[Tarantool-patches] [PATCH luajit 01/19] MIPS: Use precise search for exit jump patching.

[Sergey Kaplun]

From: Mike Pall <mike>

Contributed by Djordje Kovacevic and Stefan Pejic.

(cherry-picked from commit 7381b620358c2561e8690149f1d25828fdad6675)

Without the aforementioned checks, some non-branch instructions may be
interpreted as some branch due to memory address collisions. This patch
adds the corresponding comparisons masked values with instruction
opcodes used in the LuaJIT:
* `MIPSI_BEQ` for `beq` and `bne`,
* `MIPSI_BLTZ` for `bltz`, `blez`, `bgtz` and `bgez`,
* `MIPSI_BC1F` for `bc1f` and `bc1t`,
see <src/lj_target_mips.h> and MIPS Instruction Set Manual [1] for
details.

To reproduce this failure, we need specific memory mapping, so testcase
is omitted.

Since MIPS architecture is not supported by Tarantool (at the moment)
this patch is not necessary for backport. OTOH, it gives to us the
following benefits:
* Be in sync with the LuaJIT upstream not only for x86_64, arm64
  architectures.
* Avoid conflicts during the future backporting.
So, it's more useful to backport some of the patches to avoid conflicts
with the future patch series.

[1]: https://s3-eu-west-1.amazonaws.com/downloads-mips/documents/MD00086-2B-MIPS32BIS-AFP-6.06.pdf

Sergey Kaplun:
* added the description for the problem

Part of tarantool/tarantool#8825
---
 src/lj_asm_mips.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/lj_asm_mips.h b/src/lj_asm_mips.h
index 03417013..03215821 100644
--- a/src/lj_asm_mips.h
+++ b/src/lj_asm_mips.h
@@ -2472,7 +2472,11 @@ void lj_asm_patchexit(jit_State *J, GCtrace *T, ExitNo exitno, MCode *target)
   MCode tjump = MIPSI_J|(((uintptr_t)target>>2)&0x03ffffffu);
   for (p++; p < pe; p++) {
     if (*p == exitload) {  /* Look for load of exit number. */
-      if (((p[-1] ^ (px-p)) & 0xffffu) == 0) {  /* Look for exitstub branch. */
+      /* Look for exitstub branch. Yes, this covers all used branch variants. */
+      if (((p[-1] ^ (px-p)) & 0xffffu) == 0 &&
+	  ((p[-1] & 0xf0000000u) == MIPSI_BEQ ||
+	   (p[-1] & 0xfc1e0000u) == MIPSI_BLTZ ||
+	   (p[-1] & 0xffe00000u) == MIPSI_BC1F)) {
 	ptrdiff_t delta = target - p;
 	if (((delta + 0x8000) >> 16) == 0) {  /* Patch in-range branch. */
 	patchbranch:
-- 
2.41.0