https://sourceware.org/bugzilla/show_bug.cgi?id=34000
Bug ID: 34000
Summary: [libopcodes] [x86-64] Some instructions fail to decode
when unneeded prefixes are present
Product: binutils
Version: 2.47 (HEAD)
Status: UNCONFIRMED
Severity: normal
Priority: P2
Component: binutils
Assignee: unassigned at sourceware dot org
Reporter: jos.craaijo at ou dot nl
Target Milestone: ---
I have found the following two valid instructions that fail to decode:
1.) f20fbcff (BSF with unnecessary REP prefix)
2.) 660f01d9 (VMMCALL with unnecessary DATA prefix)
Other disassemblers do decode them:
Disassembling F20FBCFF:
libopcodes: "(bad) .byte 0xff"
capstone: (fails to decode)
XED: "bsf edi, edi"
zydis: "bsf edi, edi"
iced: "bsf edi,edi"
Disassembling 660F01D9:
libopcodes: "(bad) .byte 0xd9"
capstone: "vmmcall"
XED: "vmmcall"
zydis: "vmmcall"
iced: "vmmcall"
I have also verified that these instructions do indeed exist on my AMD 5980HS
CPU.
A test program for the first instruction is:
#include <stdio.h>
#include <stdint.h>
int main() {
__asm__ volatile(
".byte 0xF2, 0x0F, 0xBC, 0xFF"
);
printf("Executed successfully\n");
return 0;
}
The second instruction can also be tested by modifying the bytes in the inline
assembly.
However, note that the second instruction is AMD-only and only valid when
running inside a virtual machine.
Testing on bare metal will result in an undefined instruction exception.
I tested it on QEMU with KVM enabled.
I will try to write a patch to fix this.
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