arm64: relocatable: suppress R_AARCH64_ABS64 relocations in vmlinux

The linker routines that we rely on to produce a relocatable PIE binary
treat it as a shared ELF object in some ways, i.e., it emits symbol based
R_AARCH64_ABS64 relocations into the final binary since doing so would be
appropriate when linking a shared library that is subject to symbol
preemption. (This means that an executable can override certain symbols
that are exported by a shared library it is linked with, and that the
shared library *must* update all its internal references as well, and point
them to the version provided by the executable.)

Symbol preemption does not occur for OS hosted PIE executables, let alone
for vmlinux, and so we would prefer to get rid of these symbol based
relocations. This would allow us to simplify the relocation routines, and
to strip the .dynsym, .dynstr and .hash sections from the binary. (Note
that these are tiny, and are placed in the .init segment, but they clutter
up the vmlinux binary.)

Note that these R_AARCH64_ABS64 relocations are only emitted for absolute
references to symbols defined in the linker script, all other relocatable
quantities are covered by anonymous R_AARCH64_RELATIVE relocations that
simply list the offsets to all 64-bit values in the binary that need to be
fixed up based on the offset between the link time and run time addresses.

Fortunately, GNU ld has a -Bsymbolic option, which is intended for shared
libraries to allow them to ignore symbol preemption, and unconditionally
bind all internal symbol references to its own definitions. So set it for
our PIE binary as well, and get rid of the asoociated sections and the
relocation code that processes them.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[will: fixed conflict with __dynsym_offset linker script entry]
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 393103b..7670828 100644 (file)
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -15,7 +15,7 @@ CPPFLAGS_vmlinux.lds = -DTEXT_OFFSET=$(TEXT_OFFSET)
 GZFLAGS                :=-9
 
 ifneq ($(CONFIG_RELOCATABLE),)
-LDFLAGS_vmlinux                += -pie
+LDFLAGS_vmlinux                += -pie -Bsymbolic
 endif
 
 KBUILD_DEFCONFIG := defconfig

diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
index 2c6e598..b77f583 100644 (file)
--- a/arch/arm64/kernel/head.S
+++ b/arch/arm64/kernel/head.S
@@ -781,40 +781,25 @@ __primary_switch:
         * Iterate over each entry in the relocation table, and apply the
         * relocations in place.
         */
-       ldr     w8, =__dynsym_offset            // offset to symbol table
        ldr     w9, =__rela_offset              // offset to reloc table
        ldr     w10, =__rela_size               // size of reloc table
 
        mov_q   x11, KIMAGE_VADDR               // default virtual offset
        add     x11, x11, x23                   // actual virtual offset
-       add     x8, x8, x11                     // __va(.dynsym)
        add     x9, x9, x11                     // __va(.rela)
        add     x10, x9, x10                    // __va(.rela) + sizeof(.rela)
 
 0:     cmp     x9, x10
-       b.hs    2f
+       b.hs    1f
        ldp     x11, x12, [x9], #24
        ldr     x13, [x9, #-8]
        cmp     w12, #R_AARCH64_RELATIVE
-       b.ne    1f
+       b.ne    0b
        add     x13, x13, x23                   // relocate
        str     x13, [x11, x23]
        b       0b
 
-1:     cmp     w12, #R_AARCH64_ABS64
-       b.ne    0b
-       add     x12, x12, x12, lsl #1           // symtab offset: 24x top word
-       add     x12, x8, x12, lsr #(32 - 3)     // ... shifted into bottom word
-       ldrsh   w14, [x12, #6]                  // Elf64_Sym::st_shndx
-       ldr     x15, [x12, #8]                  // Elf64_Sym::st_value
-       cmp     w14, #-0xf                      // SHN_ABS (0xfff1) ?
-       add     x14, x15, x23                   // relocate
-       csel    x15, x14, x15, ne
-       add     x15, x13, x15
-       str     x15, [x11, x23]
-       b       0b
-
-2:
+1:
 #endif
        ldr     x8, =__primary_switched
        br      x8

diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
index d494921..659963d 100644 (file)
--- a/arch/arm64/kernel/vmlinux.lds.S
+++ b/arch/arm64/kernel/vmlinux.lds.S
@@ -103,6 +103,7 @@ SECTIONS
                *(.discard)
                *(.discard.*)
                *(.interp .dynamic)
+               *(.dynsym .dynstr .hash)
        }
 
        . = KIMAGE_VADDR + TEXT_OFFSET;
@@ -174,19 +175,9 @@ SECTIONS
        .rela : ALIGN(8) {
                *(.rela .rela*)
        }
-       .dynsym : ALIGN(8) {
-               *(.dynsym)
-       }
-       .dynstr : {
-               *(.dynstr)
-       }
-       .hash : {
-               *(.hash)
-       }
 
        __rela_offset   = ABSOLUTE(ADDR(.rela) - KIMAGE_VADDR);
        __rela_size     = SIZEOF(.rela);
-       __dynsym_offset = ABSOLUTE(ADDR(.dynsym) - KIMAGE_VADDR);
 
        . = ALIGN(SEGMENT_ALIGN);
        __init_end = .;