On Mon, Apr 06, 2020 at 03:29:18PM +0200, Ard Biesheuvel wrote: > > > > > > > Actually, it may be sufficient to #define __efistub_global to > > > > __section(.data) like we already do for ARM, to ensure that these > > > > global flags are always initialized correctly. (I'll wait for Sergey > > > > to confirm that the spurious enabling of the PCI DMA protection > > > > resulting from this BSS issue is causing the boot regression) > > > > Yeah I thought of that as the easiest fix, but it might be safer to > > explicitly zero-init in efi_main to avoid future problems in case > > someone adds another variable in bss and isn't aware of this obscure > > requirement. We actually already have sys_table in bss, but that one is > > always initialized. There's also other globals that aren't annotated > > (but not in bss by virtue of having initializers). What do you think? > > > > *If* we zero init BSS, I'd prefer for it to be done in the EFI > handover protocol entrypoint only. But does that fix the issue that > BSS lives outside of the memory footprint of the kernel image? > Yes, I was thinking of only doing it if we didn't come through the pe_entry. We could also avoid re-parsing the command line if we add a global flag to indicate that. Regarding the memory footprint, if there's no initrd that might be a problem, since in that case ImageSize will only cover the actual data from the bzImage, so it would be safer to move them to data (including sys_table). We could just pull the stub's bss section all into data with objcopy similar to what ARM64 does [1]? i.e. rename .bss to .bss.efistub and then pull that into .data in the linker script for the compressed kernel? There is also this scary looking comment in gnu-efi's linker script: /* the EFI loader doesn't seem to like a .bss section, so we stick it all into .data: */ I don't know what the history of that is. [1] As an aside, why doesn't ARM do this as well rather than using the section(.data) annotation? > > > What do you think of the other problem -- that's actually worse to fix, > > as it won't just be when kaslr is disabled, the startup_64 code will do > > relocation to the end of init_size and clobber the initrd before getting > > to the kaslr code, so it will break as soon as the firmware loads the > > "unified kernel image" at a 2Mb-aligned address. The only thing I can > > think of is to just unconditionally call efi_relocate_kernel if we were > > entered via handover_entry? > > > > Yes, that seems to be the most robust approach.
