[PATCH v2] x86: Propery check PC16 reloc overflow in 16-bit mode instructions

[H.J. Lu]

On Tue, May 25, 2021 at 8:34 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 25.05.2021 15:41, H.J. Lu wrote:
> > On Mon, May 24, 2021 at 11:40 PM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 25.05.2021 01:42, H.J. Lu via Binutils wrote:
> >>> commit a7664973b24a242cd9ea17deb5eaf503065fc0bd
> >>> Author: Jan Beulich <jbeulich@suse.com>
> >>> Date:   Mon Apr 26 10:41:35 2021 +0200
> >>>
> >>>     x86: correct overflow checking for 16-bit PC-relative relocs
> >>>
> >>> caused linker failure when building 16-bit program in a 32-bit ELF
> >>> container.  Update GNU_PROPERTY_X86_FEATURE_2_USED with
> >>>
> >>>  #define GNU_PROPERTY_X86_FEATURE_2_CODE16 (1U << 12)
> >>>
> >>> as in
> >>>
> >>> https://groups.google.com/g/x86-64-abi/c/UvvXWeHIGMA
> >>>
> >>> to indicate that 16-bit mode instructions are used in the object to
> >>> allow linker to properly perform relocation overflow check for 16-bit
> >>> PC-relative relocations in 16-bit mode instructions.
> >>
> >> I certainly don't mind the introduction of this flag; I think its
> >> introduction wants to be separated from the specific use in the
> >> linker, not the lease because ...
> >>
> >>> 1. Update x86 assembler to always generate the GNU property note with
> >>> GNU_PROPERTY_X86_FEATURE_2_CODE16 for .code16 in ELF object.
> >>> 2. Update i386 and x86-64 linkers to use 16-bit PC16 relocations if
> >>> input object is marked with GNU_PROPERTY_X86_FEATURE_2_CODE16.
> >>
> >> ... I don't think this is an appropriate step to take. The majority
> >> of cases of 16-bit code use that I know of is in projects where this
> >> is just a small portion of code, and the rest of the code is 32-
> >> and/or 64-bit. By taking mere presence of a tiny bit of 16-bit code
> >> as indication to relax overflow checking, you undermine the main
> >> goal of the earlier change.
> >
> > commit a7664973b24a242cd9ea17deb5eaf503065fc0bd
> > Author: Jan Beulich <jbeulich@suse.com>
> > Date:   Mon Apr 26 10:41:35 2021 +0200
> >
> >     x86: correct overflow checking for 16-bit PC-relative relocs
> >
> > is technically correct according to psABIs.  But GNU assembler
> > only generates PC16 relocations for 16-bit codes.  That is why
> > we never ran into any PC16 relocation overflow before.  My change
> > restores the old behavior only when input has 16-bit codes.
>
> When paying specific attention to resulting code size, people may
> want to encode XBEGIN with 16-bit operand size in 32- or 64-bit
> code. This not having worked properly is what initially triggered
> me touching this area. And of course assembler programmers could
> even cause data to have PC16 relocations attached to it.

As long as it doesn't .code16, PC16 relocation overflow check won't be
changed.

> >  I think it
> > is a good compromise between usability and error checking.
>
> Maybe it is; I'm not convinced though as of yet.
>

My solution isn't perfect.  But it should cover most use cases.
seabios should build out of box with binutils 2.37.

-- 
H.J.