intr: avoid random trace in (init_)call_intr

[ecm-pushbx]

Here's example output of what happens when setting the CheckDebugger configuration item, then loading the kernel in bootable lDebug and running it:

 -g
 FreeDOS kernel - GIT (build 2043 OEM:0xfd) [compiled Jun  4 2022]
 Kernel compatibility 7.10 - GNUC - FAT32 support
 (C) Copyright 1995-2022 Pasquale J. Villani and The FreeDOS Project.
 All Rights Reserved. This is free software and comes with ABSOLUTELY NO
 WARRANTY; you can redistribute it and/or modify it under the terms of the
 GNU General Public License as published by the Free Software Foundation;
 either version 2, or (at your option) any later version.
 Unexpected single-step interrupt
 AX=3173 BX=0030 CX=FF01 DX=C781 SP=24D0 BP=24FA SI=0C7C DI=5058
 DS=80E5 ES=013D SS=80E5 CS=7C80 IP=0204 NV DN DI PL NZ NA PO NC
 7C80:0204 1F                pop     ds
 -
 Unexpected single-step interrupt
 AX=8001 BX=3D75 CX=11F9 DX=76D0 SP=24F8 BP=2020 SI=8938 DI=B8C6
 DS=80E5 ES=305D SS=80E5 CS=7C80 IP=0204 OV UP DI NG NZ NA PO CY
 7C80:0204 1F                pop     ds
 -

The first of those two single-step interrupts happened to be caused by this code:

kernel/kernel/initoem.c

Lines 41 to 44 in 0e6d90c

	unsigned init_oem(void)
	{
	iregs r;
	init_call_intr(0x12, &r);

As you can see it doesn't initialise the iregs structure and just expects the function to allow arbitrary values. There appear to be many cases of this in the kernel, so I decided to fix it in the call_intr function instead of hunting down all the uninitialised uses.

[tkchia]

Hello @ecm-pushbx,

👀💭
Incidentally, in my libi86, I currently handle this by having the _intrf (...) routine load only the low byte of the flags, via sahf.

Thank you!

[ecm-pushbx]

Here is a longer debugging session depicting the caller of the first unexpected single-step interrupt. The kernel used here is from my build as of today, at https://pushbx.org/ecm/download/fdkernel.zip which will be moved to https://pushbx.org/ecm/download/old/fdkernel/20220530.zip once the next build happens.

lDebug connected to serial port. Enter KEEP to confirm.
= keep
&boot protocol freedos cmdline=1 kernel.sys //
&r
AX=0000 BX=0000 CX=F000 DX=0000 SP=7AEC BP=7C00 SI=0000 DI=0000
DS=7460 ES=0060 SS=7460 CS=0060 IP=0000 NV UP DI PL ZR NA PE NC
0060:0000 EB1E              jmp     0020
&g
Unexpected breakpoint interrupt
AX=8001 BX=0000 CX=0000 DX=0BA0 SP=2596 BP=2596 SI=0017 DI=0152
DS=80E5 ES=80E5 SS=80E5 CS=7C80 IP=00DF NV UP EI PL ZR NA PE NC
7C80:00DF 7202              jb      00E3                            not jumping
&r f CY
-g
Unexpected single-step interrupt
AX=3173 BX=0030 CX=FF01 DX=C781 SP=24D0 BP=24FA SI=0C7C DI=5058
DS=80E5 ES=013D SS=80E5 CS=7C80 IP=0204 NV DN DI PL NZ NA PO NC
7C80:0204 1F                pop     ds
-dw ss:sp
header     0    2    4    6    8    A    C    E    0123456789ABCDEF
80E5:24D0  0000 0206 80E5 0000-013D 24FA 2E8F 0012 ........=..$....
80E5:24E0  24E2 3173 0030 FF01-C781 0C7C 5058 24FA .$s10.....|.XP.$
80E5:24F0  0000 013D 358B 06B6-2500 2590 0AE7 06B6 ..=..5...%.%....
80E5:2500  17B0 00D9 0007 000A-0156 3DE8 F619 0007 ........V..=....
80E5:2510  8001 3D75 11F9 76D0-8938 B8C6 2020 8DAB ..u=...v8...  ..
80E5:2520  305D F189 D9F7 F983-7610 B903 0010 1E1F ]0.......v......
80E5:2530  AC51 D7E8 B080 AA20-F7E2 1659 F71F 83D9 Q..... ...Y.....
80E5:2540  10C1 0774 20B8 AB20-E2AA C6F9 E745 892D ..t..  .....E.-.
-u
7C80:0204 1F                pop     ds
7C80:0205 CD12              int     12
7C80:0207 9C                pushf
7C80:0208 1E                push    ds
7C80:0209 53                push    bx
7C80:020A 89E3              mov     bx, sp
7C80:020C 368E5F08          mov     ds, word [ss:bx+08]
7C80:0210 368B5F14          mov     bx, [ss:bx+14]
7C80:0214 8907              mov     [bx], ax
7C80:0216 8F4702            pop     word [bx+02]
7C80:0219 894F04            mov     [bx+04], cx
7C80:021C 895706            mov     [bx+06], dx
7C80:021F 897708            mov     [bx+08], si
7C80:0222 897F0A            mov     [bx+0A], di
-
7C80:0225 896F0C            mov     [bx+0C], bp
7C80:0228 8F470E            pop     word [bx+0E]
7C80:022B 8C4710            mov     word [bx+10], es
7C80:022E 8F4716            pop     word [bx+16]
7C80:0231 9D                popf
7C80:0232 1F                pop     ds
7C80:0233 5F                pop     di
7C80:0234 5E                pop     si
7C80:0235 5D                pop     bp
7C80:0236 C20400            retn    0004
7C80:0239 5B                pop     bx
7C80:023A 59                pop     cx
7C80:023B 07                pop     es
7C80:023C 58                pop     ax
7C80:023D 5A                pop     dx
7C80:023E 0E                push    cs
7C80:023F 53                push    bx
7C80:0240 06                push    es
7C80:0241 51                push    cx
7C80:0242 CB                retf
7C80:0243 B80043            mov     ax, 4300
-g 236
AX=020D BX=24E2 CX=FF01 DX=C781 SP=24DC BP=24FA SI=013D DI=0000
DS=80E5 ES=013D SS=80E5 CS=7C80 IP=0236 NV UP EI PL NZ NA PE NC
7C80:0236 C20400            retn    0004
-u word [ss:sp] - 20
7C80:2E6F 06                push    es
7C80:2E70 E83BE0            call    0EAE
7C80:2E73 FF8668FF          inc     word [bp+FF68]
7C80:2E77 81C68200          add     si, 0082
7C80:2E7B E93FFF            jmp     2DBD
7C80:2E7E 55                push    bp
7C80:2E7F 89E5              mov     bp, sp
7C80:2E81 83EC18            sub     sp, +18
7C80:2E84 8D46E8            lea     ax, [bp-18]
7C80:2E87 50                push    ax
7C80:2E88 B81200            mov     ax, 0012
7C80:2E8B 50                push    ax
7C80:2E8C E844D3            call    01D3
-
7C80:2E8F 8B46E8            mov     ax, [bp-18]
7C80:2E92 89EC              mov     sp, bp
7C80:2E94 5D                pop     bp
7C80:2E95 C3                retn
7C80:2E96 56                push    si
7C80:2E97 57                push    di
7C80:2E98 55                push    bp
7C80:2E99 89E5              mov     bp, sp
7C80:2E9B FF7608            push    word [bp+08]
7C80:2E9E BF0E04            mov     di, 040E
7C80:2EA1 31F6              xor     si, si
7C80:2EA3 8EC6              mov     es, si
7C80:2EA5 26FF35            push    word [es:di]
7C80:2EA8 56                push    si
7C80:2EA9 FF760A            push    word [bp+0A]
7C80:2EAC 56                push    si
7C80:2EAD E8C9D4            call    0379
-

2E7Eh clearly is the init_oem function. The code not initialising its local iregs structure is visible too. The address matches that found in kernel.map:

 .text          0x0000000000002e7e       0x9f initoem.obj
                0x0000000000002e7e                _init_oem
                0x0000000000002e96                _movebda
                0x0000000000002ec7                _ebdasize

And 01D3h is the init_call_intr function:

 INIT_TEXT      0x00000000000001d3      0x106 intr.obj
                0x00000000000001d3                INIT_CALL_INTR
                0x0000000000000239                INIT_CALL_XMSCALL
                0x0000000000000243                DETECTXMSDRIVER