Re: [syzbot] [arm?] upstream test error: KASAN: invalid-access Write in setup_arch

Mark Rutland
2024-09-19T09:16:15Z

On Thu, Sep 05, 2024 at 04:25:57PM +0200, Ard Biesheuvel wrote:
> On Thu, 5 Sept 2024 at 16:03, Mark Rutland <mark.rutland@arm.com> wrote:
> > Putting the KASAN issue aside (which I'll handle in a separate thread),
> > I think there is a real issue here with LLVM.
> >
> > What's going on here is that .idmap.text ends up more than 128M away
> > from .head.text, so the 'b primary_entry' at the start of the Image
> > isn't in range:
> >
> > | [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> > | ffff800080000000 g .head.text 0000000000000000 _text
> > | [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry
> > | ffff8000889df0e0 g .rodata.text 000000000000006c primary_entry
> >
> > ... as those are ~128MiB apart.
> >
> > When building with GCC those end up ~101MiB apart:
> >
> > | [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w _text
> > | ffff800080000000 g .head.text 0000000000000000 _text
> > | [mark@lakrids:~/src/linux]% usekorg 14.1.0 aarch64-linux-objdump -t vmlinux | grep -w primary_entry
> > | ffff8000865ae0e0 g .rodata.text 000000000000006c primary_entry
> >
> > When that happens, LLD makes the header branch to a veneer/thunk:
> >
> > | ffff800080000000 <_text>:
> > | ffff800080000000: fa405a4d ccmp x18, #0x0, #0xd, pl // pl = nfrst
> > | ffff800080000004: 14003fff b ffff800080010000 <__AArch64AbsLongThunk_primary_entry>
> >
> > ... and unfortunately, that veneer/thunk uses a literal with the
> > statically-linked TTBR1 address of primary_entry:
> >
> > | ffff800080010000 <__AArch64AbsLongThunk_primary_entry>:
> > | ffff800080010000: 58000050 ldr x16, ffff800080010008 <__AArch64AbsLongThunk_primary_entry+0x8>
> > | ffff800080010004: d61f0200 br x16
> > | ffff800080010008: 889df0e0 .word 0x889df0e0
> > | ffff80008001000c: ffff8000 .word 0xffff8000
> >
> ...
> > LLVM folk, is there any existing option to ask LLD to use ADRP+ADD for
> > the veneer/thunk? ... and if not, would it be possible to add an option
> > for that?
> >
>
> ld.lld takes --pic-veneer, which (from looking at the llvm sources)
> appears to do what we need here.

Ah; now I take another look I see that's in the man page and is also
supported by GNU LD, so I'll spin a patch to use that.

Thanks for the pointer!

Mark.