There are only 24 bytes left in this structure, and this would occupy 8 > of them for no valid reason. The *only* valid reason to put a > zero-initialized field in struct setup_header is if it used by the > 16-bit legacy BIOS boot, which is obviously not the case here. > > This field thus belongs in struct boot_params, not struct setup_header. Would you be okay with putting acpi_rsdp_addr at offset 0x0cc (_pad4)? Juergen
