[RFC V2 PATCH 8/9] riscv: add support for PR_SET_UNALIGN and PR_GET_UNALIGN

[Clément Léger]

Now that trap support is ready to handle misalignment errors in S-mode,
allow the user to control the behavior of misalignment accesses using
prctl(). Add an align_ctl flag in thread_struct which will be used to
determine if we should SIGBUS the process or not on such fault.

Signed-off-by: Clément Léger <cleger at rivosinc.com>
---
 arch/riscv/include/asm/cpufeature.h  |  8 ++++++++
 arch/riscv/include/asm/processor.h   |  9 +++++++++
 arch/riscv/kernel/process.c          | 18 ++++++++++++++++++
 arch/riscv/kernel/traps_misaligned.c |  7 +++++++
 4 files changed, 42 insertions(+)

diff --git a/arch/riscv/include/asm/cpufeature.h b/arch/riscv/include/asm/cpufeature.h
index 7e968499db49..e2fd6fc7157f 100644
--- a/arch/riscv/include/asm/cpufeature.h
+++ b/arch/riscv/include/asm/cpufeature.h
@@ -6,6 +6,8 @@
 #ifndef _ASM_CPUFEATURE_H
 #define _ASM_CPUFEATURE_H
 
+#include <asm/hwprobe.h>
+
 /*
  * These are probed via a device_initcall(), via either the SBI or directly
  * from the corresponding CSRs.
@@ -20,6 +22,12 @@ DECLARE_PER_CPU(struct riscv_cpuinfo, riscv_cpuinfo);
 
 DECLARE_PER_CPU(long, misaligned_access_speed);
 
+static inline bool misaligned_access_emulated(void)
+{
+	return per_cpu(misaligned_access_speed, 0) ==
+					RISCV_HWPROBE_MISALIGNED_EMULATED;
+}
+
 void __init misaligned_emulation_init(void);
 
 #endif
diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 94a0590c6971..4e6667d5ca68 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -7,6 +7,7 @@
 #define _ASM_RISCV_PROCESSOR_H
 
 #include <linux/const.h>
+#include <linux/prctl.h>
 
 #include <vdso/processor.h>
 
@@ -39,6 +40,7 @@ struct thread_struct {
 	unsigned long s[12];	/* s[0]: frame pointer */
 	struct __riscv_d_ext_state fstate;
 	unsigned long bad_cause;
+	unsigned long align_ctl;
 };
 
 /* Whitelist the fstate from the task_struct for hardened usercopy */
@@ -51,6 +53,7 @@ static inline void arch_thread_struct_whitelist(unsigned long *offset,
 
 #define INIT_THREAD {					\
 	.sp = sizeof(init_stack) + (long)&init_stack,	\
+	.align_ctl = PR_UNALIGN_NOPRINT,		\
 }
 
 #define task_pt_regs(tsk)						\
@@ -80,6 +83,12 @@ int riscv_of_parent_hartid(struct device_node *node, unsigned long *hartid);
 extern void riscv_fill_hwcap(void);
 extern int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
 
+extern int get_unalign_ctl(struct task_struct *, unsigned long addr);
+extern int set_unalign_ctl(struct task_struct *, unsigned int val);
+
+#define GET_UNALIGN_CTL(tsk, addr)	get_unalign_ctl((tsk), (addr))
+#define SET_UNALIGN_CTL(tsk, val)	set_unalign_ctl((tsk), (val))
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index e2a060066730..88a71359396b 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -24,6 +24,7 @@
 #include <asm/switch_to.h>
 #include <asm/thread_info.h>
 #include <asm/cpuidle.h>
+#include <asm/cpufeature.h>
 
 register unsigned long gp_in_global __asm__("gp");
 
@@ -40,6 +41,23 @@ void arch_cpu_idle(void)
 	cpu_do_idle();
 }
 
+int set_unalign_ctl(struct task_struct *tsk, unsigned int val)
+{
+	if (!misaligned_access_emulated())
+		return -EINVAL;
+
+	tsk->thread.align_ctl = val;
+	return 0;
+}
+
+int get_unalign_ctl(struct task_struct *tsk, unsigned long adr)
+{
+	if (!misaligned_access_emulated())
+		return -EINVAL;
+
+	return put_user(tsk->thread.align_ctl, (unsigned long __user *)adr);
+}
+
 void __show_regs(struct pt_regs *regs)
 {
 	show_regs_print_info(KERN_DEFAULT);
diff --git a/arch/riscv/kernel/traps_misaligned.c b/arch/riscv/kernel/traps_misaligned.c
index 243ef9314734..5fb6758b0bf9 100644
--- a/arch/riscv/kernel/traps_misaligned.c
+++ b/arch/riscv/kernel/traps_misaligned.c
@@ -9,6 +9,7 @@
 #include <linux/perf_event.h>
 #include <linux/irq.h>
 #include <linux/stringify.h>
+#include <linux/prctl.h>
 
 #include <asm/processor.h>
 #include <asm/ptrace.h>
@@ -305,6 +306,9 @@ int handle_misaligned_load(struct pt_regs *regs)
 	if (!unaligned_enabled)
 		return -1;
 
+	if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;
 
@@ -398,6 +402,9 @@ int handle_misaligned_store(struct pt_regs *regs)
 	if (!unaligned_enabled)
 		return -1;
 
+	if (user_mode(regs) && (current->thread.align_ctl & PR_UNALIGN_SIGBUS))
+		return -1;
+
 	if (get_insn(regs, epc, &insn))
 		return -1;
 
-- 
2.40.1