Zack's Kernel News
Un-inlining for Debugging
Changbin Du from Intel posted a patch for kernel developers only – it would give developers the option of preventing GCC from auto-inlining code. In the C language, inlining a function tells the compiler not to actually call the function when the user invokes it. Instead, the compiler copies the function's code directly to the place that called it. The drawback is that the compiled binary is bulked up with copies of that particular function, but the benefit is that the code runs faster because it doesn't have to jump all the way over to the function for each invocation anymore.
But GCC can also use its own judgment to inline functions that the developer never specified. It's a great feature, which allows GCC to produce faster output in general. However, as a byproduct, it makes certain Linux kernel debugging tools less effective, because something like the kernel function tracer will only trace functions that have not been inlined.
Changbin's patch prevented GCC from using its own judgment to inline functions. This meant that a whole lot more functions would be analyzed by the kernel function tracer.
The result was striking. Immediately, the function tracer started finding bugs all over the kernel, in areas it had never been able to test before.
Steven Rostedt was highly impressed by the patch's ability to uncover bugs throughout the entire kernel. Johan Hovold was also very impressed but did notice one warning produced by Changbin's patch that was a false positive. He asked if there was some way to trick the test code into just letting this one case slip through, and Steven suggested modifying the kernel code slightly to stop the warning.
Viresh Kumar took a look at the false positive and remarked, "I am not sure what would [be] the best way to get around this incorrect warning." The specific issue had to do with the way the code allocated memory in one particular spot, but didn't clear out the RAM buffer before making use of it. It also wasn't clear why the false warning showed up for only two of the four occurrences in the affected file.
Johan suggested that fixing the false positive might not be the best idea and that it was "probably best to leave things as they are, and let the GCC folks find a way to handle such false positives." Any other fix in the kernel sources, he said, would be "contrived." Viresh agreed that they might as well just leave the false positive as it was, since it did no harm.
Meanwhile Steven did actually come up with a patch to alleviate the false positive, but Johan felt it was overly complicated and remarked, "should we really be working around GCC this way? If the implementation of this new warning isn't smart enough yet, should it not just be disabled instead?"
The conversation ended there. The interesting part of all of that for me is the attention given to a benign and minor circumstance, just because it didn't fit properly. Changbin's code worked and uncovered bugs all over the place, which presumably all got patched. But a couple of false positives – not kernel bugs at all – inspired a discussion of the best way to allocate memory, possible ways to trick GCC into doing the expected thing, and ultimately the decision to allow GCC to continue to produce the false positive so that the GCC people might find and fix the compiler's own misbehavior.
« Previous 1 2
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Gnome 47.1 Released with a Few Fixes
The latest release of the Gnome desktop is all about fixing a few nagging issues and not about bringing new features into the mix.
-
System76 Unveils an Ampere-Powered Thelio Desktop
If you're looking for a new desktop system for developing autonomous driving and software-defined vehicle solutions. System76 has you covered.
-
VirtualBox 7.1.4 Includes Initial Support for Linux kernel 6.12
The latest version of VirtualBox has arrived and it not only adds initial support for kernel 6.12 but another feature that will make using the virtual machine tool much easier.
-
New Slimbook EVO with Raw AMD Ryzen Power
If you're looking for serious power in a 14" ultrabook that is powered by Linux, Slimbook has just the thing for you.
-
The Gnome Foundation Struggling to Stay Afloat
The foundation behind the Gnome desktop environment is having to go through some serious belt-tightening due to continued financial problems.
-
Thousands of Linux Servers Infected with Stealth Malware Since 2021
Perfctl is capable of remaining undetected, which makes it dangerous and hard to mitigate.
-
Halcyon Creates Anti-Ransomware Protection for Linux
As more Linux systems are targeted by ransomware, Halcyon is stepping up its protection.
-
Valve and Arch Linux Announce Collaboration
Valve and Arch have come together for two projects that will have a serious impact on the Linux distribution.
-
Hacker Successfully Runs Linux on a CPU from the Early ‘70s
From the office of "Look what I can do," Dmitry Grinberg was able to get Linux running on a processor that was created in 1971.
-
OSI and LPI Form Strategic Alliance
With a goal of strengthening Linux and open source communities, this new alliance aims to nurture the growth of more highly skilled professionals.