[MIPS] Fix $gp usage
Now we load $gp with _GLOBAL_OFFSET_TABLE_, but this is incorrect use.
As a general principle, we should use _gp for $gp.
Thanks to linker script's help we fortunately have _gp which equals to
_GLOBAL_OFFSET_TABLE_. But once _gp gets out of alignment, we will not
be able to access to GOT entires, global variables and procedure entry
points. The right thing to do is to use _gp.
This patch also introduce a new symbol `.gpword _GLOBAL_OFFSET_TABLE_'
which holds the offset from _gp. When updating GOT entries, we use this
offset and _gp to calculate the final _GLOBAL_OFFSET_TABLE_.
This patch is originally submitted by Vlad Lungu <vlad@comsys.ro>, then
I made some change to leave over num_got_entries.
Signed-off-by: Shinya Kuribayashi <shinya.kuribayashi@necel.com>
Cc: Vlad Lungu <vlad@comsys.ro>
diff --git a/cpu/mips/start.S b/cpu/mips/start.S
index e91e213..074d01d 100644
--- a/cpu/mips/start.S
+++ b/cpu/mips/start.S
@@ -234,11 +234,11 @@
li t0, CONF_CM_UNCACHED
mtc0 t0, CP0_CONFIG
- /* Initialize GOT pointer.
+ /* Initialize $gp.
*/
bal 1f
nop
- .word _GLOBAL_OFFSET_TABLE_
+ .word _gp
1:
move gp, ra
lw t1, 0(ra)
@@ -306,9 +306,9 @@
move t1, a2
/*
- * Fix GOT pointer:
+ * Fix $gp:
*
- * New GOT-PTR = (old GOT-PTR - CFG_MONITOR_BASE) + Destination Address
+ * New $gp = (Old $gp - CFG_MONITOR_BASE) + Destination Address
*/
move t6, gp
sub gp, CFG_MONITOR_BASE
@@ -341,15 +341,22 @@
j t0
nop
+ .gpword _GLOBAL_OFFSET_TABLE_ /* _GLOBAL_OFFSET_TABLE_ - _gp */
.word uboot_end_data
.word uboot_end
.word num_got_entries
in_ram:
- /* Now we want to update GOT.
+ /*
+ * Now we want to update GOT.
+ *
+ * GOT[0] is reserved. GOT[1] is also reserved for the dynamic object
+ * generated by GNU ld. Skip these reserved entries from relocation.
*/
lw t3, -4(t0) /* t3 <-- num_got_entries */
- addi t4, gp, 8 /* Skipping first two entries. */
+ lw t4, -16(t0) /* t4 <-- (_GLOBAL_OFFSET_TABLE_ - _gp) */
+ add t4, t4, gp /* t4 now holds _GLOBAL_OFFSET_TABLE_ */
+ addi t4, t4, 8 /* Skipping first two entries. */
li t2, 2
1:
lw t1, 0(t4)