From: Alexander Bluhm <bluhm@openbsd.org>
Subject: ddb disassembler call
To: tech@openbsd.org
Date: Wed, 5 Jun 2024 17:33:23 +0200

Hi,

Disassembling the amd64 call instruction in ddb creates wrong output.
The operand of e8 is only 4 bytes long, not 8.

objdump -Wl -d obj/explicit_bzero.o
/home/bluhm/openbsd/cvs/src/sys/lib/libkern/explicit_bzero.c:19
  44:   31 f6                   xor    %esi,%esi
  46:   4c 89 f2                mov    %r14,%rdx
  49:   e8 00 00 00 00          callq  4e <explicit_bzero+0x1e>
/home/bluhm/openbsd/cvs/src/sys/lib/libkern/explicit_bzero.c:20
  4e:   4c 89 ff                mov    %r15,%rdi
  51:   4c 89 f6                mov    %r14,%rsi
  54:   41 5e                   pop    %r14
  56:   41 5f                   pop    %r15
  58:   c9                      leaveq

buggy ddb
ddb> x/i explicit_bzero+0x14,7
explicit_bzero+0x14:    xorl    %esi,%esi
explicit_bzero+0x16:    movq    %r14,%rdx
explicit_bzero+0x19:    callq   memset+0x4
explicit_bzero+0x22:    movl    %esi,%esi
explicit_bzero+0x24:    popq    %r14
explicit_bzero+0x26:    popq    %r15
explicit_bzero+0x28:    leave

ddb with fix below
ddb> x/i explicit_bzero+0x14,8
explicit_bzero+0x14:    xorl    %esi,%esi
explicit_bzero+0x16:    movq    %r14,%rdx
explicit_bzero+0x19:    calll   memset
explicit_bzero+0x1e:    movq    %r15,%rdi
explicit_bzero+0x21:    movq    %r14,%rsi
explicit_bzero+0x24:    popq    %r14
explicit_bzero+0x26:    popq    %r15
explicit_bzero+0x28:    leave

ok?

bluhm

Index: arch/amd64/amd64/db_disasm.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/amd64/db_disasm.c,v
diff -u -p -r1.24 db_disasm.c
--- arch/amd64/amd64/db_disasm.c	22 Apr 2023 18:26:17 -0000	1.24
+++ arch/amd64/amd64/db_disasm.c	5 Jun 2024 15:31:05 -0000
@@ -793,7 +793,7 @@ struct inst db_inst_table[256] = {
 /*e6*/	{ "out",   0, BYTE,  op2(A, Ib),  0 },
 /*e7*/	{ "out",   0, LONG,  op2(A, Ib) , 0 },
 
-/*e8*/	{ "call",  0, QUAD,  op1(Dl),     0 },
+/*e8*/	{ "call",  0, LONG,  op1(Dl),     0 },
 /*e9*/	{ "jmp",   0, NONE,  op1(Dl),     0 },
 /*ea*/	{ "",      0, NONE,  op1(OS),     0 },
 /*eb*/	{ "jmp",   0, NONE,  op1(Db),     0 },