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

Palmer Dabbelt
2023-12-06T16:29:06Z

On Wed, 06 Dec 2023 05:44:37 PST (-0800), 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>
> 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)

Thanks.

Thomas: looks like you picked up all the commits that touched this? No
rush on my end, just LMK if you want me to pick it up -- I'm going to
leave it alone for now.