RISC-V: add support for restartable sequences

Add support for RSEQ, restartable sequences, for RISC-V.  This also adds
support for the related selftests.

Note: the selftests require a linker with 3e7bd7f2414 ("RISC-V: Fix
linker problems with tls copy relocs."), which was first released in
2.33 (from 2019).

* palmer/riscv-rseq:
  rseq/selftests: Add support for RISC-V
  RISC-V: Add support for restartable sequence

arch/riscv/Kconfig

@@ -101,6 +101,7 @@ config RISCV
 	select HAVE_FUNCTION_ARG_ACCESS_API
 	select HAVE_STACKPROTECTOR
 	select HAVE_SYSCALL_TRACEPOINTS
+	select HAVE_RSEQ
 	select IRQ_DOMAIN
 	select IRQ_FORCED_THREADING
 	select MODULES_USE_ELF_RELA if MODULES

arch/riscv/kernel/entry.S

@@ -225,6 +225,10 @@ ret_from_syscall:
 	 * (If it was configured with SECCOMP_RET_ERRNO/TRACE)
 	 */
 ret_from_syscall_rejected:
+#ifdef CONFIG_DEBUG_RSEQ
+	move a0, sp
+	call rseq_syscall
+#endif
 	/* Trace syscalls, but only if requested by the user. */
 	REG_L t0, TASK_TI_FLAGS(tp)
 	andi t0, t0, _TIF_SYSCALL_WORK

arch/riscv/kernel/signal.c

@@ -258,6 +258,8 @@ static void handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 		}
 	}
 
+	rseq_signal_deliver(ksig, regs);
+
 	/* Set up the stack frame */
 	ret = setup_rt_frame(ksig, oldset, regs);
 

tools/testing/selftests/rseq/param_test.c

@@ -207,6 +207,29 @@ unsigned int yield_mod_cnt, nr_abort;
 	"addiu " INJECT_ASM_REG ", -1\n\t" \
 	"bnez " INJECT_ASM_REG ", 222b\n\t" \
 	"333:\n\t"
+#elif defined(__riscv)
+
+#define RSEQ_INJECT_INPUT \
+	, [loop_cnt_1]"m"(loop_cnt[1]) \
+	, [loop_cnt_2]"m"(loop_cnt[2]) \
+	, [loop_cnt_3]"m"(loop_cnt[3]) \
+	, [loop_cnt_4]"m"(loop_cnt[4]) \
+	, [loop_cnt_5]"m"(loop_cnt[5]) \
+	, [loop_cnt_6]"m"(loop_cnt[6])
+
+#define INJECT_ASM_REG	"t1"
+
+#define RSEQ_INJECT_CLOBBER \
+	, INJECT_ASM_REG
+
+#define RSEQ_INJECT_ASM(n)					\
+	"lw " INJECT_ASM_REG ", %[loop_cnt_" #n "]\n\t"		\
+	"beqz " INJECT_ASM_REG ", 333f\n\t"			\
+	"222:\n\t"						\
+	"addi  " INJECT_ASM_REG "," INJECT_ASM_REG ", -1\n\t"	\
+	"bnez " INJECT_ASM_REG ", 222b\n\t"			\
+	"333:\n\t"
+
 
 #else
 #error unsupported target