From 6569580de7ae367def89b7671029cb97c1965574 Mon Sep 17 00:00:00 2001 From: Vivek Goyal Date: Thu, 7 Dec 2006 02:14:03 +0100 Subject: [PATCH] [PATCH] i386: Distinguish absolute symbols Ld knows about 2 kinds of symbols, absolute and section relative. Section relative symbols symbols change value when a section is moved and absolute symbols do not. Currently in the linker script we have several labels marking the beginning and ending of sections that are outside of sections, making them absolute symbols. Having a mixture of absolute and section relative symbols refereing to the same data is currently harmless but it is confusing. This must be done carefully as newer revs of ld do not place symbols that appear in sections without data and instead ld makes those symbols global :( My ultimate goal is to build a relocatable kernel. The safest and least intrusive technique is to generate relocation entries so the kernel can be relocated at load time. The only penalty would be an increase in the size of the kernel binary. The problem is that if absolute and relocatable symbols are not properly specified absolute symbols will be relocated or section relative symbols won't be, which is fatal. The practical motivation is that when generating kernels that will run from a reserved area for analyzing what caused a kernel panic, it is simpler if you don't need to hard code the physical memory location they will run at, especially for the distributions. [AK: and merged:] o Also put a message so that in future people can be aware of it and avoid introducing absolute symbols. Signed-off-by: Eric W. Biederman Signed-off-by: Vivek Goyal Signed-off-by: Andi Kleen --- arch/i386/kernel/vmlinux.lds.S | 113 +++++++++++++++++++++----------------- include/asm-generic/vmlinux.lds.h | 10 ++-- 2 files changed, 68 insertions(+), 55 deletions(-) diff --git a/arch/i386/kernel/vmlinux.lds.S b/arch/i386/kernel/vmlinux.lds.S index c6f84a0..cbd24860 100644 --- a/arch/i386/kernel/vmlinux.lds.S +++ b/arch/i386/kernel/vmlinux.lds.S @@ -1,5 +1,11 @@ /* ld script to make i386 Linux kernel * Written by Martin Mares ; + * + * Don't define absolute symbols until and unless you know that symbol + * value is should remain constant even if kernel image is relocated + * at run time. Absolute symbols are not relocated. If symbol value should + * change if kernel is relocated, make the symbol section relative and + * put it inside the section definition. */ #define LOAD_OFFSET __PAGE_OFFSET @@ -24,31 +30,32 @@ SECTIONS . = __KERNEL_START; phys_startup_32 = startup_32 - LOAD_OFFSET; /* read-only */ - _text = .; /* Text and read-only data */ .text : AT(ADDR(.text) - LOAD_OFFSET) { + _text = .; /* Text and read-only data */ *(.text) SCHED_TEXT LOCK_TEXT KPROBES_TEXT *(.fixup) *(.gnu.warning) - } :text = 0x9090 - - _etext = .; /* End of text section */ + _etext = .; /* End of text section */ + } :text = 0x9090 . = ALIGN(16); /* Exception table */ - __start___ex_table = .; - __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) { *(__ex_table) } - __stop___ex_table = .; + __ex_table : AT(ADDR(__ex_table) - LOAD_OFFSET) { + __start___ex_table = .; + *(__ex_table) + __stop___ex_table = .; + } RODATA . = ALIGN(4); - __tracedata_start = .; .tracedata : AT(ADDR(.tracedata) - LOAD_OFFSET) { + __tracedata_start = .; *(.tracedata) + __tracedata_end = .; } - __tracedata_end = .; /* writeable */ . = ALIGN(4096); @@ -58,10 +65,12 @@ SECTIONS } :data . = ALIGN(4096); - __nosave_begin = .; - .data_nosave : AT(ADDR(.data_nosave) - LOAD_OFFSET) { *(.data.nosave) } - . = ALIGN(4096); - __nosave_end = .; + .data_nosave : AT(ADDR(.data_nosave) - LOAD_OFFSET) { + __nosave_begin = .; + *(.data.nosave) + . = ALIGN(4096); + __nosave_end = .; + } . = ALIGN(4096); .data.page_aligned : AT(ADDR(.data.page_aligned) - LOAD_OFFSET) { @@ -75,8 +84,10 @@ SECTIONS /* rarely changed data like cpu maps */ . = ALIGN(32); - .data.read_mostly : AT(ADDR(.data.read_mostly) - LOAD_OFFSET) { *(.data.read_mostly) } - _edata = .; /* End of data section */ + .data.read_mostly : AT(ADDR(.data.read_mostly) - LOAD_OFFSET) { + *(.data.read_mostly) + _edata = .; /* End of data section */ + } #ifdef CONFIG_STACK_UNWIND . = ALIGN(4); @@ -94,54 +105,56 @@ SECTIONS /* might get freed after init */ . = ALIGN(4096); - __smp_alt_begin = .; - __smp_alt_instructions = .; .smp_altinstructions : AT(ADDR(.smp_altinstructions) - LOAD_OFFSET) { + __smp_alt_begin = .; + __smp_alt_instructions = .; *(.smp_altinstructions) + __smp_alt_instructions_end = .; } - __smp_alt_instructions_end = .; . = ALIGN(4); - __smp_locks = .; .smp_locks : AT(ADDR(.smp_locks) - LOAD_OFFSET) { + __smp_locks = .; *(.smp_locks) + __smp_locks_end = .; } - __smp_locks_end = .; .smp_altinstr_replacement : AT(ADDR(.smp_altinstr_replacement) - LOAD_OFFSET) { *(.smp_altinstr_replacement) + __smp_alt_end = .; } . = ALIGN(4096); - __smp_alt_end = .; /* will be freed after init */ . = ALIGN(4096); /* Init code and data */ - __init_begin = .; .init.text : AT(ADDR(.init.text) - LOAD_OFFSET) { + __init_begin = .; _sinittext = .; *(.init.text) _einittext = .; } .init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { *(.init.data) } . = ALIGN(16); - __setup_start = .; - .init.setup : AT(ADDR(.init.setup) - LOAD_OFFSET) { *(.init.setup) } - __setup_end = .; - __initcall_start = .; + .init.setup : AT(ADDR(.init.setup) - LOAD_OFFSET) { + __setup_start = .; + *(.init.setup) + __setup_end = .; + } .initcall.init : AT(ADDR(.initcall.init) - LOAD_OFFSET) { + __initcall_start = .; INITCALLS + __initcall_end = .; } - __initcall_end = .; - __con_initcall_start = .; .con_initcall.init : AT(ADDR(.con_initcall.init) - LOAD_OFFSET) { + __con_initcall_start = .; *(.con_initcall.init) + __con_initcall_end = .; } - __con_initcall_end = .; SECURITY_INIT . = ALIGN(4); - __alt_instructions = .; .altinstructions : AT(ADDR(.altinstructions) - LOAD_OFFSET) { + __alt_instructions = .; *(.altinstructions) + __alt_instructions_end = .; } - __alt_instructions_end = .; .altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) { *(.altinstr_replacement) } @@ -150,32 +163,32 @@ SECTIONS .exit.text : AT(ADDR(.exit.text) - LOAD_OFFSET) { *(.exit.text) } .exit.data : AT(ADDR(.exit.data) - LOAD_OFFSET) { *(.exit.data) } . = ALIGN(4096); - __initramfs_start = .; - .init.ramfs : AT(ADDR(.init.ramfs) - LOAD_OFFSET) { *(.init.ramfs) } - __initramfs_end = .; + .init.ramfs : AT(ADDR(.init.ramfs) - LOAD_OFFSET) { + __initramfs_start = .; + *(.init.ramfs) + __initramfs_end = .; + } . = ALIGN(L1_CACHE_BYTES); - __per_cpu_start = .; - .data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) { *(.data.percpu) } - __per_cpu_end = .; + .data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) { + __per_cpu_start = .; + *(.data.percpu) + __per_cpu_end = .; + } . = ALIGN(4096); - __init_end = .; /* freed after init ends here */ - __bss_start = .; /* BSS */ - .bss.page_aligned : AT(ADDR(.bss.page_aligned) - LOAD_OFFSET) { - *(.bss.page_aligned) - } .bss : AT(ADDR(.bss) - LOAD_OFFSET) { + __init_end = .; + __bss_start = .; /* BSS */ + *(.bss.page_aligned) *(.bss) + . = ALIGN(4); + __bss_stop = .; + _end = . ; + /* This is where the kernel creates the early boot page tables */ + . = ALIGN(4096); + pg0 = . ; } - . = ALIGN(4); - __bss_stop = .; - - _end = . ; - - /* This is where the kernel creates the early boot page tables */ - . = ALIGN(4096); - pg0 = .; /* Sections to be discarded */ /DISCARD/ : { diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index e60d6f2..9f47477 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -11,8 +11,8 @@ #define RODATA \ . = ALIGN(4096); \ - __start_rodata = .; \ .rodata : AT(ADDR(.rodata) - LOAD_OFFSET) { \ + VMLINUX_SYMBOL(__start_rodata) = .; \ *(.rodata) *(.rodata.*) \ *(__vermagic) /* Kernel version magic */ \ } \ @@ -119,17 +119,17 @@ *(__ksymtab_strings) \ } \ \ + /* Unwind data binary search table */ \ + EH_FRAME_HDR \ + \ /* Built-in module parameters. */ \ __param : AT(ADDR(__param) - LOAD_OFFSET) { \ VMLINUX_SYMBOL(__start___param) = .; \ *(__param) \ VMLINUX_SYMBOL(__stop___param) = .; \ + VMLINUX_SYMBOL(__end_rodata) = .; \ } \ \ - /* Unwind data binary search table */ \ - EH_FRAME_HDR \ - \ - __end_rodata = .; \ . = ALIGN(4096); #define SECURITY_INIT \ -- 2.7.4