Re: [PATCH v4 2/2] riscv: selftests: Add a ptrace test to verify syscall parameter modification

["Dmitry V. Levin" <ldv@strace.io>]

On Thu, Dec 26, 2024 at 06:52:52PM +0800, Celeste Liu wrote:
> This test checks that orig_a0 allows a syscall argument to be modified,
> and that changing a0 does not change the syscall argument.
> 
> Co-developed-by: Quan Zhou <zhouquan@iscas.ac.cn>
> Signed-off-by: Quan Zhou <zhouquan@iscas.ac.cn>
> Co-developed-by: Charlie Jenkins <charlie@rivosinc.com>
> Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
> Signed-off-by: Celeste Liu <uwu@coelacanthus.name>
[...]
> diff --git a/tools/testing/selftests/riscv/abi/ptrace.c b/tools/testing/selftests/riscv/abi/ptrace.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..023695352215bb5de3f91c1a6f5ea3b4f9373ff9
> --- /dev/null
> +++ b/tools/testing/selftests/riscv/abi/ptrace.c
[...]
> +	if (ptrace(PTRACE_GET_SYSCALL_INFO, pid, PTRACE_SYSCALL_INFO_ENTRY, &syscall_info_entry))
> +		perr_and_exit("failed to get syscall info of entry\n");
> +	result->orig_a0 = syscall_info_entry->entry.args[0];
> +	if (ptrace(PTRACE_GET_SYSCALL_INFO, pid, PTRACE_SYSCALL_INFO_EXIT, &syscall_info_exit))
> +		perr_and_exit("failed to get syscall info of exit\n");
> +	result->a0 = syscall_info_exit->exit.rval;

I'm sorry but this is not how PTRACE_GET_SYSCALL_INFO should be used.

PTRACE_GET_SYSCALL_INFO operation takes a pointer and a size,
and in this example instead of size you pass constants 1 and 2, which
essentially means that both syscall_info_entry->entry.args[0] and
syscall_info_exit->exit.rval are not going to be assigned
and would just contain some garbage from the stack.

Also, PTRACE_GET_SYSCALL_INFO operation returns the number of bytes
available to be written by the kernel, which is always nonzero on any
PTRACE_GET_SYSCALL_INFO-capable kernel.  In other words, this example
will always end up with perr_and_exit() call.

I wonder how this test was tested before the submission.


-- 
ldv