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

On Tue, Jul 14, 2026 at 09:29:13AM +0200, Michal Suchánek wrote:

On Tue, Jul 14, 2026 at 12:20:49AM +0200, Thomas Gleixner wrote:

quoted

On Mon, Jul 13 2026 at 10:44, Michal Suchánek wrote:

quoted

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

quoted

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.

That's an implementation choice of PPC/S390 as I explained before, which
could trivially be solved by having an explicit pt_regs->return_val
member,

Sorry, I got confused, and was looking at the previous revision of the
patchset.

In this revision I do not see any obvious problem that did not exist
before.

Thanks

Michal