kernel: fix an issue with infinite stack traces on MIPS

Reproduced by enabling lockdep on MT7621

Signed-off-by: Felix Fietkau <nbd@nbd.name>

diff --git a/target/linux/generic/pending-4.14/102-MIPS-only-process-negative-stack-offsets-on-stack-tr.patch b/target/linux/generic/pending-4.14/102-MIPS-only-process-negative-stack-offsets-on-stack-tr.patch
new file mode 100644 (file)
index 0000000..cc17053
--- /dev/null
+++ b/target/linux/generic/pending-4.14/102-MIPS-only-process-negative-stack-offsets-on-stack-tr.patch
@@ -0,0 +1,57 @@
+From: Felix Fietkau <nbd@nbd.name>
+Date: Wed, 18 Apr 2018 10:50:05 +0200
+Subject: [PATCH] MIPS: only process negative stack offsets on stack traces
+
+Fixes endless back traces in cases where the compiler emits a stack
+pointer increase in a branch delay slot (probably for some form of
+function return).
+
+[    3.475442] BUG: MAX_STACK_TRACE_ENTRIES too low!
+[    3.480070] turning off the locking correctness validator.
+[    3.485521] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.14.34 #0
+[    3.491475] Stack : 00000000 00000000 00000000 00000000 80e0fce2 00000034 00000000 00000000
+[    3.499764]         87c3838c 80696377 8061047c 00000000 00000001 00000001 87c2d850 6534689f
+[    3.508059]         00000000 00000000 80e10000 00000000 00000000 000000cf 0000000f 00000000
+[    3.516353]         00000000 806a0000 00076891 00000000 00000000 00000000 ffffffff 00000000
+[    3.524648]         806c0000 00000004 80e10000 806a0000 00000003 80690000 00000000 80700000
+[    3.532942]         ...
+[    3.535362] Call Trace:
+[    3.537818] [<80010a48>] show_stack+0x58/0x100
+[    3.542207] [<804c2f78>] dump_stack+0xe8/0x170
+[    3.546613] [<80079f90>] save_trace+0xf0/0x110
+[    3.551010] [<8007b1ec>] mark_lock+0x33c/0x78c
+[    3.555413] [<8007bf48>] __lock_acquire+0x2ac/0x1a08
+[    3.560337] [<8007de60>] lock_acquire+0x64/0x8c
+[    3.564846] [<804e1570>] _raw_spin_lock_irqsave+0x54/0x78
+[    3.570186] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.574770] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.579257] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.583839] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.588329] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.592911] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.597401] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.601983] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.606473] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.611055] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.615545] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.620125] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.624619] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.629197] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.633691] [<801b618c>] kernfs_notify+0x94/0xac
+[    3.638269] [<801b7b10>] sysfs_notify+0x74/0xa0
+[    3.642763] [<801b618c>] kernfs_notify+0x94/0xac
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+
+--- a/arch/mips/kernel/process.c
++++ b/arch/mips/kernel/process.c
+@@ -357,6 +357,8 @@ static inline int is_sp_move_ins(union m
+ 
+       if (ip->i_format.opcode == addiu_op ||
+           ip->i_format.opcode == daddiu_op) {
++              if (ip->i_format.simmediate > 0)
++                      return 0;
+               *frame_size = -ip->i_format.simmediate;
+               return 1;
+       }