Re: [PATCH] arm64: vdso: disable .eh_frame_hdr via /DISCARD/ instead of --no-eh-frame-hdr

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 30 Dec 2020 at 23:20, Peter Collingbourne <pcc@google.com> wrote:
>
> Currently with ld.lld we emit an empty .eh_frame_hdr section (and a
> corresponding program header) into the vDSO. With ld.bfd the section
> is not emitted but the program header is, with p_vaddr set to 0. This
> can lead to unwinders attempting to interpret the data at whichever
> location the program header happens to point to as an unwind info
> header. This happens to be mostly harmless as long as the byte at
> that location (interpreted as a version number) has a value other
> than 1, causing both libgcc and LLVM libunwind to ignore the section
> (in libunwind's case, after printing an error message to stderr),
> but it could lead to worse problems if the byte happened to be 1 or
> the program header points to non-readable memory (e.g. if the empty
> section was placed at a page boundary).
>
> Instead of disabling .eh_frame_hdr via --no-eh-frame-hdr (which
> also has the downside of being unsupported by older versions of GNU
> binutils), disable it by discarding the section, and stop emitting
> the program header that points to it.
>
> I understand that we intend to emit valid unwind info for the vDSO
> at some point. Once that happens this patch can be reverted.
>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/If745fd9cadcb31b4010acbf5693727fe111b0863

Acked-by: Ard Biesheuvel <ardb@kernel.org>

although I am slighty concerned that we haven't been able to converge
on a proper fix yet.


> ---
>  arch/arm64/kernel/vdso/Makefile   | 3 +--
>  arch/arm64/kernel/vdso/vdso.lds.S | 5 +----
>  2 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> index a8f8e409e2bf..cd9c3fa25902 100644
> --- a/arch/arm64/kernel/vdso/Makefile
> +++ b/arch/arm64/kernel/vdso/Makefile
> @@ -24,8 +24,7 @@ btildflags-$(CONFIG_ARM64_BTI_KERNEL) += -z force-bti
>  # routines, as x86 does (see 6f121e548f83 ("x86, vdso: Reimplement vdso.so
>  # preparation in build-time C")).
>  ldflags-y := -shared -nostdlib -soname=linux-vdso.so.1 --hash-style=sysv       \
> -            -Bsymbolic $(call ld-option, --no-eh-frame-hdr) --build-id=sha1 -n \
> -            $(btildflags-y) -T
> +            -Bsymbolic --build-id=sha1 -n $(btildflags-y) -T
>
>  ccflags-y := -fno-common -fno-builtin -fno-stack-protector -ffixed-x18
>  ccflags-y += -DDISABLE_BRANCH_PROFILING -DBUILD_VDSO
> diff --git a/arch/arm64/kernel/vdso/vdso.lds.S b/arch/arm64/kernel/vdso/vdso.lds.S
> index d808ad31e01f..61dbb4c838ef 100644
> --- a/arch/arm64/kernel/vdso/vdso.lds.S
> +++ b/arch/arm64/kernel/vdso/vdso.lds.S
> @@ -40,9 +40,6 @@ SECTIONS
>         PROVIDE (_etext = .);
>         PROVIDE (etext = .);
>
> -       .eh_frame_hdr   : { *(.eh_frame_hdr) }          :text   :eh_frame_hdr
> -       .eh_frame       : { KEEP (*(.eh_frame)) }       :text
> -
>         .dynamic        : { *(.dynamic) }               :text   :dynamic
>
>         .rodata         : { *(.rodata*) }               :text
> @@ -54,6 +51,7 @@ SECTIONS
>                 *(.note.GNU-stack)
>                 *(.data .data.* .gnu.linkonce.d.* .sdata*)
>                 *(.bss .sbss .dynbss .dynsbss)
> +               *(.eh_frame .eh_frame_hdr)
>         }
>  }
>
> @@ -66,7 +64,6 @@ PHDRS
>         text            PT_LOAD         FLAGS(5) FILEHDR PHDRS; /* PF_R|PF_X */
>         dynamic         PT_DYNAMIC      FLAGS(4);               /* PF_R */
>         note            PT_NOTE         FLAGS(4);               /* PF_R */
> -       eh_frame_hdr    PT_GNU_EH_FRAME;
>  }
>
>  /*
> --
> 2.29.2.729.g45daf8777d-goog
>

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