[PATCH 3/12] i386: Force section size to be non-zero to prevent a symbol becoming absolute

Vivek Goyal
2006-10-03T17:32:48Z

o Relocation patches for i386, moved the symbols in vmlinux.lds.S inside
sections so that these symbols become section relative and are no more
absolute. If these symbols become absolute, its bad as they are not
relocated if kernel is not loaded at the address it has been compiled
for.

o Ironically, just moving the symbols inside the section does not
gurantee that symbols inside will not become absolute. Recent
versions of linkers, do some optimization, and if section size is
zero, it gets rid of the section and makes any defined symbol as absolute.

o This leads to a failure while second kernel is booting.
arch/i386/alternative.c frees any pages present between __smp_alt_begin
and __smp_alt_end. In my case size of section .smp_altinstructions is
zero and symbol __smpt_alt_begin becomes absolute and is not relocated
and system crashes while it is trying to free the memory starting
from __smp_alt_begin.

o This issue is being fixed by the linker guys and they are making sure
that linker does not get rid of an empty section if there is any
section relative symbol defined in it. But we need to fix it at
kernel level too so that people using the linker version without fix,
are not affected.

o One of the possible solutions is that force the section size to be
non zero to make sure these symbols don't become absolute. This
patch implements that.

Signed-off-by: Vivek Goyal <vgoyal@in.ibm.com>
---

arch/i386/kernel/vmlinux.lds.S | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff -puN arch/i386/kernel/vmlinux.lds.S~i386-reloc-non-zero-size-section arch/i386/kernel/vmlinux.lds.S
--- linux-2.6.18-git17/arch/i386/kernel/vmlinux.lds.S~i386-reloc-non-zero-size-section 2006-10-02 13:17:58.000000000 -0400
+++ linux-2.6.18-git17-root/arch/i386/kernel/vmlinux.lds.S 2006-10-02 14:36:32.000000000 -0400
@@ -40,6 +40,7 @@ SECTIONS
__start___ex_table = .;
*(__ex_table)
__stop___ex_table = .;
+ LONG(0)
}

RODATA
@@ -49,6 +50,7 @@ SECTIONS
__tracedata_start = .;
*(.tracedata)
__tracedata_end = .;
+ LONG(0)
}

/* writeable */
@@ -64,6 +66,7 @@ SECTIONS
*(.data.nosave)
. = ALIGN(4096);
__nosave_end = .;
+ LONG(0)
}

. = ALIGN(4096);
@@ -81,6 +84,7 @@ SECTIONS
.data.read_mostly : AT(ADDR(.data.read_mostly) - LOAD_OFFSET) {
*(.data.read_mostly)
_edata = .; /* End of data section */
+ LONG(0)
}

#ifdef CONFIG_STACK_UNWIND
@@ -89,6 +93,7 @@ SECTIONS
__start_unwind = .;
*(.eh_frame)
__end_unwind = .;
+ LONG(0)
}
#endif

@@ -104,17 +109,20 @@ SECTIONS
__smp_alt_instructions = .;
*(.smp_altinstructions)
__smp_alt_instructions_end = .;
+ LONG(0)
}
. = ALIGN(4);
.smp_locks : AT(ADDR(.smp_locks) - LOAD_OFFSET) {
__smp_locks = .;
*(.smp_locks)
__smp_locks_end = .;
+ LONG(0)
}
.smp_altinstr_replacement : AT(ADDR(.smp_altinstr_replacement) - LOAD_OFFSET) {
*(.smp_altinstr_replacement)
. = ALIGN(4096);
__smp_alt_end = .;
+ LONG(0)
}

/* will be freed after init */
@@ -124,6 +132,7 @@ SECTIONS
_sinittext = .;
*(.init.text)
_einittext = .;
+ LONG(0)
}
.init.data : AT(ADDR(.init.data) - LOAD_OFFSET) { *(.init.data) }
. = ALIGN(16);
@@ -131,6 +140,7 @@ SECTIONS
__setup_start = .;
*(.init.setup)
__setup_end = .;
+ LONG(0)
}
.initcall.init : AT(ADDR(.initcall.init) - LOAD_OFFSET) {
__initcall_start = .;
@@ -142,11 +152,13 @@ SECTIONS
*(.initcall6.init)
*(.initcall7.init)
__initcall_end = .;
+ LONG(0)
}
.con_initcall.init : AT(ADDR(.con_initcall.init) - LOAD_OFFSET) {
__con_initcall_start = .;
*(.con_initcall.init)
__con_initcall_end = .;
+ LONG(0)
}
SECURITY_INIT
. = ALIGN(4);
@@ -154,6 +166,7 @@ SECTIONS
__alt_instructions = .;
*(.altinstructions)
__alt_instructions_end = .;
+ LONG(0)
}
.altinstr_replacement : AT(ADDR(.altinstr_replacement) - LOAD_OFFSET) {
*(.altinstr_replacement)
@@ -167,12 +180,14 @@ SECTIONS
__initramfs_start = .;
*(.init.ramfs)
__initramfs_end = .;
+ LONG(0)
}
. = ALIGN(L1_CACHE_BYTES);
.data.percpu : AT(ADDR(.data.percpu) - LOAD_OFFSET) {
__per_cpu_start = .;
*(.data.percpu)
__per_cpu_end = .;
+ LONG(0)
}
. = ALIGN(4096);
/* freed after init ends here */
_