On November 10, 2018 7:22:29 AM PST, Juergen Gross <jgross@xxxxxxxx> wrote: >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 I'd prefer if you used __pad3 offset 0x70 to keep the large block, and that way your field is also aligned. However, if you have some specific reason to prefer __pad4 it's no big deal, although I'm curious what it would be. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
