1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
|
<think>
Okay, let's tackle this bug report. The user is saying that in QEMU's RISC-V user mode, the signal frame is misaligned. Specifically, it's aligned to 4 bytes instead of 16 bytes, which causes issues with UBSan's pointer alignment checks.
First, I need to understand what a signal frame is. When a signal is delivered, the OS (or in this case, QEMU emulating the OS) sets up a signal handler frame on the stack. This frame contains the signal information, context, etc. Proper alignment is crucial here because RISC-V, especially 64-bit, requires 16-byte alignment for the stack pointer. If the alignment is wrong, accessing those structures could lead to misaligned memory accesses, which are problematic and caught by tools like UBSan.
Looking at the steps to reproduce: The user created a program that sets up a signal handler with SA_SIGINFO. Then they deliberately misalign the stack by subtracting 4 from the stack pointer (sp). When a SIGINT is sent, the handler prints the addresses of info and context. The addresses are 0x400080025c and 0x40008002dc. Let me check the alignment. The last hex digit is 'c' which is 12 in decimal. Since 12 mod 16 is 12, which isn't 0, but the alignment required is 16 bytes. Wait, maybe I should check if the addresses are aligned to 16 bytes. The addresses end in 5c and dc. Let's see: 5c mod 16 is 12, same for dc. So both are misaligned by 12 bytes. But why? Because QEMU is setting up the signal frame with a 4-byte alignment instead of 16.
The user points to QEMU's source code in signal.c where a constant is used. The comment there mentions aligning the signal frame, and changing the constant from 4 to 15 (which would make the alignment 16 bytes when adding 1?) might fix it.
Now, classifying this bug. The options are instruction, syscall, or runtime.
- Instruction bugs are about incorrect CPU instructions being executed or emulated. This doesn't seem to be about instruction execution, but about the setup of the signal frame.
- Syscall errors involve system calls. The problem here isn't a syscall being handled wrong, but the setup of the signal handler's stack frame, which is part of handling signals, not a syscall itself.
- Runtime errors are other issues during program execution. Since the problem occurs when the signal handler is invoked, leading to misaligned pointers during runtime, this seems like a runtime issue.
The core issue is that the signal frame setup in QEMU isn't properly aligning the stack, which affects the runtime behavior of the program. Therefore, the category is "runtime".
</think>
runtime
|