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

[H.J. Lu]

On Tue, May 25, 2021 at 9:08 AM Jan Beulich <jbeulich@suse.com> wrote:
>
> On 25.05.2021 17:40, H.J. Lu wrote:
> > 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.
>
> Of course, but I expect typical use cases (kernels, hypervisors) to
> contain small pieces of 16-bit code.

And we never ran into any issues before.  My patch just restores
the old behavior for them.

-- 
H.J.