A recent Linux kernel change inadvertently broke eBPF programs relying on PT_REGS_RC(regs). Intended to optimize register access for x86, this change accidentally cleared the return value register before eBPF programs using kprobe and kretprobe could access it. This resulted in eBPF tools like bpftrace and bcc showing garbage data instead of expected return values. The issue primarily affects x86 systems running kernel versions 6.5 and later and has already been fixed in 6.5.1, 6.4.12, and 6.1.38. Users of affected kernels should update to receive the fix.
Tanel Poder's blog post, "When eBPF pt_regs reads return garbage on the latest Linux kernels, blame Fred," discusses a perplexing issue encountered while using extended Berkeley Packet Filter (eBPF) programs to trace system calls on recent Linux kernels. The problem manifested as seemingly random garbage data being read from the pt_regs structure, which holds CPU register values at the time of a system call. This structure is crucial for eBPF programs to understand the context of the call and access arguments passed to it.
Poder meticulously details his troubleshooting process, beginning with the observation of inconsistent data when attempting to read the system call number from pt_regs->ax. He suspected a kernel bug, initially focusing on potential issues with the relatively new instruction pointer value caching mechanism introduced to enhance performance. To isolate the problem, Poder employed several debugging techniques, including:
kprobe tracing: He used kprobes, another kernel tracing facility, to directly examine the contents of pt_regs inside the kernel, confirming the corruption wasn't occurring within the eBPF program itself but rather in the data being provided to it.- Kernel debugging with
printk: He added print statements within the kernel code to track the values of pt_regs at various points, helping him pinpoint the location where the corruption occurred.
- Examining kernel source code: Poder delved into the kernel source code, meticulously tracing the flow of execution related to system call entry and the handling of
pt_regs, ultimately identifying a suspicious code path.
His investigation ultimately revealed that the culprit wasn't the instruction pointer caching but rather a seemingly innocuous optimization introduced by a developer named "Fred." This optimization involved reusing a stack variable previously used for the system call number within the __sysvec_tail function, which is part of the system call handling logic. This reuse inadvertently corrupted the pt_regs structure because the stack variable was not properly cleared or reinitialized before being reused for a different purpose.
The consequence of this optimization was that the original system call number within pt_regs was overwritten, leading to the "garbage" data observed by Poder. He explains that this issue was particularly tricky to identify due to its timing sensitivity and dependency on the specific path taken through the optimized code. The problem didn't always manifest, making it appear intermittent and further complicating the debugging process.
The post concludes with Poder highlighting the importance of thorough testing, even for seemingly minor optimizations, and emphasizes the complexity of modern kernel development. He also notes the value of persistent debugging and the use of various tools and techniques to pinpoint the root cause of elusive bugs. He applauds the responsiveness of the kernel developers, who acknowledged and swiftly addressed the issue once identified.
Summary of Comments ( 9 )
https://news.ycombinator.com/item?id=43214576
The Hacker News comments discuss the complexities and nuances of the issue presented in the article about
pt_regsreturning garbage in recent Linux kernels due to changes introduced by "Fred." Several commenters express sympathy for Fred, highlighting the challenging trade-offs inherent in kernel development, especially when balancing performance optimizations with backward compatibility. Some point out the difficulties of maintaining eBPF programs across kernel versions and the lack of clear documentation or warnings about these breaking changes. Others delve into the technical specifics, discussing register context, stack unwinding, and the implications for debuggers and profiling tools. The overall sentiment seems to be one of acknowledging the difficulty of the situation and the need for better communication and tooling to navigate such kernel-level changes. A few users also suggest potential workarounds and debugging strategies.The Hacker News post titled "When eBPF pt_regs reads return garbage on the latest Linux kernels, blame Fred" has generated a moderate number of comments, most of which delve into the technical details of the issue and offer further insights or related experiences.
Several commenters discuss the complexities of the
pt_regsstructure and its usage within the eBPF (extended Berkeley Packet Filter) context. One user highlights the inherent fragility of relying on the layout ofpt_regs, as it is architecture-specific and subject to change. They point out that accessingpt_regsdirectly from eBPF programs is essentially working with a "private, unstable ABI" and that a more robust solution would involve explicitly passing the needed register values to the eBPF program. This echoes the sentiment expressed in the original article about the need for a more stable interface for eBPF programs to access register data.Another comment chain focuses on the challenges of maintaining compatibility in the Linux kernel, especially when dealing with low-level structures like
pt_regs. One commenter mentions the difficulty of keeping track of all the implicit dependencies and the potential for unintended side effects when making changes to core kernel components. They express sympathy for the developers involved, acknowledging the difficulty of balancing performance optimization with maintaining stable ABIs.A couple of commenters share their own experiences with similar issues related to kernel updates and ABI compatibility. One recounts a story of encountering unexpected behavior after a kernel upgrade, which ultimately traced back to changes in internal kernel structures. This anecdote reinforces the point about the inherent risks associated with relying on undocumented or unstable interfaces.
One commenter questions the use of "blame" in the title, suggesting that it is perhaps too strong a word, given that the change was likely unintentional and a consequence of complex system interactions. They advocate for a more understanding approach, acknowledging the difficulty of maintaining such a large and intricate project as the Linux kernel.
The comments also touch upon related topics such as the use of kernel tracing tools, the benefits and drawbacks of eBPF technology, and the trade-offs between performance and stability. While not directly related to the core issue, these comments provide additional context and enrich the discussion.
Overall, the comments on Hacker News provide valuable insights into the complexities of kernel development, the challenges of maintaining ABI compatibility, and the delicate balance between performance and stability. They also offer practical advice for developers working with eBPF and highlight the importance of using stable interfaces whenever possible.