[PATCH v5 0/2] PTRACE_SET_SYSCALL_INFO: add support for seccomp syscall skipping

[Renzo Davoli]

PTRACE_SET_SYSCALL_INFO is a generic ptrace API that complements
PTRACE_GET_SYSCALL_INFO by allowing a tracer to modify details of a
system call in which the tracee is currently blocked.

The API is designed to let tracers inspect and modify system call
information in a simple, architecture-agnostic manner.

The current implementation only supports modifying the subset of
system call information needed by strace: the system call number,
arguments, and return value.

This patch set extends PTRACE_SET_SYSCALL_INFO with support for
skipping a system call.

When a seccomp filter returns SECCOMP_RET_TRACE, the tracer receives,
via PTRACE_GET_SYSCALL_INFO, a struct ptrace_syscall_info with
op == PTRACE_SYSCALL_INFO_SECCOMP.

The tracer can skip the system call by setting the system call number
to -1. However, the current PTRACE_SET_SYSCALL_INFO interface does not
provide a way to specify the return value or error code that should be
reported to the tracee after skipping the call.

This patchset extends PTRACE_SET_SYSCALL_INFO to support skipping a system call
triggered via seccomp.

When a tracer retrieves a ptrace_syscall_info structure with 'op' set to
PTRACE_SYSCALL_INFO_SECCOMP, it can now choose to skip the system call.  To do
this, the tracer changes 'op' to PTRACE_SYSCALL_INFO_EXIT and populates the
exit union fields (rval and is_error) to define the return value and error
status for the tracee.

System call suppression via PTRACE_SYSCALL_INFO_ENTRY is currently not
implemented.  On some architectures (e.g. MIPS), when a system call is
skipped by setting the syscall number to -1 at the entry stop, the
architecture entry path unconditionally overwrites the return value
register with -ENOSYS, clobbering any custom return value set by the
tracer at the entry stop.

This patchset is a new version of the proposed patchset entitled:
ptrace_set_syscall_info: add support for seccomp syscall skipping and
instruction pointer modification
The patchset has been split in two:
syscall skipping(this)
instruction pointer modification (it will be updated soon)

Changes in v5:
* reworded the explanation for not supporting PTRACE_SYSCALL_INFO_ENTRY yet.
* selftests/ptrace: removed a redundant check.

Changes in v4:
* Reworded the commit messages for clarity.
* Renamed the local variable child_op to op in ptrace_set_syscall_info()
* Clean up and improve the coding style of selftests/ptrace/set_syscall_info.c
  (suggested by Dmitry V. Levin)

Changes in v3:
* restrict the syscall skipping feature to PTRACE_SYSCALL_INFO_SECCOMP

Changes in v2:
bugfix: _NONE -> _EXIT transition was erroneously permitted

Changes since the previous patchset v2:
* bugfix: skip_syscall init value
* fix comments

Changes in (previous patchset) v2:
* use PTRACE_SYSCALL_INFO_EXIT instead of a new tag
* fixed most of the comments from sashiko.dev

Renzo Davoli (2):
  ptrace: add PTRACE_SET_SYSCALL_INFO syscall skipping support
  selftests/ptrace: add a test case for PTRACE_SET_SYSCALL_INFO syscall
    skipping

 kernel/ptrace.c                               |  27 ++-
 .../selftests/ptrace/set_syscall_info.c       | 172 +++++++++++++++++-
 2 files changed, 193 insertions(+), 6 deletions(-)

-- 
2.53.0