John Reiser:
On x86, the byte sequence {0xc7,0310,1,2,3,4} superficially looks like "move
immediate to r/m dword" because of the opcode 0xC7. Actually, it is an illegal
instruction because 0!=(070 & mod_rm); namely, the 0310 should be 0300. Gdb
disassembly should report illegal instruction, but instead says "movl
$0x4030201,%eax".
Please see the original post:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=172034
Steps to Reproduce:
1. Compile and run this program under gdb:
-----foo.S
_start: .globl _start
nop; int3
.byte 0xc7,0310,1,2,3,4
nop; nop
-----
$ gcc -o foo -nostartfiles -nostdlib foo.S
$ gdb foo
(gdb) run
2.
3.
Actual Results: Program received signal SIGTRAP, Trace/breakpoint trap. ## as
expected for 'int3'
0x08048076 in _start ()
(gdb) x/i $pc
0x8048076 <_start+2>: mov $0x4030201,%eax ## gdb says it's OK
(gdb) stepi
Program received signal SIGILL, Illegal instruction. ## CPU rejects
0x08048076 in _start ()
0x8048076 <_start+2>: mov $0x4030201,%eax
(gdb)
Expected Results: Program received signal SIGTRAP, Trace/breakpoint trap.
0x08048076 in _start ()
(gdb) x/i $pc
0x8048076 <_start+2>: (bad) ## modrm of 0310 is illegal for opcode 0xC7
--
Summary: bad disassembly for 0xC7 /1 "movl $___,"
Product: binutils
Version: 2.18 (HEAD)
Status: NEW
Severity: minor
Priority: P2
Component: binutils
AssignedTo: unassigned at sources dot redhat dot com
ReportedBy: web-sources dot redhat dot com at jankratochvil dot net
CC: bug-binutils at gnu dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://sourceware.org/bugzilla/show_bug.cgi?id=2829
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