[PATCH] LoongArch: make the users of larch_insn_gen_break() constant

[Oleg Nesterov <oleg@redhat.com>]

LoongArch defines UPROBE_SWBP_INSN as a function call and this breaks
arch_uprobe_trampoline() which uses it to initialize a static variable.

Add the new "__builtin_constant_p" helper, __emit_break(), and redefine
the current users of larch_insn_gen_break() to use it.

The patch adds check_emit_break() into kprobes.c and uprobes.c to test
this change. They can be removed if LoongArch boots at least once, but
otoh these 2 __init functions will be discarded by free_initmem().

Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20240614174822.GA1185149@thelio-3990X/
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/loongarch/include/asm/inst.h    |  3 +++
 arch/loongarch/include/asm/uprobes.h |  4 ++--
 arch/loongarch/kernel/kprobes.c      | 12 ++++++++++--
 arch/loongarch/kernel/uprobes.c      |  8 ++++++++
 4 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/arch/loongarch/include/asm/inst.h b/arch/loongarch/include/asm/inst.h
index c3993fd88aba..944482063f14 100644
--- a/arch/loongarch/include/asm/inst.h
+++ b/arch/loongarch/include/asm/inst.h
@@ -532,6 +532,9 @@ static inline void emit_##NAME(union loongarch_instruction *insn,	\
 
 DEF_EMIT_REG0I15_FORMAT(break, break_op)
 
+/* like emit_break(imm) but returns a constant expression */
+#define __emit_break(imm)	((u32)((imm) | (break_op << 15)))
+
 #define DEF_EMIT_REG0I26_FORMAT(NAME, OP)				\
 static inline void emit_##NAME(union loongarch_instruction *insn,	\
 			       int offset)				\
diff --git a/arch/loongarch/include/asm/uprobes.h b/arch/loongarch/include/asm/uprobes.h
index c8f59983f702..99a0d198927f 100644
--- a/arch/loongarch/include/asm/uprobes.h
+++ b/arch/loongarch/include/asm/uprobes.h
@@ -9,10 +9,10 @@ typedef u32 uprobe_opcode_t;
 #define MAX_UINSN_BYTES		8
 #define UPROBE_XOL_SLOT_BYTES	MAX_UINSN_BYTES
 
-#define UPROBE_SWBP_INSN	larch_insn_gen_break(BRK_UPROBE_BP)
+#define UPROBE_SWBP_INSN	__emit_break(BRK_UPROBE_BP)
 #define UPROBE_SWBP_INSN_SIZE	LOONGARCH_INSN_SIZE
 
-#define UPROBE_XOLBP_INSN	larch_insn_gen_break(BRK_UPROBE_XOLBP)
+#define UPROBE_XOLBP_INSN	__emit_break(BRK_UPROBE_XOLBP)
 
 struct arch_uprobe {
 	unsigned long	resume_era;
diff --git a/arch/loongarch/kernel/kprobes.c b/arch/loongarch/kernel/kprobes.c
index 17b040bd6067..78cfaac52748 100644
--- a/arch/loongarch/kernel/kprobes.c
+++ b/arch/loongarch/kernel/kprobes.c
@@ -4,8 +4,16 @@
 #include <linux/preempt.h>
 #include <asm/break.h>
 
-#define KPROBE_BP_INSN		larch_insn_gen_break(BRK_KPROBE_BP)
-#define KPROBE_SSTEPBP_INSN	larch_insn_gen_break(BRK_KPROBE_SSTEPBP)
+#define KPROBE_BP_INSN		__emit_break(BRK_KPROBE_BP)
+#define KPROBE_SSTEPBP_INSN	__emit_break(BRK_KPROBE_SSTEPBP)
+
+static __init int check_emit_break(void)
+{
+	BUG_ON(KPROBE_BP_INSN      != larch_insn_gen_break(BRK_KPROBE_BP));
+	BUG_ON(KPROBE_SSTEPBP_INSN != larch_insn_gen_break(BRK_KPROBE_SSTEPBP));
+	return 0;
+}
+arch_initcall(check_emit_break);
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe);
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
diff --git a/arch/loongarch/kernel/uprobes.c b/arch/loongarch/kernel/uprobes.c
index 87abc7137b73..90462d94c28f 100644
--- a/arch/loongarch/kernel/uprobes.c
+++ b/arch/loongarch/kernel/uprobes.c
@@ -7,6 +7,14 @@
 
 #define UPROBE_TRAP_NR	UINT_MAX
 
+static __init int check_emit_break(void)
+{
+	BUG_ON(UPROBE_SWBP_INSN  != larch_insn_gen_break(BRK_UPROBE_BP));
+	BUG_ON(UPROBE_XOLBP_INSN != larch_insn_gen_break(BRK_UPROBE_XOLBP));
+	return 0;
+}
+arch_initcall(check_emit_break);
+
 int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe,
 			     struct mm_struct *mm, unsigned long addr)
 {
-- 
2.25.1.362.g51ebf55