Re: [patch 16/18] x86/entry: Get rid of the sys_ni_syscall() indirection

[H. Peter Anvin]

On October 23, 2026 7:24:22 AM PDT, Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>On Fri, Oct 23 2026 at 15:09, Thomas Gleixner wrote:
>> On Thu, Oct 22 2026 at 17:22, H. Peter Anvin wrote:
>> Tired brain misread the test output:
>>
>> [RUN]   Running tests under ptrace: modifying the default return
>> [RUN]       Checking system calls with msb = 0 (0x0)
>> [OK]                x32 syscalls 0:-64..-2 returned -ENOSYS as expected
>> [OK]                x32 syscall 0:-1 returned MODIFIED_BY_PTRACE as expected
>> [OK]                x64 syscalls 0:-64..-1 returned -ENOSYS as expected
>>
>> But that's because the "-1" on x32 is actually 0xFFFFFFFF while on x64
>> it's 0x7FFFFFFF.
>>
>> The whole mess comes from using -1 as indicator for syscall abort which
>> is undistinguishable from a syscall(nr=-1).
>>
>> Bah. Let me restore this nonsense.
>
>The real question is how we ended up there. By design or as an
>implementation detail which got later set in stone and the selftest was
>written to test the implementation instead of testing against a
>specification.
>
>I went back in history and in 2.6.12 the behavior was for both i386 and
>x86_64:
>
>  regs->ax = -ENOSYS;
>  syscall_trace_enter();
>  if (syscall_nr < MAX)
>  	do_syscall();
>  ....
>
>IOW, a tracer modification of regs->ax is not overwritten when the
>syscall number is out of range.
>
>Between 2.6.19-rc2 and -rc3 x86_64 was changed to _always_ overwrite
>regs->ax when the syscall number is out of range.
>
>In the 3.8 merge window seccomp introduced the -1 == skip bogosity which
>got fixed up and made "consistent" in several changes afterwards. Just
>that it never reached a state of consistency due to the overloading.
>
>So are we stuck with this behavior or do we sit down and define what the
>correct behavior should be?
>
>Thanks,
>
>        tglx
>
>

Going back on something from 3.8 that is actively used seems unwise. I fear that that ship sailed a long time ago, even though I hate it just as much as you do.