Re: [PATCH v5 6/7] x86/boot: Remove run-time relocations from head_{32,64}.S

From: Sedat Dilek
Date: Wed Jul 15 2020 - 05:10:47 EST

On Wed, Jul 15, 2020 at 11:03 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Wed, 15 Jul 2020 at 11:58, Sedat Dilek <sedat.dilek@xxxxxxxxx> wrote:
> >
> > On Wed, Jul 15, 2020 at 2:41 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> > >
> > > The BFD linker generates run-time relocations for z_input_len and
> > > z_output_len, even though they are absolute symbols.
> > >
> > > This is fixed for binutils-2.35 [1]. Work around this for earlier
> > > versions by defining two variables input_len and output_len in addition
> > > to the symbols, and use them via position-independent references.
> > >
> > > This eliminates the last two run-time relocations in the head code and
> > > allows us to drop the -z noreloc-overflow flag to the linker.
> > >
> > > Move the -pie and --no-dynamic-linker LDFLAGS to LDFLAGS_vmlinux instead
> > > of KBUILD_LDFLAGS. There shouldn't be anything else getting linked, but
> > > this is the more logical location for these flags, and modversions might
> > > call the linker if an EXPORT_SYMBOL is left over accidentally in one of
> > > the decompressors.
> > >
> >
> > Tested-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> > Reported-by: Sedat Dilek <sedat.dilek@xxxxxxxxx>
> >
> > Reported breakage with LLD in previous patchset version.
> >
>
> Please drop the bogus reported-bys. The patch does *not* address an
> issue you reported, so recording this in the commit log would be
> incorrect.
>
> Your review and testing is appreciated, and resulted in substantial
> improvements. So feel free to give your reviewed-by in addition to
> your tested-by. But reported-by is inappropriate here.
>

Correct, in a hurry I mixed it up as said in the next patch 7/7.
Please drop this Reported-by.

- Sedat -

>
> >
> > > [1] https://sourceware.org/bugzilla/show_bug.cgi?id=25754
> > >
> > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > > Reviewed-by: Fangrui Song <maskray@xxxxxxxxxx>
> > > Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
> > > ---
> > > arch/x86/boot/compressed/Makefile | 12 ++----------
> > > arch/x86/boot/compressed/head_32.S | 17 ++++++++---------
> > > arch/x86/boot/compressed/head_64.S | 4 ++--
> > > arch/x86/boot/compressed/mkpiggy.c | 6 ++++++
> > > 4 files changed, 18 insertions(+), 21 deletions(-)
> > >
> > > diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> > > index c829d874dcac..ae2c0dc98a6a 100644
> > > --- a/arch/x86/boot/compressed/Makefile
> > > +++ b/arch/x86/boot/compressed/Makefile
> > > @@ -51,16 +51,8 @@ UBSAN_SANITIZE :=n
> > > KBUILD_LDFLAGS := -m elf_$(UTS_MACHINE)
> > > # Compressed kernel should be built as PIE since it may be loaded at any
> > > # address by the bootloader.
> > > -ifeq ($(CONFIG_X86_32),y)
> > > -KBUILD_LDFLAGS += $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> > > -else
> > > -# To build 64-bit compressed kernel as PIE, we disable relocation
> > > -# overflow check to avoid relocation overflow error with a new linker
> > > -# command-line option, -z noreloc-overflow.
> > > -KBUILD_LDFLAGS += $(shell $(LD) --help 2>&1 | grep -q "\-z noreloc-overflow" \
> > > - && echo "-z noreloc-overflow -pie --no-dynamic-linker")
> > > -endif
> > > -LDFLAGS_vmlinux := -T
> > > +LDFLAGS_vmlinux := $(call ld-option, -pie) $(call ld-option, --no-dynamic-linker)
> > > +LDFLAGS_vmlinux += -T
> > >
> > > hostprogs := mkpiggy
> > > HOST_EXTRACFLAGS += -I$(srctree)/tools/include
> > > diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
> > > index 8c1a4f5610f5..659fad53ca82 100644
> > > --- a/arch/x86/boot/compressed/head_32.S
> > > +++ b/arch/x86/boot/compressed/head_32.S
> > > @@ -178,18 +178,17 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
> > > /*
> > > * Do the extraction, and jump to the new kernel..
> > > */
> > > - /* push arguments for extract_kernel: */
> > > - pushl $z_output_len /* decompressed length, end of relocs */
> > > + /* push arguments for extract_kernel: */
> > >
> > > - pushl %ebp /* output address */
> > > -
> > > - pushl $z_input_len /* input_len */
> > > + pushl output_len@GOTOFF(%ebx) /* decompressed length, end of relocs */
> > > + pushl %ebp /* output address */
> > > + pushl input_len@GOTOFF(%ebx) /* input_len */
> > > leal input_data@GOTOFF(%ebx), %eax
> > > - pushl %eax /* input_data */
> > > + pushl %eax /* input_data */
> > > leal boot_heap@GOTOFF(%ebx), %eax
> > > - pushl %eax /* heap area */
> > > - pushl %esi /* real mode pointer */
> > > - call extract_kernel /* returns kernel location in %eax */
> > > + pushl %eax /* heap area */
> > > + pushl %esi /* real mode pointer */
> > > + call extract_kernel /* returns kernel location in %eax */
> > > addl $24, %esp
> > >
> > > /*
> > > diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> > > index 11429092c224..9e46729cf162 100644
> > > --- a/arch/x86/boot/compressed/head_64.S
> > > +++ b/arch/x86/boot/compressed/head_64.S
> > > @@ -534,9 +534,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
> > > movq %rsi, %rdi /* real mode address */
> > > leaq boot_heap(%rip), %rsi /* malloc area for uncompression */
> > > leaq input_data(%rip), %rdx /* input_data */
> > > - movl $z_input_len, %ecx /* input_len */
> > > + movl input_len(%rip), %ecx /* input_len */
> > > movq %rbp, %r8 /* output target address */
> > > - movl $z_output_len, %r9d /* decompressed length, end of relocs */
> > > + movl output_len(%rip), %r9d /* decompressed length, end of relocs */
> > > call extract_kernel /* returns kernel location in %rax */
> > > popq %rsi
> > >
> > > diff --git a/arch/x86/boot/compressed/mkpiggy.c b/arch/x86/boot/compressed/mkpiggy.c
> > > index 7e01248765b2..52aa56cdbacc 100644
> > > --- a/arch/x86/boot/compressed/mkpiggy.c
> > > +++ b/arch/x86/boot/compressed/mkpiggy.c
> > > @@ -60,6 +60,12 @@ int main(int argc, char *argv[])
> > > printf(".incbin \"%s\"\n", argv[1]);
> > > printf("input_data_end:\n");
> > >
> > > + printf(".section \".rodata\",\"a\",@progbits\n");
> > > + printf(".globl input_len\n");
> > > + printf("input_len:\n\t.long %lu\n", ilen);
> > > + printf(".globl output_len\n");
> > > + printf("output_len:\n\t.long %lu\n", (unsigned long)olen);
> > > +
> > > retval = 0;
> > > bail:
> > > if (f)
> > > --
> > > 2.26.2
> > >