SUMMARY Memcheck emulates incorrectly the 'notrack' instruction prefix of i386 code when run on amd64. STEPS TO REPRODUCE 0. $ cat notrack.S # the i386 assembly code _start: .globl _start push $0 # keep stack 2-word aligned after 'call' call jmp_back hlt jmp_back: push %esp pop %ebp .byte 0x3e; jmp *(%ebp) 1. gcc -m32 -g -nostartfiles -nostdlib -o notrack notrack.S # compile and build for i386 on amd64 2. valgrind ./notrack # run on amd64 with Linux support for executing i386 programs 3. gdb notrack # detailed examination of code; see below OBSERVED RESULT $ valgrind ./notrack ==748797== Memcheck, a memory error detector ==748797== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al. ==748797== Using Valgrind-3.24.0 and LibVEX; rerun with -h for copyright info ==748797== Command: ./notrack ==748797== ==748797== ==748797== Process terminating with default action of signal 11 (SIGSEGV): dumping core ==748797== General Protection Fault ==748797== at 0x804900A: ??? (notrack.S:8) ==748797== $ gdb ./notrack # step-by-step run without valgrind (memcheck) (gdb) b jmp_back Breakpoint 1 at 0x8049008: file notrack.S, line 7. (gdb) run Starting program: ./notrack Breakpoint 1, jmp_back () at notrack.S:7 7 push %esp (gdb) x/i $pc => 0x8049008 <jmp_back>: push %esp (gdb) p $esp $1 = (void *) 0xffffcc78 (gdb) stepi 8 pop %ebp => 0x8049009 <jmp_back+1>: pop %ebp (gdb) stepi 0x0804900a in jmp_back () at notrack.S:8 8 pop %ebp => 0x804900a <jmp_back+2>: notrack jmp *0x0(%ebp) ## the address of memcheck complaint (gdb) x/xw $ebp 0xffffcc78: 0x08049007 (gdb) x/i 0x08049007 0x8049007 <_start+7>: hlt (gdb) stepi _start () at notrack.S:4 4 hlt ## successful return to the instruction after the 'call' (gdb) q EXPECTED RESULT No complaint SOFTWARE/OS VERSIONS Windows: macOS: (available in the Info Center app, or by running `kinfo` in a terminal window) Linux/KDE Plasma: Linux 6.12.13 (Fedora 40) KDE Plasma Version: KDE Frameworks Version: Qt Version: ADDITIONAL INFORMATION $ uname -a Linux fedora 6.12.13-100.fc40.x86_64 #1 SMP PREEMPT_DYNAMIC Sat Feb 8 17:10:01 UTC 2025 x86_64 GNU/Linux