Re: [PATCH v9 2/3] ptrace: add PTRACE_SET_SYSCALL_INFO syscall skipping support

[sashiko-bot@kernel.org]

> Extend 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.
> 
> The skip request is passed to seccomp with seccomp_ptrace_skip_syscall()
> rather than by setting the system call number to -1, so that it does not
> depend on how the architecture lays out the syscall number and the
> return value in the registers.  The tracee then skips the system call
> the same way as for SECCOMP_RET_ERRNO.
> 
> Once the system call has been skipped, a further PTRACE_SET_SYSCALL_INFO
> with op == PTRACE_SYSCALL_INFO_SECCOMP during the same stop fails with
> -EINVAL, as the return value may already have overwritten registers
> holding the syscall number or arguments; op ==
> PTRACE_SYSCALL_INFO_EXIT can still be used to change the return value.
> 
> 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.
> 
> Signed-off-by: Renzo Davoli <renzo@cs.unibo.it>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261020152945.575664-1-renzo@cs.unibo.it?part=2