Re: [PATCH v2 0/4] x86/boot: Remove runtime relocations from compressed kernel

[Ard Biesheuvel]

On Tue, 26 May 2020 at 14:29, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> On Tue, May 26, 2020 at 12:59 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> >
> > The compressed kernel currently contains bogus runtime relocations in
> > the startup code in head_{32,64}.S, which are generated by the linker,
> > but must not actually be processed at runtime.
> >
> > This generates warnings when linking with the BFD linker, and errors
> > with LLD, which defaults to erroring on runtime relocations in read-only
> > sections. It also requires the -z noreloc-overflow hack for the 64-bit
> > kernel, which prevents us from linking it as -pie on an older BFD linker
> > (<= 2.26) or on LLD, because the locations that are to be apparently
> > relocated are only 32-bits in size and so cannot normally have
> > R_X86_64_RELATIVE relocations.
> >
> > This series aims to get rid of these relocations. It is based on
> > efi/next, where the latest patches touch the head code to eliminate the
> > global offset table.
> >
> > The first patch is an independent fix for LLD, to avoid an orphan
> > section in arch/x86/boot/setup.elf.
> >
> > The second patch gets rid of almost all the relocations. It uses
> > standard PIC addressing technique for 32-bit, i.e. loading a register
> > with the address of _GLOBAL_OFFSET_TABLE_ and then using GOTOFF
> > references to access variables. For 64-bit, there is 32-bit code that
> > cannot use RIP-relative addressing, and also cannot use the 32-bit
> > method, since GOTOFF references are 64-bit only. This is instead handled
> > using a macro to replace a reference like gdt with (gdt-startup_32)
> > instead. The assembler will generate a PC32 relocation entry, with
> > addend set to (.-startup_32), and these will be replaced with constants
> > at link time. This works as long as all the code using such references
> > lives in the same section as startup_32, i.e. in .head.text.
> >
> > The third patch addresses a remaining issue with the BFD linker, which
> > insists on generating runtime relocations for absolute symbols. We use
> > z_input_len and z_output_len, defined in the generated piggy.S file, as
> > symbols whose absolute "addresses" are actually the size of the
> > compressed payload and the size of the decompressed kernel image
> > respectively. LLD does not generate relocations for these two symbols,
> > but the BFD linker does, prior to the upcoming 2.35. To get around this,
> > piggy.S is extended to also define two u32 variables (in .rodata) with
> > the lengths, and the head code is modified to use those instead of the
> > symbol addresses.
> >
> > An alternative way to handle z_input_len/z_output_len would be to just
> > include piggy.S in head_{32,64}.S instead of as a separate object file,
> > since the GNU assembler doesn't generate relocations for symbols set to
> > constants.
> >
> > The last patch adds a check in the linker script to ensure that no
> > runtime relocations get reintroduced. Since the GOT has been eliminated
> > as well, the compressed kernel has no runtime relocations whatsoever any
> > more.
> >
> > Changes from v1:
> > - Add .text.* to setup.ld instead of just .text.startup
> > - Rename the la() macro introduced in the second patch for 64-bit to
> >   rva(), and rework the explanatory comment.
> > - In the last patch, check both .rel.dyn and .rela.dyn, instead of just
> >   one per arch.
> >
>
> Hi,
>
> I would like to test this patchset v2 on top of Linux v5.7-rc7 together with:
>
> [1] x86/boot: Discard .discard.unreachable for arch/x86/boot/compressed/vmlinux
> [2] x86/boot: Correct relocation destination on old linkers
>
> I tried to pull efi/next on top of Linux v5.7-rc7 and cleaned up the
> merge problems, but I am not sure I did it correctly.
> So, which patches are really relevant from efi/next?
>
> What's your suggestions?
>

efi/next is here:

https://git.kernel.org/pub/scm/linux/kernel/git/efi/efi.git/log/?h=next

You'll need the top 3 patches.