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

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>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Link: https://linux-review.googlesource.com/id/If745fd9cadcb31b4010acbf5693727fe111b0863
Link: https://lore.kernel.org/r/20201230221954.2007257-1-pcc@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index a8f8e40..cd9c3fa 100644 (file)
--- 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 d808ad3..61dbb4c 100644 (file)
--- 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;
 }
 
 /*