[PATCH 1/2] MIPS: detect sibling call in get_frame_info

[Tony Wu <tung7970@gmail.com>]

Given a function, get_frame_info() analyzes its instructions
to figure out frame size and return address. get_frame_info()
works as follows:

1. analyze up to 128 instructions if the function size is unknown
2. search for 'addiu/daddiu sp,sp,-immed' for frame size
3. search for 'sw ra,offset(sp)' for return address
4. end search when it sees jr/jal/jalr

This leads to an issue when the given function is a sibling
call, example given as follows.

801ca110 <schedule>:
801ca110:       8f820000        lw      v0,0(gp)
801ca114:       8c420000        lw      v0,0(v0)
801ca118:       080726f0        j       801c9bc0 <__schedule>
801ca11c:       00000000        nop

801ca120 <io_schedule>:
801ca120:       27bdffe8        addiu   sp,sp,-24
801ca124:       3c028022        lui     v0,0x8022
801ca128:       afbf0014        sw      ra,20(sp)

In this case, get_frame_info() cannot properly detect schedule's
frame info, and eventually returns io_schedule's info instead.

This patch adds sibling call check by detecting out of range jump.

Signed-off-by: Tony Wu <tung7970@gmail.com>
---
 arch/mips/kernel/process.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
index cfc742d..a794eb5 100644
--- a/arch/mips/kernel/process.c
+++ b/arch/mips/kernel/process.c
@@ -223,6 +223,9 @@ struct mips_frame_info {
 	int		pc_offset;
 };
 
+#define J_TARGET(pc,target)	\
+		(((unsigned long)(pc) & 0xf0000000) | ((target) << 2))
+
 static inline int is_ra_save_ins(union mips_instruction *ip)
 {
 	/* sw / sd $ra, offset($sp) */
@@ -250,11 +253,25 @@ static inline int is_sp_move_ins(union mips_instruction *ip)
 	return 0;
 }
 
+static inline int is_sibling_j_ins(union mips_instruction *ip,
+				   unsigned long func_begin, unsigned long func_end)
+{
+	if (ip->j_format.opcode == j_op) {
+		unsigned long addr;
+
+		addr = J_TARGET(ip, ip->j_format.target);
+		if (addr < func_begin || addr > func_end)
+			return 1;
+	}
+	return 0;
+}
+
 static int get_frame_info(struct mips_frame_info *info)
 {
 	union mips_instruction *ip = info->func;
 	unsigned max_insns = info->func_size / sizeof(union mips_instruction);
 	unsigned i;
+	unsigned long func_begin, func_end;
 
 	info->pc_offset = -1;
 	info->frame_size = 0;
@@ -266,10 +283,15 @@ static int get_frame_info(struct mips_frame_info *info)
 		max_insns = 128U;	/* unknown function size */
 	max_insns = min(128U, max_insns);
 
+	func_begin = (unsigned long) info->func;
+	func_end = func_begin + max_insns * sizeof(union mips_instruction);
+
 	for (i = 0; i < max_insns; i++, ip++) {
 
 		if (is_jal_jalr_jr_ins(ip))
 			break;
+		if (is_sibling_j_ins(ip, func_begin, func_end))
+			break;
 		if (!info->frame_size) {
 			if (is_sp_move_ins(ip))
 				info->frame_size = - ip->i_format.simmediate;
-- 
1.7.10.2 (Apple Git-33)