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<think>
Alright, let's try to figure out whether this bug is related to user-mode or system-mode in QEMU.
First, I'll read the problem description carefully. The issue occurs when setreuid() or setegid() are called from x86_64 target code in user mode. It causes a crash inside NPTL signal handlers within QEMU.
I know that setreuid and setegid are system calls used to change the real or effective user/group IDs of a process. In x86 targets, these calls aren't handled directly with a syscall; instead, they use a temporary data region (__xidcmd) and send a signal (SIGRT1) to all threads. The signal handler then issues the actual syscall.
In this case, under QEMU, __xidcmd remains null, leading to a segmentation fault when accessing it in the signal handler. This suggests that the setup for handling these setuid/setgid operations isn't properly initialized within the user-mode emulation environment provided by QEMU.
Looking at the backtrace, it shows that the crash happens in the signal handler (sighandler_setxid) which is part of the NPTL (Native POSIX Thread Library) implementation. The threads involved are dealing with syscalls and futex operations, which are common in user-space threading implementations.
Since QEMU's user-mode emulation runs unprivileged processes without full OS support, it must correctly emulate all necessary system calls and their associated behaviors. Here, the problem arises because __xidcmd isn't set up, implying that the user-mode code in QEMU doesn't handle these specific system calls or their signal-based implementation properly.
This leads me to conclude that the issue is within the user-mode emulation provided by QEMU, as it's failing to correctly manage the thread-specific setup required for setreuid/setegid. Therefore, this bug should be classified under 'user' mode.
</think>
The bug report describes an issue occurring in the user-mode environment of QEMU, specifically with signal handling and system calls related to changing user IDs. The crash happens within the NPTL implementation's signal handler when these system calls are made, indicating a problem with how user-mode processes are being emulated.
user
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