On 15 September 2015 at 17:24, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > On 15 September 2015 at 16:46, Will Deacon <will.deacon@xxxxxxx> wrote: >> On Tue, Sep 15, 2015 at 11:11:43AM +0100, Ard Biesheuvel wrote: >>> Since arm64 does not use a builtin decompressor, the EFI stub is built >>> into the kernel proper. So far, this has been working fine, but actually, >>> since the stub is in fact a PE/COFF relocatable binary that is executed >>> at an unknown offset in the 1:1 mapping provided by the UEFI firmware, we >>> should not be seamlessly sharing code with the kernel proper, which is a >>> position dependent executable linked at a high virtual offset. >>> >>> So instead, separate the contents of libstub and its dependencies, by >>> putting them into their own namespace by prefixing all of its symbols >>> with __efistub. This way, we have tight control over what parts of the >>> kernel proper are referenced by the stub. >> >> Could we add an __efistub annotation to spit out warnings if the stub >> calls into unexpected kernel code, like we do for __init/__ref? >> > > Currently, it will break the build rather than warn if you use a > disallowed symbol, which I think is not unreasonable. > > But I suppose that the objcopy step in this patch could rename the > sections to .efistub.xxx sections, which would allow us to reuse some > of the section mismatch code. > However, this would involve marking things like the generic string and > memcpy routines __efistub as well, which I think may be too much. > Also, note that the logic is inverted here: with __init, we disallow > normal code to call __init functions, but with __efistub, it will be > the other way around, which may be more difficult to accomplish > (Rutland and I did discuss this option when we talked about this over > IRC) > OK, I have given this a go, and as it turns out, it implies that we go and mark generic pieces of lib/ as __section(.text.efistubok) in order for modpost.c to accept it. Tweaking modpost.c itself seems quite doable, since the logic is fairly flexible and can easily be augmented to complain about unauthorized references from the stub to the kernel proper. So what we could do is fold libfdt and lib/sort.c (which are the primary generic dependencies) into the stub, but we would still need to retain the symbol prefixing bit to prevent the stub's symbols from clashing with the ones from the kernel proper. And with the symbol prefixing in place, we have something that is even stronger than section mismatch, which is to error out on undefined references rather than warn about section mismatches. I think the current approach is better, but only if we agree that we should do something in the first place. (Currently, there are no known issues, just the awareness that things are not quite as tidy as they should be) -- Ard. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
