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

[Jan Beulich]

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.

Jan