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

From: Sedat Dilek
Date: Tue May 26 2020 - 10:49:13 EST

On Tue, May 26, 2020 at 2:44 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
>
> On Tue, May 26, 2020 at 2:33 PM Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> >
> > On Tue, May 26, 2020 at 2:30 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > 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.
> >
> > Thanks /o\.
> >
> > - Sedat -
>
> Are those diffs correct when using "x86/boot: Correct relocation
> destination on old linkers"?
>
> $ cat ../head_32_S.diff
> diff --cc arch/x86/boot/compressed/head_32.S
> index 064e895bad92,03557f2174bf..000000000000
> --- a/arch/x86/boot/compressed/head_32.S
> +++ b/arch/x86/boot/compressed/head_32.S
> @@@ -49,13 -49,17 +49,14 @@@
> * Position Independent Executable (PIE) so that linker won't optimize
> * R_386_GOT32X relocation to its fixed symbol address. Older
> * linkers generate R_386_32 relocations against locally defined symbols,
> - * _bss, _ebss, in PIE. It isn't wrong, just suboptimal compared
> - * _bss, _ebss, _got, _egot and _end, in PIE. It isn't wrong, just less
> - * optimal than R_386_RELATIVE. But the x86 kernel fails to properly handle
> ++ * _bss, _ebss, _end in PIE. It isn't wrong, just suboptimal compared
> + * to R_386_RELATIVE. But the x86 kernel fails to properly handle
> * R_386_32 relocations when relocating the kernel. To generate
> - * R_386_RELATIVE relocations, we mark _bss and _ebss as hidden:
> - * R_386_RELATIVE relocations, we mark _bss, _ebss, _got, _egot and _end as
> - * hidden:
> ++ * R_386_RELATIVE relocations, we mark _bss, _ebss and _end as hidden:
> */
> .hidden _bss
> .hidden _ebss
> - .hidden _got
> - .hidden _egot
> + .hidden _end
>
> __HEAD
> SYM_FUNC_START(startup_32)
>
> $ cat ../head_64_S.diff
> diff --cc arch/x86/boot/compressed/head_64.S
> index 4b7ad1dfbea6,76d1d64d51e3..000000000000
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@@ -40,34 -40,11 +40,35 @@@
> */
> .hidden _bss
> .hidden _ebss
> - .hidden _got
> - .hidden _egot
> + .hidden _end
>
> __HEAD
> +
> +/*
> + * This macro gives the relative virtual address of X, i.e. the offset of X
> + * from startup_32. This is the same as the link-time virtual address of X,
> + * since startup_32 is at 0, but defining it this way tells the
> + * assembler/linker that we do not want the actual run-time address of X. This
> + * prevents the linker from trying to create unwanted run-time relocation
> + * entries for the reference when the compressed kernel is linked as PIE.
> + *
> + * A reference X(%reg) will result in the link-time VA of X being stored with
> + * the instruction, and a run-time R_X86_64_RELATIVE relocation entry that
> + * adds the 64-bit base address where the kernel is loaded.
> + *
> + * Replacing it with (X-startup_32)(%reg) results in the offset being stored,
> + * and no run-time relocation.
> + *
> + * The macro should be used as a displacement with a base register containing
> + * the run-time address of startup_32 [i.e. rva(X)(%reg)], or as an immediate
> + * [$ rva(X)].
> + *
> + * This macro can only be used from within the .head.text section, since the
> + * expression requires startup_32 to be in the same section as the code being
> + * assembled.
> + */
> +#define rva(X) ((X) - startup_32)
> +
> .code32
> SYM_FUNC_START(startup_32)
> /*
>
> Thanks.
>

With LLVM/Clang/LLD I see:

mycompiler -Wp,-MD,arch/x86/boot/compressed/.kernel_info.o.d
-nostdinc -isystem
/home/dileks/src/llvm-toolchain/install/lib/clang/10.0.1rc1/include
-I./arch/x86/include -I./arch/x86/include/generated -I./include
-I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi
-I./include/uapi -I./include/generated/uapi -include
./include/linux/kconfig.h -D__KERNEL__ -Qunused-arguments -m64 -O2
-fno-strict-aliasing -fPIE -DDISABLE_BRANCH_PROFILING -mcmodel=small
-mno-mmx -mno-sse -ffreestanding -fno-stack-protector
-Wno-address-of-packed-member -Wno-gnu -Wno-pointer-sign
-fmacro-prefix-map=./= -fno-asynchronous-unwind-tables -include
hidden.h -D__ASSEMBLY__ -c -o
arch/x86/boot/compressed/kernel_info.o
arch/x86/boot/compressed/kernel_info.S
<built-in>:345:10: fatal error: 'hidden.h' file not found
#include "hidden.h"
^~~~~~~~~~
1 error generated.
make[5]: *** [scripts/Makefile.build:349:
arch/x86/boot/compressed/kernel_info.o] Error 1
make[4]: *** [arch/x86/boot/Makefile:114:
arch/x86/boot/compressed/vmlinux] Error 2
make[4]: *** Waiting for unfinished jobs....

mycompiler is a wrapper-script to use ccache * clang-10 as compiler.

patchset to previous build:

$ git log --no-merges --oneline 5.7.0-rc7-1-amd64-clang..5.7.0-rc7-2-amd64-clang
8b74901cb9e5 (for-5.7/x86-boot-remove-runtime-relocations-from-compressed-kernel-v2-nivedita76)
x86/boot: Check that there are no runtime relocations
83fb1bc3b076 x86/boot: Remove runtime relocations from head_{32,64}.S
fede23dacbbd x86/boot: Remove run-time relocations from .head.text code
3e5ea481b8fb x86/boot: Add .text.* to setup.ld
bec910ba3d67 x86/boot/compressed: Get rid of GOT fixup code
be834cee6f39 x86/boot/compressed: Force hidden visibility for all
symbol references
9b7c7d8d2d7b x86/boot/compressed: Move .got.plt entries out of the .got section
ba6a49f806a0 (for-5.7/kbuild-compressed-debug-info) Makefile: support
compressed debug info
a1fb11944d11 (for-5.7/x86-boot-nivedita76) x86/boot: Correct
relocation destination on old linkers
c70e3890058f (for-5.7/x86-build-maskray) x86/boot: Discard
.discard.unreachable for arch/x86/boot/compressed/vmlinux

$ find ./ -name hidden.h
./drivers/firmware/efi/libstub/hidden.h
./arch/x86/boot/compressed/hidden.h

Any thoughts?

- Sedat -