On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> From: Arvind Sankar <[email protected]>
>
> The x86-64 psABI [0] specifies special relocation types
> (R_X86_64_[REX_]GOTPCRELX) for indirection through the Global Offset
> Table, semantically equivalent to R_X86_64_GOTPCREL, which the linker
> can take advantage of for optimization (relaxation) at link time. This
> is supported by LLD and binutils versions 2.26 onwards.
>
> The compressed kernel is position-independent code, however, when using
> LLD or binutils versions before 2.27, it must be linked without the -pie
> option. In this case, the linker may optimize certain instructions into
> a non-position-independent form, by converting foo@GOTPCREL(%rip) to $foo.
>
> This potential issue has been present with LLD and binutils-2.26 for a
> long time, but it has never manifested itself before now:
> - LLD and binutils-2.26 only relax
> movq foo@GOTPCREL(%rip), %reg
> to
> leaq foo(%rip), %reg
> which is still position-independent, rather than
> mov $foo, %reg
> which is permitted by the psABI when -pie is not enabled.
> - gcc happens to only generate GOTPCREL relocations on mov instructions.
> - clang does generate GOTPCREL relocations on non-mov instructions, but
> when building the compressed kernel, it uses its integrated assembler
> (due to the redefinition of KBUILD_CFLAGS dropping -no-integrated-as),
> which has so far defaulted to not generating the GOTPCRELX
> relocations.
>
> Nick Desaulniers reports [1,2]:
> A recent change [3] to a default value of configuration variable
> (ENABLE_X86_RELAX_RELOCATIONS OFF -> ON) in LLVM now causes Clang's
> integrated assembler to emit R_X86_64_GOTPCRELX/R_X86_64_REX_GOTPCRELX
> relocations. LLD will relax instructions with these relocations based
> on whether the image is being linked as position independent or not.
> When not, then LLD will relax these instructions to use absolute
> addressing mode (R_RELAX_GOT_PC_NOPIC). This causes kernels built with
> Clang and linked with LLD to fail to boot.
>
> Patch series [4] is a solution to allow the compressed kernel to be
> linked with -pie unconditionally, but even if merged is unlikely to be
> backported. As a simple solution that can be applied to stable as well,
> prevent the assembler from generating the relaxed relocation types using
> the -mrelax-relocations=no option. For ease of backporting, do this
> unconditionally.
>
> [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> [1] https://lore.kernel.org/lkml/[email protected]/
> [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> [4] https://lore.kernel.org/lkml/[email protected]/
>
> Reported-by: Nick Desaulniers <[email protected]>
> Signed-off-by: Arvind Sankar <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
--
Kees Cook
On Thu, Sep 03, 2020 at 02:44:41PM -0700, Kees Cook wrote:
> On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> > From: Arvind Sankar <[email protected]>
> >
> > Patch series [4] is a solution to allow the compressed kernel to be
> > linked with -pie unconditionally, but even if merged is unlikely to be
> > backported. As a simple solution that can be applied to stable as well,
> > prevent the assembler from generating the relaxed relocation types using
> > the -mrelax-relocations=no option. For ease of backporting, do this
> > unconditionally.
> >
> > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> > [1] https://lore.kernel.org/lkml/[email protected]/
> > [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> > [4] https://lore.kernel.org/lkml/[email protected]/
> >
> > Reported-by: Nick Desaulniers <[email protected]>
> > Signed-off-by: Arvind Sankar <[email protected]>
>
> Reviewed-by: Kees Cook <[email protected]>
>
> --
> Kees Cook
Note that since [4] is now in tip, assuming it doesn't get dropped for
some reason, this patch isn't necessary unless you need to backport this
LTO series to 5.9 or below.
Thanks.
On Thu, Sep 03, 2020 at 07:42:15PM -0400, Arvind Sankar wrote:
> On Thu, Sep 03, 2020 at 02:44:41PM -0700, Kees Cook wrote:
> > On Thu, Sep 03, 2020 at 01:30:26PM -0700, Sami Tolvanen wrote:
> > > From: Arvind Sankar <[email protected]>
> > >
> > > Patch series [4] is a solution to allow the compressed kernel to be
> > > linked with -pie unconditionally, but even if merged is unlikely to be
> > > backported. As a simple solution that can be applied to stable as well,
> > > prevent the assembler from generating the relaxed relocation types using
> > > the -mrelax-relocations=no option. For ease of backporting, do this
> > > unconditionally.
> > >
> > > [0] https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/linker-optimization.tex#L65
> > > [1] https://lore.kernel.org/lkml/[email protected]/
> > > [2] https://github.com/ClangBuiltLinux/linux/issues/1121
> > > [3] https://reviews.llvm.org/rGc41a18cf61790fc898dcda1055c3efbf442c14c0
> > > [4] https://lore.kernel.org/lkml/[email protected]/
> > >
> > > Reported-by: Nick Desaulniers <[email protected]>
> > > Signed-off-by: Arvind Sankar <[email protected]>
> >
> > Reviewed-by: Kees Cook <[email protected]>
> >
> > --
> > Kees Cook
>
> Note that since [4] is now in tip, assuming it doesn't get dropped for
> some reason, this patch isn't necessary unless you need to backport this
> LTO series to 5.9 or below.
>
> Thanks.
It is still necessary for tip of tree LLVM to work properly
(specifically clang and ld.lld) regardless of whether or not LTO is
used.
[4] also fixes it but I don't think it can be backported to stable so it
would still be nice to get it picked up so that it can be sent back
there. We have been carrying it in our CI for a decent amount of time...
Cheers,
Nathan