Re: [patch 4/4] entry, treewide: Make syscall_enter_from_user_mode[_work]() indicate syscall execution

Hello,

On Sun, Jul 12, 2026 at 11:25:32PM +0200, Thomas Gleixner wrote:

The return values of syscall_enter_from_user_mode[_work]() are
non-intuitive. Both functions return the syscall number which should be
invoked by the architecture specific syscall entry code. The returned
number can be:

  - the unmodified syscall number which was handed in by the caller

  - a modified syscall number (ptrace, seccomp, trace/probe/bpf)

That has an additional twist. If the return value is -1L then the caller is
not allowed to modify the return value as that indicates that the modifying
entity requests to abort the syscall and set the return value already. That
can obviously not be differentiated from a syscall which handed in -1 as
syscall number.

The most trivial way to deal with that is:

    set_return_value(regs, -ENOSYS);
    nr = syscall_enter_from_user_mode(regs, nr);
    if (valid(nr))
    	handle_syscall(regs, nr);

That's what LOONGARCH, RISCV, and X86 do. But PowerPC and S390 do not
preset the return value, so when user space hands in -1 and there is
nothing setting the return value in the entry work code, then the syscall
is skipped but the return value is whatever random data has been in the
return value register.

The reason why PowerPC and S390 do not preset the return value is that
the return value uses the same register as the syscall number. There are
apparently other architectures on which the return value overlaps with
the arguments which also do not preset the return value for that reason.
If they would use the generic entry the same problem would arise.

Change the return values of syscall_enter_from_user_mode[_work]() to
boolean and return false, when either ptrace or seccomp request to skip the

There is a difference between seccomp and ptrace.

When seccomp indicates to skip the syscall it has also set the syscall
return value.

However, when the syscall number is -1 and the return value is not
preset that does not indicate anything.

The return value can still hold garbage. ptrace does not have the
ability to indicate that a syscall is to be skipped, at least on the
entry trace. It needs to be skipped based on the syscall number being
invalid.

Thanks

Michal