Re: [PATCH] x86: make hypervisor build with gcc11

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.05.2021 10:48, Roger Pau Monné wrote:
> On Wed, May 19, 2021 at 05:39:50PM +0200, Jan Beulich wrote:
>> Gcc 11 looks to make incorrect assumptions about valid ranges that
>> pointers may be used for addressing when they are derived from e.g. a
>> plain constant. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100680.
>>
>> Utilize RELOC_HIDE() to work around the issue, which for x86 manifests
>> in at least
>> - mpparse.c:efi_check_config(),
>> - tboot.c:tboot_probe(),
>> - tboot.c:tboot_gen_frametable_integrity(),
>> - x86_emulate.c:x86_emulate() (at -O2 only).
>> The last case is particularly odd not just because it only triggers at
>> higher optimization levels, but also because it only affects one of at
>> least three similar constructs. Various "note" diagnostics claim the
>> valid index range to be [0, 2⁶³-1].
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks.

> This is all quite ugly, but I don't have any recommendation short of
> getting gcc fixed (or being able to disable those heuristics).

Indeed.

>> --- a/tools/tests/x86_emulator/x86-emulate.c
>> +++ b/tools/tests/x86_emulator/x86-emulate.c
>> @@ -8,6 +8,13 @@
>>  
>>  #define ERR_PTR(val) NULL
>>  
>> +/* See gcc bug 100680, but here don't bother making this version dependent. 
>> */
> 
> Might be worth also referencing 99578 which seems to be the parent
> bug? (as 100680 has been closed as a duplicate)

Anyone going there will immediately find the xref to that supposed
parent bug. Personally I'm not convinced of it truly being a
duplicate.

Jan