Re: [PATCH] x86/build32: Discard all orphaned sections

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 13/05/2020 10:13, Jan Beulich wrote:
> [CAUTION - EXTERNAL EMAIL] DO NOT reply, click links, or open attachments unless you have verified the sender and know the content is safe.
>
> On 12.05.2020 21:11, Andrew Cooper wrote:
>> @@ -47,6 +47,14 @@ SECTIONS
>>           *
>>           * Please check build32.mk for more details.
>>           */
>> -        /* *(.got.plt) */
>> +        *(.got.plt)
>> +  }
>> +
>> +  /DISCARD/ : {
>> +        /*
>> +         * Discard everything else, to prevent linkers from putting
>> +         * orphaned sections ahead of .text, which needs to be first.
>> +         */
>> +        *(*)
>>    }
>>  }
> To be honest I'm not sure if this isn't going too far. Much
> depends on what would happen if a new real section appeared
> that needs retaining.

Anything which is important enough will result in a linker error.

> Granted the linker may then once again
> put it at the beginning of the image, and we'll be screwed
> again, just like we'll be screwed if a section gets discarded
> by mistake.

This way is more likely to result in a build failure than an inability
to boot the resulting build of Xen.

> But it would be really nice if we had a way to
> flag the need to play with the linker script. Hence perhaps
> on new enough tool chains we indeed may want to make use of
> --orphan-handling= ? And then discard just .note and .note.*
> here?

The only valid option would be =error, but experimenting with that yields

ld: error: unplaced orphan section `.comment' from `cmdline.o'
ld: error: unplaced orphan section `.note.GNU-stack' from `cmdline.o'
ld: error: unplaced orphan section `.note.gnu.property' from `cmdline.o'
ld: error: unplaced orphan section `.rel.got' from `cmdline.o'
ld: error: unplaced orphan section `.got' from `cmdline.o'
ld: error: unplaced orphan section `.got.plt' from `cmdline.o'
ld: error: unplaced orphan section `.iplt' from `cmdline.o'
ld: error: unplaced orphan section `.rel.iplt' from `cmdline.o'
ld: error: unplaced orphan section `.igot.plt' from `cmdline.o'

which I think is going to get us massively bogged down in toolchain
specifics.  I'm not entirely convinced this would be a good move.

~Andrew