ptrace: add PTRACE_GET_SYSCALL_INFO request
PTRACE_GET_SYSCALL_INFO is a generic ptrace API that lets ptracer obtain details of the syscall the tracee is blocked in. There are two reasons for a special syscall-related ptrace request. Firstly, with the current ptrace API there are cases when ptracer cannot retrieve necessary information about syscalls. Some examples include: * The notorious int-0x80-from-64-bit-task issue. See [1] for details. In short, if a 64-bit task performs a syscall through int 0x80, its tracer has no reliable means to find out that the syscall was, in fact, a compat syscall, and misidentifies it. * Syscall-enter-stop and syscall-exit-stop look the same for the tracer. Common practice is to keep track of the sequence of ptrace-stops in order not to mix the two syscall-stops up. But it is not as simple as it looks; for example, strace had a (just recently fixed) long-standing bug where attaching strace to a tracee that is performing the execve system call led to the tracer identifying the following syscall-exit-stop as syscall-enter-stop, which messed up all the state tracking. * Since the introduction of commit 84d77d3f ("ptrace: Don't allow accessing an undumpable mm"), both PTRACE_PEEKDATA and process_vm_readv become unavailable when the process dumpable flag is cleared. On such architectures as ia64 this results in all syscall arguments being unavailable for the tracer. Secondly, ptracers also have to support a lot of arch-specific code for obtaining information about the tracee. For some architectures, this requires a ptrace(PTRACE_PEEKUSER, ...) invocation for every syscall argument and return value. ptrace(2) man page: long ptrace(enum __ptrace_request request, pid_t pid, void *addr, void *data); ... PTRACE_GET_SYSCALL_INFO Retrieve information about the syscall that caused the stop. The information is placed into the buffer pointed by "data" argument, which should be a pointer to a buffer of type "struct ptrace_syscall_info". The "addr" argument contains the size of the buffer pointed to by "data" argument (i.e., sizeof(struct ptrace_syscall_info)). The return value contains the number of bytes available to be written by the kernel. If the size of data to be written by the kernel exceeds the size specified by "addr" argument, the output is truncated. [ldv@altlinux.org: selftests/seccomp/seccomp_bpf: update for PTRACE_GET_SYSCALL_INFO] Link: http://lkml.kernel.org/r/20190708182904.GA12332@altlinux.org Link: http://lkml.kernel.org/r/20190510152842.GF28558@altlinux.org Signed-off-by:Elvira Khabirova <lineprinter@altlinux.org> Co-developed-by: