[PATCH v2] selftests: sud_test: return correct emulated syscall value on RISC-V

["Clément Léger" <cleger@rivosinc.com>]

Currently, the sud_test expects the emulated syscall to return the
emulated syscall number. This assumption only works on architectures
were the syscall calling convention use the same register for syscall
number/syscall return value. This is not the case for RISC-V and thus
the return value must be also emulated using the provided ucontext.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>

---

Changes in V2:
 - Changes comment to be more explicit
 - Use A7 syscall arg rather than hardcoding MAGIC_SYSCALL_1

---
 .../selftests/syscall_user_dispatch/sud_test.c     | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/tools/testing/selftests/syscall_user_dispatch/sud_test.c b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
index b5d592d4099e..d975a6767329 100644
--- a/tools/testing/selftests/syscall_user_dispatch/sud_test.c
+++ b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
@@ -158,6 +158,20 @@ static void handle_sigsys(int sig, siginfo_t *info, void *ucontext)
 
 	/* In preparation for sigreturn. */
 	SYSCALL_DISPATCH_OFF(glob_sel);
+
+	/*
+	 * The tests for argument handling assume that `syscall(x) == x`. This
+	 * is a NOP on x86 because the syscall number is passed in %rax, which
+	 * happens to also be the function ABI return register.  Other
+	 * architectures may need to swizzle the arguments around.
+	 */
+#if defined(__riscv)
+/* REG_A7 is not defined in libc headers */
+# define REG_A7 (REG_A0 + 7)
+
+	((ucontext_t *)ucontext)->uc_mcontext.__gregs[REG_A0] =
+			((ucontext_t *)ucontext)->uc_mcontext.__gregs[REG_A7];
+#endif
 }
 
 TEST(dispatch_and_return)
-- 
2.43.0