[llvm-dev] Linking Linux kernel with LLD

[George Rimar via llvm-dev]

> I still didn't do anything with "Setup too big!" problem, just commented out the assert.
>

So I investigated this today.
(It is about https://github.com/torvalds/linux/blob/master/arch/x86/boot/setup.ld#L52)

Script ends with:
.bss		:
{
	__bss_start = .;
	*(.bss)
	__bss_end = .;
}
. = ALIGN(16);
_end = .;

/DISCARD/ : { *(.note*) }

. = ASSERT(_end <= 0x8000, "Setup too big!");

And what LLD do here. It makes command list to be:
Place .bss, Place non-allocated orphans, Assign _end.
So places command "_end = " after commands for orphan sections.
There are plenty of them: debug_*, .comment, .shstrtab, .symtab, .strtab.
And _end value is set to not what expected finally.

Fix on script side can be very easy, just add one more line before "/DISCARD/":
.debug_info : { *(.debug_info) }
That way LLD will attach that non-allocated orphans after .debug_info section. 
And _end will be calculated earlier and be correct.

I just had a quick look about how to fix that. We have shouldSkip(const BaseCommand &Cmd) with comment:
// We don't want to go over alignments, since doing so in
//  rx_sec : { *(rx_sec) }
//  . = ALIGN(0x1000);
//  /* The RW PT_LOAD starts here*/
//  rw_sec : { *(rw_sec) }
// would mean that the RW PT_LOAD would become unaligned.

I think probably fix could be to allow go over alignments if we placing non-allocatable orphans,
them are placed the last and should not cause such issue to happen.

George.