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SubjectRe: [PATCH] selftests: sud_test: return correct emulated syscall value on RISC-V
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On Wed, 13 Sep 2023 07:07:11 PDT (-0700), cleger@rivosinc.com wrote:
> 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>
> ---
> tools/testing/selftests/syscall_user_dispatch/sud_test.c | 8 ++++++++
> 1 file changed, 8 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..1b5553c19700 100644
> --- a/tools/testing/selftests/syscall_user_dispatch/sud_test.c
> +++ b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
> @@ -158,6 +158,14 @@ static void handle_sigsys(int sig, siginfo_t *info, void *ucontext)
>
> /* In preparation for sigreturn. */
> SYSCALL_DISPATCH_OFF(glob_sel);
> +
> + /*
> + * Modify interrupted context returned value according to syscall
> + * calling convention
> + */
> +#if defined(__riscv)
> + ((ucontext_t*)ucontext)->uc_mcontext.__gregs[REG_A0] = MAGIC_SYSCALL_1;
> +#endif
> }
>
> TEST(dispatch_and_return)

I'm not sure if I'm just tired, but it took me a while to figure out why
this was necessary. I think this is a better explanation:

diff --git a/tools/testing/selftests/syscall_user_dispatch/sud_test.c b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
index b5d592d4099e..a913fd90cfa3 100644
--- a/tools/testing/selftests/syscall_user_dispatch/sud_test.c
+++ b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
@@ -158,6 +158,16 @@ 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)
+ (ucontext_t*)ucontext)->uc_mcontext.__gregs[REG_A0] =
+ (ucontext_t*)ucontext)->uc_mcontext.__gregs[REG_A7];
+#endif
}

TEST(dispatch_and_return)

but also

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>

as I agree this is correct.

also: wouldn't arm64 also need to move x8 into x0 here, for essentially
the same reason as we do?

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